QUEENSLAND UNIVERSITY OF TECHNOLOGY

SCHOOL OF PHYSICAL SCIENCES

THE INHIBITION OF COPPER CORROSION

IN AQUEOUS ENVIRONMENTS WITH

HETEROCYCLIC COMPOUNDS

Submitted by NGOC HUU HUYNH, School of Physical and Chemical Sciences, Queensland University of Technology in partial fulfilment of the requirements of the degree of Doctor of Philosophy

February 2004

Examination of PhD Thesis Memo

QUEENSLAND UNIVERSITY OF TECHNOLOGY

DOCTOR OF PHILOSOPHY THESIS EXAMINATION

CANDIDATE NAME: Ngoc Huu Huynh

FACULTY: Science

SCHOOL: Physical and Chemical Sciences

CENTRE: Science Research Centre

PRI NCl PAL SUPERVISOR: Dr Steven Bottle

ASSOCIATE S U PE RVI SO R: Dr Paul Schweinsberg

THESIS TITLE: The Inhibition of Copper Corrosion in Aqueous Environmnets with Heterocyclic Compounds

Under the requirements of Ph D regulations, Section 16, it is hereby certified that the thesis of the above-named candidate has been examined. On advice from the Principal Supervisor and Head of School, I recommend on behalf of the University that the thesis be accepted in fulfilment of the conditions for the award of the degree of Doctor of Philosophy.

. . . . . . . ..................... Professor R M&er Chair of Research Degrees Committee

KEYWORDS

Corrosion

Inhibition

Inhibitor

Copper

Alkyl esters of carboxybenzotriazole

Polarisation

Weight-loss

SERS

EIS

Molecular modelling

5

ABSTRACT

Benzotriazole (BTAH) has been used as a corrosion inhibitor for copper and copper-

based alloys for more than 40 years. It has been successfully employed for the

prevention of both atmospheric corrosion and particularly for the protection of

copper under immersed conditions. Whilst BTAH is an excellent inhibitor in alkaline

solution its efficiency drops off markedly as the pH decreases. It was hypothesized

that a possible way to increase surface adsorption and subsequent better inhibition

over a wide pH range might be through the preparation of derivatives, particularly

carboxybenzotriazoles and alkyl esters of these compounds.

In this work the following techniques: weight loss measurements, potentiodynamic

polarisation, SERS spectroscopy, electrochemical impedance spectroscopy and

coulometry were employed to investigate the inhibition efficiency of 4- and 5-

carboxybenzotriazole and their alkyl ester for copper corrosion. Molecular modelling

was also investigated as a tool for inhibitor design.

Studies on 4- and 5- carboxybenzotriazole (CBT) showed that the inhibition

efficiency for copper corrosion in aerated acidic sulphate solution of each isomer was

pH, concentration and time dependant. At lower pH the 5-isomer is the better

inhibitor and this behaviour continues at higher pH. The anti-tarnishing test showed

that whilst both isomers exhibited these properties, 5-CBT was once again the

superior inhibitor.

It was found that a commercial mixture of the octyl esters of 4- and 5-

carboxybenzotriazole inhibits copper corrosion in sulphate environments open to air.

The inhibition efficiency of the ester mixture at the lxlO-' M level (pH - 0) is 98%

which compares very favourably with that for BTAH (- 50%). With respect to other

alkyl esters of 4- and 5-carboxybezotriazole, hexyl, butyl and methyl, it was found

that all of these inlibited copper corrosion in sulphate environments open to air. In

each case the inhibition efficiency is concentration, pH and time dependent. Both

coupon tests and EIS measurements indicate that inhibition efficiency depends on the

length of the alkyl chain. At pH - 0 the inhibition efficiency decreased in the order

octyl >hexyl >butyl >methyl. At higher pH (- 8) the order is reversed. At the 1x104

M level (pH - 0) the inhibition efficiency of each of the alkyl esters is equal to or

better than that for BTAH. At higher pH (- 8) the inhibition efficiency in each case is

7

reduced in comparison to BTAH. but is still good enough for practical use ( 2 75%).

The inhibitive behaviour of the alkyl esters at low pH can be attributed to

chemisorption through an azole nitrogen of the protonated alkyl esters. The

hydrocarbon chain is also physically adsorbed and the increase in physical adsorption

as the chain is lengthened accounts for the improved inhibition efficiency.

Dry films formed by immersing copper in solutions of alkyl esters of

carboxybenzotriazole also inhibit copper corrosion in both strongly acidic (pH - 0)

and near neutral (pH - S) sulphate corrodents. The inhibition efficiency depends on

the solvents used to dissolve the esters, solution temperature and immersion time.

Aqueous coating solutions furnish the most protective films. Films formed by

CBTAH-BU, CBTAH-HE and CBTAH-OE are more protective than that formed by

BTAH. The inhibition efficiency of the alkyl ester film increases as the alkyl chain is

made longer.

Molecular modeling showed that the optimum crude binding energy (Eblnd)

between each protonated ester molecule and the surface varied linearly with the alkyl

chain length. The resulting linear correlation between IE% and E bind for compounds

that are structurally similar suggested that the crude binding energy of a single

molecule with copper may be used to predict the inhibition performance of other

compounds constituting a series.

8

LIST OF PUBLICATIONS

V. Otieno-Alego, N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg ; Inhibitive

effect of 4- and 5-carboxybenzotriazole on copper corrosion in acidic sulphate and

hydrogen sulphide solutions, Corrosion Science 11 (1999) 685-697

N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg ; Inhibitive action of the octyl

esters of 4- and 5- carboxybenzotriazole for copper corrosion in sulphate solution,

Corrosion Science 42 (2000) 259-274

N. Huynh, S.E. Bottle, T. Notoya, A. Trueman, B. Hinton, D.P. Schweinsberg;

Studies on alkyl esters of carboxybenzotriazole as inhibitors for 'copper, corrosion,

Corrosion Science 44 (2002) 1257-1276

N. Huynh, S.E. Bottle, T. Notoya; D.P. Schweinsberg; Inhibition of copper corrosion

by coatings of alkyl esters of carboxybenzotriazole, Corrosion Science 44 (2002)

2583-2596

J. Bartley, N. Huynh, S.E. Bottle, T. Notoya, D.P. Schweinsberg; Computer

simulation of the corrosion inhibitor of copper in acidic solution by alkyl esters of 5-

carboxybenzotriazole, Corrosion Science 44 (2003) 8 1-96

9

TABLE OF CONTENT

11

CHAPTER 3. INHIBITIVE EFFECT OF 4- AND 5- CARBOXY-

BENZOTRIAZOLE ON COPPER CORROSION IN ACIDIC SULPHATE

AND HYDROGEN SULPHIDE SOLUTION ................................... 85

CHAPTER 4. INHIBITIVE ACTION OF THE OCTYL ESTERS OF 4- AND

CHAPTER 5. STUDIES ON ALKYL ESTERS OF CARBOXY-

BENZOTRIAZOLE AS INHIBITORS FOR COPPER CORROSION ---- 119

CHAPTER 6. INHIBITION OF COPPER CORROSION BY COATINGS OF

ALKYL ESTERS OF CARBOXYBENZOTRIAZOLE ...................... 141

12

STATEMENT OF ORIGINAL AUTHORSHIP

The work contained in this thesis has not been previously submitted for a degree or

diploma at any other tertiary educational institution. To the best of my knowledge

and belief, the thesis contains no material previously published or written by another

person except where due reference is made.

Signed ~

Date

13

ACKNOWLEDGEMENTS

I would like to thank the following people and organisations for their support

throughout this research:

My supervisors Dr D.P. Schweinsberg and Dr Steve Bottle who provided the

invaluable vision and guidance throughout this work;

Dr T. Notoya , Hokkaido University, Japan, and Johoku R&D Co., Japan for the

supply of inhibitors;

Dr Harvey Flitt and Dr John Bartley for their work on computer simulation;

Dr Bruce Hinton and Dr Tony Trueman from Aeronautical and Maritime Research

Laboratory, Melbourne for their support in the EIS measurements

Dr Vincent Otieno-Alego for initial work on the carboxybenzotriazoles

My colleagues and friends from School of Physical Sciences, QUT

The Centre for Instrumental and Developmental Chemistry, QUT for their guidance

and financial support

And finally, my wife Phi and my children, Quoc and Minh for their love and

invaluable support throughout my work.

15

CHAPTER 1

INTRODUCTION AND LITERATURE REVIEW

17

1.1 Significance of corrosion

Corrosion is the destructive result of chemical reaction between a material and its

environment’. The most common form of corrosion (discussed in this thesis) is

aqueous metallic corrosion in which the material is a metal or metal alloy and the

environment (corrodent) is an aqueous solution. In daily life, such corrosion is

present in various forms: corroded nails, tools, reddish-orange spots in car bodies,

leaking hot-water tanks, murky potable water are common examples2.

The economic cost of corrosion is enormous, and has been estimated to be in the

range of 2-4% of an industrialised country’s gross national product?. In addition to

these direct costs, there are also indirect costs associated with plant shutdown, lower

efficiency of equipment, contamination and overdesign. Parts and labour to replace

corroded equipment are often minor compared to the loss of production while the

plant is non-operational. Thus leaks in pipelines and tanks result in loss of costly

products and these leaks can also pose a serious environmental problem.

Accumulation of undesirable corrosion products on heat exchanger tubing and

pipelines decreases the efficiency of heat transfer and reduces pumping capacity.

Soluble corrosion products can contaminate a system and decontamination is costly.

In the absence of adequate corrosion rate information (metal weight loss/ unit area /

unit time) overdesign (e.g., thicker tube wall) is required to ensure reasonable service

life. This leads to waste of resources and, for moving parts, greater power

requirements.

1.2. Common methods of corrosion prevention2

In most industrial situations it is virtually impossible to “prevent” corrosion. The

general strategy is to use measures that reduce the corrosion rate to an economically

sustainable level. The most important corrosion mitigation procedures are as follows:

1.2.1. Selection of materials and design

Select materials for the particular working environment (composition,

temperature, ‘velocity) taking into account mechanical and physical properties,

availabiiity, method of fabrication and overall cost of component or structure. It

must be decided whether or not an expensive corrosion-resistant alloy is more

economical than a cheaper metal that requires protection or possible periodic

replacement.

Avoid geometrical configurations that facilitate corrosive conditions such as

19

a) Features that trap dust. dirt and water;

b) Crevices and situations where deposits can form on a metal surface;

c) Designs that lead to erosion-corrosion or to cavitation damage in flowing

systems;

d) Designs with inaccessible areas that cannot be re-protected, e.g., by

maintenance painting;

e) Designs that lead to heterogeneity in the metal or in the environment.

Avoid metal-metal or metal-non metallic contacting materials that facilitate

corrosion such as

a) Bimetallic couple;

b) A metal in contact with absorbent materials that maintain constantly wet

conditions;

c) Contact with substances that gives off corrosive vapours.

Avoid stresses that lead to stress corrosion, corrosion fatigue or fretting

corrosion.

1.2.2. Changing interfacial potential

Protect metal cathodically by making the interfacial (metal/solution) potential

sufficiently negative by means of either (i) sacrificial anode or (ii) impressed

current, i.e. by cathodic protection.

Protect metal by making the interfacial potential sufficiently positive to cause

passivation (formation of a protective film on the metal). This method is confined

to metals that passivate in the corrodent under consideration).

*

1.2.3. Protective coatings

Form metal reaction products, e.g., anodic oxide films on Al, phosphate coatings

on steel, chromate films on light metals (Zn, Al, Cd);

Generate metallic coatings that form protective barriers (Ni, Cr) or protect the

substrate by sacrificial action (Zn, Al, Cd on steel);

Use inorganic coatings, e.g., vitreous enamel, glasses, ceramics;

Apply organic coatings, e.g., paint, plastics, greases.

1.2.4. Changing the environment

20

0 For aqueous corrosion, make the environment less aggressive by removing

constituents or modifying conditions that facilitate corrosion: decrease

temperature, decrease velocity, prevent access of water and moisture, remove

dissolved 0 2 , increase pH (for steel)

For atmospheric corrosion, dehumidify the air and remove solid particles. 0

1.2.5. Adding inhibitors

Corrosion inhibition means the reduction of the corrosion rate of the metal by the

addition of a chemical compound to the solution in contact with the metal3. In

general, an inhibitor forms a protective film in situ by reaction with the corroding

surface. As a result, the rate of the anodic and/or cathodic corrosion reactions are

retarded. Normally, only a small quantity of the inhibiting compound is needed to be

effective (e.g., -104M). Corrosion inhibition is reversible and a minimum

concentration of the inhibiting compound is required to maintain the inhibiting

surface film. Good circulation and the absence of any stagnant areas are also needed

to maintain inhibitor concentration. There is often a synergism between different

inhibitors and commercial formulations usually consist of mixtures. If two or more

alloys are present in the system, specially designed mixtures are required.

Inhibitors are used mostly in recirculating systems. In once-through systems the

consumption of inhibitors is usually too high to be commercially feasible. The

effectiveness of inhibitors depends on solution corrosivity, concentration and

temperature. Many are effective for more than one type of alloy, but an inhibitor for

one metal may be corrosive to others. Both inorganic and organic compounds are

employed and many inhibitors are toxic (chromate, arsenic, hydrazine) and their use

has been limited by environmental regulations. Nevertheless, inhibitors still play a

critical role in corrosion prevention.

Inhibitors are most commonly used in three types of environments:

1. Cooling waters in the near neutral (pH 5 to pH 9) range.

2. Pickling acid solutions for removal of dust and mill scale during the

production and fabrication of metal parts or post-service cleaning of these

parts.

3. Primary and secondary production of crude oil and subsequent processes.

21

1.3. The corrosion of copper and organic inhibitors for copper corrosion

Copper and its alloys exhibit high electrical and thermal conductivity. high

formability, machinability and strength and as a result of these favourable properties

are extensively used in potable water pipes, valves, heat exchanger tubes and tube

sheets, wire, screens, shafts, roofing, bearings, stills, tanks. and printed circuits.

These materials also have good corrosion resistance to de-aerated non-oxidizing

acids such as < 10% HC1 at < 75OC. <70% HF at <lOO°C, 60% HzSO4 at <lOO°C,

H ~ P O J and acetic acid at room temperatures. Copper is not resistant to HNO;, hot

aerated H2S04 and >10% HC1.

It is important to know the mechanism of copper corrosion in order to design,

select and use inhibitors that affect the corrosion rate. In the absence of C1- ions or

NH3, reference to the Pourbaix potential/pH diagram for copper4 shows that the

anodic dissolution of copper occurs (depending on the pH) according to the

following reactions:

cu + cu’ + e-

CU+ -+ CU*+ + e-

The cathodic reaction involves reduction of oxygen and the Pourbaix diagram for

copper shows that hydrogen evolution is not part of the dissolution process.

It is also important to have a good knowledge of the nature and composition of

any corrosion product layer in order to design, select and properly use an inhibitor.

Thus for 10-6 M dissolved ion activity at alkaline pH, cuprous oxide is formed

initially which on oxidation, gives a cupric oxide film which is not stable and

protective. In contrast in acidic solution the inhibitor could be expected to be

adsorbed on a clean copper ~ur face”~ .

Many organic compounds have been used for the inhibition of copper

corrosion and historically one of the most effective is benzotriazole (B TAH).

Because this thesis is concerned with the action of some BTAH derivatives, the

following review concentrates on the inhibiting action of benzotriazole towards

copper and related compounds. The action of other types of surface active organic

compounds is also outlined.

Table 1.1 summarised the main types of organic inhibitors that were studied as

inhibitors for copper corrosion. More details were outlined in sections 1.3.1 to

1.3.36.

22

Type of inhibitors Benzotriazole and analogous compounds

Table 1.1. Types of compounds studied as inhibitors for copper corrosion

Corrosive environments 5% NaC1, HzS, NH,Cl, IN NaOH, cooling water,

NaC104

1.3.1 Benzotriazole and analogous compounds

As early . a s 1967, Cotton et aL5 studied corrosion inhibition of Cu by

benzotriazole and analogous compounds where one or two of the N atoms of

benzotriazole are substituted by C or the labile H atom is replaced by a CH3 group,

i.e., indazole, benzimidazole, indole, and methyl benzotriazole. Salt-spray testing

showed only, benzotriazole and indazole prevented staining and only the former

retained tarnish-resistant properties after the treated Cu surface was washed with

organic solvents. Chemisorption through an azole nitrogen resulted in an insoluble

complex. Substitutional studies in the benzene ring showed C1 or nitro-groups in the

5-position increased acidity of the labile H atom but did not affect inhibition effects;

naphthotriazole behaved similarly but 5-hydroxy indazole and 4-methyl

benzimidazole were less effective than the parent compounds. Generally 5-

membered heterocyclics, imidazole, 1,2,4-triazole, and pyrazole and their derivatives

showed little inhibitive effect. The surface film apparently forms a true metal surface

bond and the complex is polymeric.

Mansfeld6 studied the effect of benzotriazole on the corrosion of Cu in 5% NaC1.

Here benzotriazole is a good inhibitor. It is chemisorbed through N in the azole ring

on the surface and this prevents adsorption of 0 and formation of a prenucleation

layer which is the forerunner of oxide formation. In acid solution, film thickness

appears high7, but for a short exposure time this is a result of a porous salt film which

23

is wiped off easily. For longer exposure times, this underlying film becomes thicker

and cannot be wiped off.

Walker8 reviewed the use of BTAH as a corrosion inhibitor and the theory of its

mode of protection of Cu in aqueous systems. BTAH is a good inhibitor for Cu and

brass when added to many neutral and alkaline solutions and acts as a weak buffer.

Cu surfaces pretreated with a hot solution of BTAH had an increased resistance to

staining in the atmosphere and also to corrosion in many solutions of salts. The

inhibitor reduced the dissolution of Cu in water.

The corrosion of Cu by H2S was inhibited by dipping the metal into aqueous

solutions containing Zn2+, benzotriazole and H2SO:.

The oxidation behavior of Cu dipped in BTAH was compared with that of

untreated Cu in air at 400 OC Io. The oxidation rate of the dipped Cu decreased with

immersion time, benzotriazole concentration and solution temperature.

A packing for Cu is prepared by impregnating paper or plastics with a mixture of

4-methylbenzotriazole and 5-methylbenzotriazole' * . The packing protects Cu from

corrosion in atmospheres containing H2SO3 or H2S.

Reaction products of BTAH with oleic acid, linoleic acid dimer,

di(nonylpheny1)phosphonate or mono(nonylpheny1) phosphate were prepared and

used as copper corrosion inhibitors in lubricating oils12.

Walker13 studied the corrosion of Cu in acidic, neutral and alkaline solutions

containing triazole, benzotriazole and naphtotriazole. Triazole was a poor inhibitor,

while benzotriazole and naphthotriazole were better. Naphthotriazole gave the best

protection. When used as a pretreatment for Cu surfaces, naphthotriazole was also

the most effective.

The inhibitive effect of BTAH as a corrosion inhibitor for Cu in 0.2M acetic,

monochloroacetic, dichloroacetic and trichloroacetic acid solution was studied I 4 - l 5 .

The inhibitor action was satisfactory and afforded adequate protection at a

concentration of 200 ppm. In trichloroacetic acid, a maximum inhibition of 98% was

found.

2-mercaptobenzimidazole, benzotriazole, 2-methylbenzothiazole and indole were

effective inhibitors for corrosion of Cu in HNo3 solutionsI6. A Langmuir isotherm

was obeyed in each case.

Indole, 2-mercaptobenzothiazole, 2-mercaptobenzimidazo1e, benzotriazole,

benzimidazole and 2-mercaptobenzoxazole at 200 ppm in 1N NH&1 were effective

24

corrosion inhibitors for 99.6% Cu and brass” . The inhibitor effectiveness increased

with increasing inhibitor concentration. The ionic species formed insoluble

protective complex salts.

X-ray photoelectron spectroscopy’* showed that on Cu and Cu-Ni alloys

benzotriazole forms a Cu(I) surface complex, which oxidizes rapidly to a Cu(II)

species on removal from the liquid phase. Results suggested that Cu20 facilitates

formation of the surface film.

BTAH, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, benzimidazole and

indole were investigated as corrosion inhibitors for Cu in IN NaOH”. Their

performance was satisfactory.

Azoles and triazines were found to be useful corrosion inhibitors for Cu,

especially in contact with water2’.

The corrosion inhibition of Cu by 1,2,4-triazole in 0.2M NHJCl solutions was

studied by photoelectron spectroscopy2’ . Corrosion inhibition was effected by the

formation of a Cu(II) oxychloride-I complex which produced a weakly adherent

scale.

The inhibitive effect of BTAH and tolytriazole (TTA) as corrosion inhibitors for

Cu and brass in 3% NaCl solutions were investigated22. The protection afforded by

pretreatment was tested in chloride solutions in the presence and absence of BTAH

and/or TTA. TTA by itself was found to be equally as effective as BTAH.

BTAH, 2-mercaptobenzothiazole, and 2,5-dimercapto-1,3,4-thiadiazole were

added to motor oil TB-20 to prevent corrosion of Cu and bronze engine parts by S-

containing antiseizing additives 23. The corrosion inhibitors decreased corrosion of

the engine parts and also improved antiseizing and antioxidation properties of the

motor oil.

BTAH, 2-mercaptobenzothiazole, 2-mercaptobenzimidazole, benzimidazole, and

2-methylbenzothiazole were investigated as corrosion inhibitors for copper in

Na2S0~ and various acid solutions24. All the inhibitors were generally effective in

the Na2S04 solution. 2-Mercaptobenzothiazole was usually effective in the acid

solutions.

Cu foil was treated with a mixture of BTAH, aminosilane and EtOH for increased

corrosion resistance”.

Cu parts for engine cooling systems were immersed in a solution containing

corrosion inhibitor(s) such as mercaptobenzothiazole, BTAH and tolyltriazole26.

25

Under typical cooling water conditions BTAH and tolytriazole (TTA) exhibit

significantly different inhibition b e h a ~ i o r ~ ~ . TTA forms a much thinner but more

hydrophobic film than does BTAH. The inhibitory effect of BTAH and TTA may be

partly explained by the stabilisation of the +1 oxidation state of Cu in the film against

further oxidation via an electronic effect of the triazole.

Tolyltriazole and benzoisothiazolone or its lower alkyl-substituted derivatives

were used as corrosion inhibitors for copper2'. The compounds showed excellent

corrosion inhibiting properties in cooling and boiler-water systems of petroleum

refineries, chemical plants and air conditioning plants.

The inhibition efficiency of BTAH, 5-aminotetrazole monohydrate , 2,5-diphenyl-

3(4-~hlorophenyl)tetrazolium nitrate, tetrazine B, a- and P-naphthylamine,

phenylthiourea, xanthane hydride, 2-mercaptothiazoline, and cupferron was

monitored over time as a function of inhibitor structure, corrosive media

(concentration, pH), concentration of inhibitor and Cu2+ ions, and Cu shape2'. In

general, the inhibitor efficiency decreased with increasing Cu2' concentration.

The inhibition efficiency and mechanism of BTAH, tolyltriazole,

mercaptobenzothiazole Na salt (MBT), and 2-(5-aminopentyl)benzimidazole (ABI)

for the corrosion of Cu in drinking water and synthetic seawater were studied3'.

MBT indicated mostly anodic action compared to the preferentially cathodic

mechanism of the other three.

The adsorption by Cu of the corrosion inhibitors BTAH, 2-

mercaptobenzothiazole, 2-mercaptobenzimidazole and 2-mercaptobenzoxazole was

studied in both neutral and acid chloride solutions by electrochemical techniques and

surface enhanced Raman scattering (SERS) 31. The undissociated inhibitors and their

anions are adsorbed simultaneously, the surface concentration ratio depending on the

pH and electrode potential. At low pH, BTAH is adsorbed weakly and it is displaced

from surface sites by both C1- and the strongly adsorbed 2-mercaptobenzothiazole.

The spectroscopic results explain the low inhibition efficiency of BTAH in acid

solution.

A mixture of mercaptobenzothiazole (I) and tolyltriazole (11) was used as a

corrosion inhibitor for Cu and Cu alloys in aqueous media32. The mixture has a

synergistic effect in aerated water (containing 0.087 % NaC1, pH 7). No synergism

was found in seawater containing 3.0% NaC1.

26

The corrosion rate of a Cu electrode in deaerated 1 . O M HC1 by Fe(III) ions, in the

absence and presence of BTAH was evaluated through weight loss measurement

using a rotating disc electrode (RDE) 33.

BTAH, tolyltriazole (TTAH), mercaptobenzothiazole (MBT), and 2-(5-

penty1amino)-benzimidazole at concentration - 10 ppm effectively inhibit Cu

corrosion in aerated seawater 34. MBT loses effectiveness at - 7OoC and it may not

be the optimum inhibitor for closed cooling units. The thickness of BTAH and

TTAH films formed under cooling water conditions are similar, as are the structures

of BTAH, TTAH and MBT films.

Cu corrosion in a boiler condensate system operating at - 40 atm is inhibited by

feeding a triazole (e.g., benzotriazole, tolyltriazole, or their alkali metal salts) into the

steam header 35. The triazoles are optionally used in combination with neutralizing

or film-forming amines.

BTAH, 2,2'-iminodiethanol-~aprylic acid (1 :2) condensate, and 2-aminoethanol

show a synergistic effect in protecting steel, copper, brass, and silver sheets from

corrosion in aqueous NaCl and/or NalS04 solution36. The caprylic acid Can be

substituted by lauric and oleic acid.

A mixture containing 2-mercapto-benzotriazole, Na tripolyphosphate,

polyethoxylated propyleneglycol ether and alkylpolyglycol ether phosphate prevents

fouling of circuits with corrosive deposits and inhibits biological conversion of

absorbed NH3 to aggressive NO3- 37.

Triazole derivatives and other heterocyclics were studied as corrosion inhibitors

for copper". The inhibitive effect depends on inhibitor concentration, potential, pH,

and temperature. 3-Amino-5-alkyl- 1,2,4- triazoles were the best inhibitors in the

whole pH range since they can form molecular compound layers in neutral and

alkaline solutions but heteropolar-compound layers in acid solutions An

intermediate aliphatic chain length gives the best efficiency since it allows sufficient

solubility and an effective hydrophobicity of the inhibitor layer. The similar

heterocyclic compounds have much smaller efficiencies. Benzotriazole is

comparable in neutral and weak alkaline solutions but it is less effective in acid

solutions and fails at high temperatures.

The cathodic and anodic behavior of Cu in tap water at pH values of 5 , 7 and 9

was studied at 50 OC in the presence different concentrations of 2-aminothiazole

(ATZ) and 2-amino-4,6- dimethylpyrimidine (ADMP)39. The chemical nature of the

27

complex formed between Cu and ATZ or ADMP was found in acidic solution to be a

Cu(II) complex and to a lesser extent Cu(I) complex. The concentration of Cu(I)

complex seems to increase as the solution becomes more basic. Polarisation studies

of the inhibition process suggest that the surface of the Cu in both cases in largely

covered by the complex.

A comprehensive theory on the bonding mechanism of the benzotriazole family of

inhibitors to Cu in neutral. aqueous systems was proposed4’ based on a 7c-bonding.

sandwich structure. This model explains the stereochemistry of the surface film and

its affinity for CU’ in preference to CU’+ ions.

A mixture containing benzotriazole, dioctyl phthalate. and balance iso-Pr alcohol

was applied on a twisted Cu wire and then the wire was coated with polyethylene4’.

The obtained Cu wire showed excellent corrosion resistances in aqueous solution

containing 100 ppm Na2S.

Discoloration of twisted hard-drawn Cu .wires due to Cu corrosion on exposure to

water is prevented without impairing the adhesion of the insulation to the Cu by

coating with corrosion inhibitor solutions containing benzotriazole and phosphate

plasticizers before applying the i n s u l a t i ~ n ~ ~ .

SERS measurements were performed for Cu-benzotriazole and Cu-6-tolyltriazole

interfaces with NaOH-containing electrolyte^^^. The effect of the solution pH was

examined. The suppression of the SERS signals was discussed. The adsorption of

corrosion inhibitors on Cu was assumed.

Alkylbenzotriazoles having the C6-12 alkyl group are corrosion inhibitors for Cu

or Cu alloys in aqueous systems44. The preferred inhibitors are heptylbenzotriazole

and octylbenzotriazole. and form stable hydrophobic films in aqueous cooling

systems and similar applications.

Corrosion inhibitors benzimidazole (BIMH), benzotriazole (BTAH), 2-

mercaptobenzimidazole (SBIMH), and polybenzimidazole (PBIMH) on Cu surfaces

were examined by SERS 4s. The corrosion protection against air oxidation followed

this order: SBIMH > BTAH > PBIMH > BIMH. Oxidation of Cu in 3% HC1,

NaOH. and NaCl was studied. No oxide was found after 12 h of exposure to a salt

solution. Good inhibition was attributed to the heteroatom at the 2-position of the

imidazole ring.

Cu or Cu alloys in aqueous systems are inhibited from corrosion with C3-18

alkoxy alkoxybenzotriazoles, e.g., butoxybenzotriazole. pentyloxybenzotriazole,

28

and/or hexyl-oxybenzothriazole46.

(penty1oxy)-benzotriazole.

A 99% inhibition was achieved with 5-

Near-IR Fourier transform surface-enhanced Raman scattering (FT-SERS) was

used in the study of competitive adsorption of benzotriazole and tolyltriazole on Cu

electrodes". There is coadsorption and benzotriazole displaces tolyltriazole when its

concentration is increased.

Corrosion inhibitors for Cu or Cu-alloy parts in aqueous systems comprise C3-12

alkylbenzotriazole and tolyltriazole, benzotriazole, mercaptobenzotriazole, and/or 1 - phenyl-5-mercaptotetrazole 48,49.

Butylbenzotriazole and tetrasodium EDTA as chelating agent inhibits copper

corrosion in aqueous bath containing NaC1, NaZS04 and NaHC03 (pH 8.5)

Corrosion inhibitors suitable for the protection of Cu or Cu-alloy parts in aqueous

systems (circulating hard water at pH - 7.8) contain a polyphosphate and an azole at

50:l to 1:50 weight ratio ' I . The azole is selected from C2-12-alkyl- or

alkoxybenzotriazoles, tolyltriazole, benzotriazole (with optimal substitution),

mercaptobenzothiazole, and/or 1-phenyl-5-mercaptotetrazole (or its isomers and

salts). The polyphosphate is optionally phosphorylate polyol.

The pitting corrosion of Cu has been investigated in plain NaC104, 7-11 pH

range, and in solutions containing benzotriazole 5 2 . Relationships between the

breakdown potential, the diameter of pits, the spatial pit distribution, the solution pH

and the applied potential have been establisheds2.

The in-situ measurement of open circuit photovoltage and AES technique were

used to investigate the corrosion of Cu in 3% NaCl solution and the inhibition

efficiency of TTA53.

Triazoles, such as tolyltriazole, have become the industry standard corrosion

inhibitors for copper metallurgy found in cooling system heat exchangerss4.

Tolyltriazole (TT) and butylbenzotriazole (BBT) - the most recent addition to the

triazole family - are excellent inhibitors of copper corrosion under a wide variety of

cooling water conditions. The common oxidizing biocide, chlorine, as made

available by the hypochlorite ion, may disrupt rhe triazole inhibitor film if applied in

sufficient concentration and for extended exposure times. Surface analysis studies,

supported by electrochemical data54, were used to examine the copper surface in an

effort to provide a more clear understanding of the triazole/chlorine interaction. The

triazole inhibitor film on copper is not a static barrier; it can be penetrated by water

29

and chlorine molecules, as well as by protons. chloride, and metal ions. Thus. a shot-

feed of chlorine could result in a brief corresponding increase in copper corrosion but

does not disrupt the triazole inhibitor film. However. longer exposures and higher

concentrations of hypochlorite can cause a weakening and ultimately a destruction of

the inhibitor film. resulting in a loss of copper surface protection. This is particularly

the case when there is no residual inhibitor available in the bulk water to repassivate

areas of the film affected by chlorine. This film disruption can be observed. by

surface analysis, from the decrease in intensity of both nitrogen (Nls) and large

cluster-ions. which are characteristic of the protective triazole film. Even after the

attack of chlorine on the triazole inhibitor film. nitrogen-containing species can still

be found on the copper surface. These triazole "remnants" may retard the corrosion

process but they are not a substitute for the original inhibitor film with its unique

protection properties. The BBT inhibitor film is somewhat more stable than that of

TT in the presence of chlorine, this advantage is of a short-term duration only.

Corrosion of Cu alloys in aqueous cooling systems can also be prevented by

adding mixed-isomer tolyltriazole (containing >45% 4-methylbenzotriazole, the

concentration of which is - 0.01-100 ppm), optionally with other corrosion

inhibitors". The 5-methylbenzotriazole isomer or is aerobically degraded in the

presence of bacteria. but the 4- isomer is stable. This mixed-isomer corrosion

inhibitor has proved suitable for service in a cooling-water tower.

The resistance of copper to pitting corrosion from aqueous solutions containing

chloride ions is greatly enhanced when mixtures of benzotriazole and potassium Et

xanthate are present in the bulk solutionj6. The existence of a synergistic effect for

copper corrosion inhibition is supported by electrochemical and surface analysis

data. This effect is attributed to the anodic formation of a continuous protective

chemisorbed layer on the corroding metallic surface in which CuzO oxide becomes

further stabilized. The critical limit at which chloride ion starts breaking the film

causing pitting was also determined.

A photoelectrochemical approach was applied to the characterisation of oxide

layers grown on copper, under open circuit corrosion conditions, in 0.5M NaCl

solutions, containing 1,2,3-benzotriazole or 6-tolyltriazole as corrosion inhibitors".

The influence of the inhibitors on the spectral response of a thin copper oxide layer is

shown. and interpreted with the support of a theoretical model.

30

A correlation between inhibiting efficiency of inhibitor, mainly azole derivatives,

and the photoelectrochemical behavior of the Cu(I) oxide layer was reviewed”. The

copper/benzotriazole system is one for which the contribution of the surface oxide

layer has been examined in conjunction with the effect of the inhibitor. The

mechanism seems to be relatively well resolved when the inhibiting molecular reacts

on a clean surface. It is much more complicated if the molecule has to react in

realistic practical conditions, i.e., for a Cu/Cu20/corrosive layer system.

The influence of azole derivatives, on the corrosion of copper in 3% NaCl over a

range of temperatures has been examined through steady state and weight loss

measurements5’. The results showed that the compounds are mixed type inhibitors.

In the anodic range, they act by forming a passive film on the copper surface, the

existence of the film being more evident for BTAH and diaminotriazole.

The efficiency of 2-amino-5-mercapto- 1,3,4,-thiadiazole (AMT) as a corrosion

inhibitor for copper in 5% (w/w) citric, sulphuric, and hydrochloric acids was studied

60. AMT showed greater inhibition efficiency than BTA in sulphuric and

hydrochloric acids. In citric acid, AMT and BTA show similar behaviour.

Traditional electrochemical tests and the contact electrical resistance technique

were used to investigate the effect on corrosion of 99.999 wt.% Cu by adding

benzotriazole and 1 -hydroxybenzotriazole (1 -0HBTAH) to H2S04/ Na2S04 solutions

at pH 1.7 61. This technique permitted growth of oxide and/or salt films as well as

adsorption of the organic inhibitors on the Cu surface to be evaluated. Formation of

(Cu20)2, CuS04.5H20, CuSCN, CuI, CuBr, and CuCl films on Cu electrodes was

followed in-situ in sulphate solutions at various pH values under low overpotentials.

The effects of pH, solution anion content, and/or amount of corrosion inhibitor on the

electrical resistance of the surface films formed on Cu electrodes were investigated.

BTAH acted as a more efficient corrosion inhibitor than 1-OHBTAH and reached an

inhibition efficiency of -90%, compared to that of 1-BTAH which reached a

maximum of -76% in 2x10” M solutions. It was possible to distinguish between

maximum resistance of the surface film found in solutions containing BTAH

associated with the adsorption of neutral inhibitor molecules and the sharp rise in

resistance attributable to Cu-I complex formation.

The cathodic and anodic behavior of copper in 30% ethylene glycol containing

SO:-, C1- and HCO3- were studied in the presence of 3-phenyl-l,2,4-triazolin-5-one

(PTR)62. The chemical nature of the complex formed between Cu and PTR was

31

found to be a Cu(II) complex as detected by XPS.

inhibition process suggested that the surface is largely covered by the complex.

Polarisation studies of the

The inhibitive action of 2-mercaptobenzo-thiazole (MBT) against the corrosion of

Cu, brass, and AI in 0.1 N AcOH, ClCH2C02H: Cl2CHCO2H. Cl3CC02H was

studied in 63. MBT was a fairly satisfactory inhibitor, affording adequate protection

to Cu and brass in these environments. MBT is predominantly an anodic inhibitor.

Patel@ reported the inhibiting action of MBT, 2-benzimidazolethiol (BI) and

sulfathiazole (ST) against corrosion of Cu in dichloroacetic acid solution. BI and

MBT are effective inhibitors with 80 and 77% inhibition respectively at 0.02%

concentration. The inhibiting action is due to chelate formation. Patel also studied the

protection of Cu against 3% NaCl by p-thiocresol (TC), BI and SI 65. TC gave 58%

inhibition at 0.001% concentration and 99% at 0.02%, BI gave 27% at 0.001%

concentration and 90% at 0.004%, and SI gave 32% at 0.001% concentration and

98% at 0.02%. Thus all are efficient inhibitors, owing to a film of insoluble chelate

with Cu ions formed at the metal surface.

The effectiveness of 2-amino-5-mercapto- 1,3,4-thiadiazole as a corrosion inhibitor

for Cu in acidic, neutral, and alkaline solutions was investigated66. The inhibitor

forms a polymeric complex layer on the Cu surface in solutions having different pH

values and completely inhibits corrosion. A Cu sheet with the polymeric complex

layer was subjected to different corrosive atmospheres, and the complex layer was

resistant to corrosion. Formation of a complex layer was confirmed by IR spectral

studies.

The impedance technique was used to study the adsorption properties of

mercaptobenzothiazole (MBT) in the corrosion inhibition of Cu in 1% aqueous

Na2S04 (pH - 6 ) 67. Stable adsorption of MBT was observed within the wide region

of potentials (* 0.3 V near the corrosion potential). The maximum degree of

coverage of the metal surface was at the MBT concentration - l ~ l O - ~ mol/L. High

adsorption energy was indicative of the chemisorption of inhibitor on Cu. The

inhibitive effect of MBT was mainly anodic.

Surface films formed by two structurally related corrosion inhibitors, 2-

mercapto benzothiazole and 2-mercaptobenzimidazole, on Cu were studied by XPS

and X-ray induced Auger spectroscopy6*. Under all conditions these corrosion

inhibitors form Cu' surface films of various stoichiometries. The film thickness were

closely related to the pH of treatment solutions, which reflects the stability of Cu20,

32

and the thickness of the original CuzO layer. A precipitation mechanism is suggested

for film formation.

The retarding effect of benzimidazole, 2-mercaptobenzimidazole, 2-methylbenz-

imidazole, 5,6-dimethylbenzimidazole and 2-mercaptobenzothiazole on the corrosion

of Cu in chlorinated water was investigated at 3OoC 69. The 5,6-

dimethylbenzimidazole gave 97% protection for 10 days.

Corrosion of Cu and 63/37 brass in 0.1N HOAc, CIH2CC02H, C12HCC02H and

C13CC02H at 3OoC was studied by the weight-loss method 70. 2-

Methylbenzothiazole was found to be a good inhibitor in C13CC02H.

Corrosion of copper in diluted acids in presence of chelating agents, 2-amino-5-

mercapto- 1,3,4-thiadiazole, 2-amino-4-benzothiazole and imidazoline-2-thione was

studiedat 30 C . 0 71

Imidazole compounds were used as corrosion inhibitors for preventing nest-form

corrosion of Cu materials (e.g., Cu pipe) caused by lubricants containing O-

containing organic compounds72.

Imidazole derivatives, which are secondary products in Vitamin B6 production,

are used as atmospheric corrosion inhibitors for copper in electronic devices industry

and in the printed circuit board p r o d ~ c t i o n ~ ~ .

Addition of 200 ppm benzoxazole to 0.1N trichloroacetic acid gave corrosion

inhibition of - 85 to 88% for immersed Cu and 63/37 brass, re~pect ively~~. Addition

of 2-mercaptobenzoxazole gave 100% protection for both metals.

1.3.2. Mercaptan

The relationship between the amounts of Bu2O and cetyl mercaptan was

investigated with respect to the corrosion inhibitive rate7j. Mercaptan is adsorbed on

Cu only when the ether, a proton-accepting substance, has removed the water

adsorbed on Cu by H-bonding. However, when excess of the ether was present in

the filming solution, the ether was preferentially adsorbed on the metal, and

mercaptan could not be adsorbed.

Other experiments were carried out using Bu2O as a proton-accepting substance

and dodecyl mercaptan as an i n h i b i t ~ r ~ ~ . Mercaptan acts as a filming agent only in

the presence of strongly proton-accepting substances.

1.3.3. Carbamate

33

Sodium diethylcarbamate was used to inhibit copper corrosion in various buffer

solutions of NaOH and citric acid (pH 4.3-5.9) 77.

A mixture of thiocarbamate and a benzotriazole prevents copper corrosion and

barnacle adhesion in a cooling system using sea water78.

The inhibition effect of di-ethyl dithiocarbamate, piperidine dithiocarbamate and

morpholine dithiocarbamate on copper in acids was in~est igated~~. Di-ethyl

dithiocarbamate, piperidine dithiocarbamate are more effective inhibitors than

morpholine dithiocarbamate in 0.1N H2S04, HC1 and HNO3.

The effect of sodium diethyldithiocarbamate on the corrosion of copper and brass

in seawater was evaluated The pronounced inhibition action of

diethyldithiocarbamate is attributed to metal-sulphur bond formation and it acts as an

anodic inhibitor.

The organic compound consisting of phenothiazinyl and/or the thiocarbamic

group is used for the prevention of corrosion of Cu by the 0-containing components

of lubricating oils". The inhibited lubricating oils are especially suitable for bending

fabrication of copper tubes for a i rconditioners.

1.3.4. Nitrobenzoates and nitronaphtolates

Dinitrobenzoates of hexamethyleneimine were studied as corrosion inhibitors for

copperg2. The protection of the metal is due primarily to anodic retardation. The 3,5-

dinitrobenzoate of hexamethyleneimine is a more effective inhibitor than oxidized

amines, p-nitrobenzoate, benzoate, and o-nitrobenzoate.

Nitrophenolates of primary amines (methyl- and trimethylammonium 2,4-

dinitrophenolates) are effective corrosion inhibitors for Cu and Zn in H20g3.

The effect of polar groups in naphthalene compounds on their protective

efficiency and protection mechanism was investigated with respect to the corrosion

of steel, Zn, and Cu in distilled H2OS4. The most efficient inhibition was obtained

with Na nitronaphtholates. The protection mechanism of Na nitronaphtholates is

based on a chemical adsorption and that of Na naphtholates on a physical adsorption.

1.3.5. p-Thiocresol

The corrosion inhibition of Cu in 0.5N HC1 and 0.5N HOAc by 2-

benzimidazolethiol was studied 'j. 2-benzimidazolethiol inhibited the corrosion in

34

both acids. The inhibitive action was attributed to chelate formation with the metal

ion. The inhibition is predominantly controlled by cathodic polarisation.

The inhibiting effect of p-thiocresol and thiobenzoic acid towards Cu corrosion in

0.5N HNO3 is due to the formation of chemisorbed 2-dimensional barriers of a

molecular thicknesss6.

Thiophenol and p-thiocresol were found to inhibit the corrosion of Cu by NH4C1

solutions7.

The inhibitive action of 2-mercaptobenzoic acid, phenylthiourea, and

thiosemicarbazide on the corrosion of Cu in HOAc solution was studieds8.

Thiosemicarbazide gave the best protection, which is due to the formation of an

insoluble Cu complex.

The protective action of thiosemicarbazide, Na diethyldithiocarbamide, and 2-

mercaptobenzothiazole for Cu in 3% NaCl solution was investigated8’. 2-

mercaptobenzothiazole was more effective than Na diethyldithiocarbamide and

thiosemicarbazide offered the least protection

1.3.6. Thioglycolic acid

The adsorbed layer of thioglycolic acid formed as a corrosion inhibitor on a Cu

surface in diluted HNo3 was studied by IR spectrometrygO.

1.3.7.2-Mercapto-2-thiazoline

The effect of 2-mercapto-2-thiazoline as a corrosion inhibitor for Cu in HOAc,

C~CHZCCO~H, C12HCC02H, and C13CC02Hy was studied’’. The protective power of

2-mercapto-2-thiazoline increased with increase in acidic character of the acid, that

is, C13CC02H > C12HCC02H > ClH$2CO2H > HOAc. The reaction is cathodically

controlled.

A mixture of polyethylene polyamines and 1,2-benzoisothiazolin-3-0ne is suitable

for corrosion prevention of cu pipes in aqueous systemg2.

1.3.8. Cyclohexylamine carbonate

Cyclohexylamine carbonate is a volatile corrosion inhibitor for Fe and Cu

surfaces 93. In the case of Cu, cyclohexylamine carbonate does not act as an inhibitor

in the liquid phase but on the contrary accelerates the corrosion.

35

1.3.9. Acetylenic alcohols

Acetylenic alcohols were effective as inhibitors for copper in simulated industrial

atmospheres containing SO2 and humid air94. Certain alcohols of this type, in

particular 1 -ethynylcyclohexanol protect Cu and brass satisfactorily in acid

atmospheres.

1.3.10. Pyridine derivatives

The inhibiting effect of pyridine derivatives on Cu corrosion in 0.1M K2S208 was

as follows: 2,6-lutidine > 2-picoline > pyridine and m-anisidine > p-phenetidine >

aniline where aniline is less effective than pyridine".

The inhibition of the corrosion of A1-4% Cu alloy (B26S) in HC1 solutions by

some N-heterocyclic compounds was in the order: 4-picoline < 3-picoline < 2-

picoline < pyridine < piperidine < acridine 96. The inhibitors function through

general adsorption, following the Langmuir adsorption isotherm.

1.3.1 1. Dihydroxybenzene

Dihydroxybenzenes such as catechol, hydroquinone and resorcinol inhibit the

corrosion of Al-4% Cu alloy (AI B 26S) in NaOH and

The effect of some 4-amino-4'-nitroazobenzene derivatives as corrosion inhibitors

for Cu in HN02 solution was investigated by using thermometric and polarization

techniques98. All the inhibitors follow the Frumkin adsorption isotherm.

1.3.12. Phenols

Corrosion inhibition of AI B26S by phenols in NaOH was studied99. The

inhibition efficiency increases as follows: PhOH < o-cresol < m-cresol < p-cresol.

The better inhibition qualities of the latter cresol may be traced to the absence of

steric hindrance.

A general corrosion study was made on the aminophenol-NaOH- Al/Cu alloy

system"'. The inhibitor efficiency increases in the order: p-aminophenol < m-amino-

phenol < PhOH < o-aminophenol. Chelate formation and the Freundlich adsorption

isotherm are suggested as the main factor in the corrosion inhibition mechanism.

The inhibition of corrosion of AI-Cu alloy by p-substituted phenols was studied

'O1. At 2.0% inhibitor concentration in 0.1M NaOH, the efficiency of the inhibitors

increased in the order p-hydroxydiphenyl < p-aminophenol < p-bromophenol <

36

hydroquinone < p-chlorophenol < p-cresol< p-hydroxyacetophenone < p-

nitrophenol< PhOH.

Inhibition of corrosion of B26S (Al-4%Cu alloy) in solutions of NaOH by

polyhydric phenols was studiedIo2. At 1% concentration in 0.1M NaOH the

efficiency of the inhibitors increases in the order: phenol (83.3%) < hydroquinone <

catechol < resorcinol < pyrogallol and phloroglucinol (98.9%); i.e.: monohydric <

dihydric < trihydric phenols. Nearly 100% inhibition is achieved with 2% inhibitor

in 0.1 M NaOH. The increase in steady-state potentials and galvanostatic polarisation

data suggest that the inhibitors are of the mixed type.

Inhibition by nitrophenols of the corrosion of B26S (Al-4% Cu alloy) in solutions

of NaOH was studiedio3. At an inhibitor concentration of 0.1-2.0% in 0.1M NaOH,

the efficiency increases in the order: p-nitrophenol <m-nitrophenol 5 o-nitrophenol <

phenol. The inhibitors appear to function through adsorption, following the

Langmuir adsorption isotherm. Galvanostatic polarisation data and open-circuit

potentials suggest that all 4 compounds are mixed-type inhibitors.

The dissolution of Cu in W O 3 solution in the presence of resorcinol, 0-, p-

aminophenols, catechol, o-cresol, and salicylaldehyde as corrosion inhibitors was

studied Io4. The inhibitors appear to function through general adsorption following

the Langmuir adsorption isotherm. The inhibiting effect of the tested compounds is

due to the decomposition of HNOz formed and its interference with the cathodic

reaction.

1.3.13. Colloids

Colloidal corrosion inhibitors for copper-aluminium alloy (B26S) in aqueous

0.1M NaOH were studiedIo5. The inhibition efficiency was due to adsorption and the

efficiency increased in the order: gelatin < dextrin < glue < agar agar < acacia <

tragacanth.

B26S A1 alloy was also exposed to Cl-substituted acetic acids containing colloids

as inhibitorsIo6. The colloid inhibitor efficiency (1.5% in 0.1 N CI~CCOZH) increased

in the order dextrin (42%) < gelatin < agar-agar < acacia < glue (89%). In HCI the

efficiency of the inhibitor increases in the order: dextrin (40%) < gelatin (56%) <

agar-agar (64.3%) < acacia (77.5%) and glue (81%) Io7. All five substances are

effective mixed inhibitors.

37

1.3.14. Benzimidazole

Benzimidazole was investigated as a corrosion inhibitor for brass in

trichloroacetic acid solutionio8. Its efficiency increased with increasing C1 content of

acetic acid i.e., CC13COOH

> CHC12COOH > CH2ClCOOH > AcOH. The inhibitive effect of benzimidazole is

mainly attributed to its chelate-forming tendency with Cu ions.

Thiocarbonyl compounds were effective corrosion inhibitors in acidic media' 09.

The most effective compound was 2-mercaptobenzimidazole.

2-(4-thiazolyl)-benzimidazole was used as corrosion inhibitor in water containing

C1- (500ppm) and S042- (500 ppm) ' I o .

Cu and Cu alloys were coated with 5-Me benzimidazole 5-60 nm thick and this

was effective against oxidation and corrosion" I .

The structure of various imidazoles on the surfaces of Cu and Au mirrors was

studied by Fourier transform IR reflection-absorption spectroscopy in order to

elucidate the molecular mechanisms of corrosion inhibition of metal surfaces by

imidazoles"*.

Fourier-transform IR reflection-absorption spectroscopy and measurement of Cu

oxide formation showed that poly(N-vinyl-, or 4(5)-vinylimidazoles) are effective

antioxidants (better than benzotriazole and imidazoles) for Cu at elevated temps' 1 3 .

Below 25OoC, no major degradation of the coated films on Cu was observed.

1 .3.15. Hydrazines

2,4-dinitrophenylhydrazine functions as a corrosion inhibitor for Cu in HzS04 by

inhibiting the cathodic reaction ". Corrosion inhibitors, prepared by adding N2H4 to phytic acid or its salt, were used

in a coolant for Cu vessels used for rapid cooling of foodIi6. The N2H4 inhibits

growth of oxide film on the Cu in contact with the coolant.

The effect of tosylhydrazine (THy) and 4-nitrobenzoylhydrazine (4-NBHy) on the

corrosion of Pb, Al, and Cu in NaOH solution was studied"'. These inhibitors

facilitate the conversion of Cu(OH)2 to black CuO film and thus inhibit corrosion of

Cu in alkaline solutions.

A mixture containing N-alkyl or -arylisoquinolinium bromide, hydrazine hydrate,

DMF, and still residues (from scrubbing of coking gas with monoethanolamine) is

38

used as corrosion inhibitor for Cu, Cu-containing alloys- and austenitic stainless steel

in aqueous acidic media"'.

1.3.16. Anilines

The action of aniline and twelve derivatives as inhibitors for the corrosion of

B26S aluminum in H3P04 solutions was e~amined ' '~ . At 0.5% inhibitor

concentration in 0.033M H3P04, p-tbluidine was the most effective.

The effect of substituted anilines on corrosion of Cu in 2N HNO3 was studied by

using weight loss and polarisation techniques'20. The relative inhibition efficiency of

these compounds was determined by the nature of the substituent group and its

position in the aromatic ring, as well as its concentration. These substituted anilines

act mainly as cathodic inhibitors. Both diazotization (removal of HN02) and

adsorption characteristics of amines (or diazonium salts) on the metal surface play a

significant role in inhibition.

1.3.17. Cyclohexylamine and amine

Salts prepared by reacting carboxylic acids R-CONH-R1 CO2H (R = C6- 12 aryl or

alkaryl, R1 = C1-5 alkylene) with alkanolamine and/or cyclohexylamine are used as

indispensable components for corrosion inhibition of Fe, Cu, and their alloys'2'.

C2-8-alkanolamine mercaptobenzothiazolate or alkali metal

mercaptobenzothiazolate, C 1 -8-alkanolamine phosphate, and a surfactant were used

as corrosion inhibitor in a cooling circuit'22. The amount of deposits, steel corrosion

and brass corrosion were decreased by 80,70 and 65 %, respectively.

Hexamethylenetetramine (HMTA) is a corrosion inhibitor for Cu in a bromide

medium containing Fe(III) ion or oxygen as oxidants'23. The passive film was

characterized by fluorescence spectroscopy as being a Cu(1)-HMTA-Br-complex.

Alkali metal salts of 2-(NYN-dialkylamino)-4,6-dimercapto-l ,3,5-triazine are

suitable corrosion inhibitors for Cu in electricity-conducting cables'24.

The inhibitive effects of aminoacids, e.g. d-alanine (AL), 1 -phenylalanine (PAL),

1-cystine (CYS), and 1-methionine (MET) on the corrosion of copper in nitric acid

have been investigated"'. Inhibitive efficiencies were in the order: MET > CYS >

PAL > AL.

Amine series organic compounds are used for prevention of corrosion of Cu by

the 0-containing components of lubricating oils'26. The inhibitors have general

39

formulas R(NH2)n, R I R ~ ( N H ) ~ , and RlRzR;N, where R, R1, Rz, and R3 = alkyl, Ph,

naphthyl, or alkyl Ph group, n = 1-3, and m = 1-2. The inhibitors are added to

lubricating oils containing an 0-containing organic. The lubricating oils are

especially suitable for bending fabrication of copper tubes for air conditioners.

1.3.1 8. Piperazines

A mixture containing 10-30% N-acylpiperazines and/or N-

(acylaminoalky1)piperazines , 70-90% alkylimidazolines and alkylamidoamines is

effective corrosion inhibitor for copper in aqueous media at pH 7-1 1 12'.

1.3.19. Benzoxazole

Ag or Cu is protected from tarnishing with 2-mercaptobenzoxazole or 1 -phenyl-

lH-tetrazole-5-thiol, which may be included in a resin solution for forming a wear-

resistant coating'28.

1.3.20. Thiourea

Thiourea was a more effective inhibitor than Na benzoate for Cu corrosion in

0.2 5 -N NaCl solutions1 29.

Corrosion inhibitors for preventing nest-form corrosion of Cu materials (e.g., Cu

pipe) caused by lubricants containing 0-containing organic compounds are urea or

thiourea compounds having general formulas: RNHCONH:! (R=H, alkyl, Ph,

naphthyl, alkylphenyl, or acetyl), RlNHCONHR2 (Rl,Rz=alkyl, Ph, naphthyl,

alkylphenyl, or acetyl), RNHCSNH2 (R=H, alkyl, Ph, naphthyl, alkylphenyl, or

acetyl), and RlNHCSNHRz (RI ,Rz=same as above)13'.

Thiourea, allylthiourea and phenylthiourea have been investigated as corrosion

inhibitors for AI-4% Cu alloy in trichloroacetic acid'31. Substitution of amino H

atoms by phenyl or allyl groups affects the inhibitive action and suggests that the

resonance effect increases more with phenyl than with allyl groups.

1.3.2 1. Benzoic acid

The corrosion of Cu in HC104 solution containing various concentrations of

benzoic acid, p-toluic acid, p-nitrobenzoic acid, phthalic acid, and terephthalic acid

was studied132. These compounds inhibit corrosion effectively even in trace

concentrations.

40

Salts of amines and amino alcohols with nitrobenzoic and dinitrobenzoic acids are

effective inhibitors which practically suppress the atmospheric corrosion under

conditions of 100% relative humidity of the surrounding^'^^.

1.3.22. Aminopyrimidine

The protective coating produced during the corrosion of Cu in the presence of 2-

aminopyrimidine (I) was studied"'. The individual units form a 3-dimensional

macro-structure via chloride bridging and H bonding.

The mechanism of the corrosion inhibition of Cu by 2-mercaptopyrimidine and

other pyrimidine derivatives were disc~ssed'~'. These compounds formed a

complexed coating on Cu and this can be simulated by their reaction with CuCl in a

solution.

1.3.23. Triphenylmethane and dyes

The inhibition of the corrosion of Cu-A1 alloy (B26S) in HC1 solutions by

triphenylmethane, anthraquinoid, and acridine dyes was studied'36. At an inhibitor

concentration of 0.1% in 0.5 M HC1, the efficiency increases in the order: fuchsine

acid (32%) < acridine orange 5 fuchsine base (39%) < alizarin red S (46%) < methyl

violet 6B (50%) < malachite green (64%) < crystal violet (70%) < light green (80%).

At 0.5% inhibitor concentration in 0.033M H3PO4, the efficiency increases in the

order: methyl violet 6B (15%) <crystal violet <fuchsine acid <fuchsine base (36%)

<malachite green (68%) <victoria blue (75%) <light green <fast green (88%) 137. In

both media the inhibitors function through adsorption following the Langmuir

adsorption isotherm.

1.3.24. Xanthenes and azo dyes

The corrosion inhibition of AI-4% Cu alloy (B26S) by xanthene and azo dyes in

HC1 solutions was studied'38. At 0.1% inhibitor concentration in 0.5M HC1, the

efficiency of inhibitors increased in the order: methyl red (-40%) < dimethyl yellow

(34%) < catechol violet < eriochrome black T < rhodamine B < fast sulfon black F <

bromocresol purple < bromocresol green (88%) The inhibitors functioned by

adsorption following the Langmuir isotherm. In 5.0 M HC1 acid the inhibition

efficiencies are in the order: bromocresol purple (1 6.8%) <<fuchsine acid(94.4%) 5

41

bromocresol green I methyl red (-99%) <crystal violet I fuchsine base 5 dimethyl

yellow I light green I malachite green

The effect of 1 -ethyl-2-(o-hydroxystyryl)pyridinium iodide cyanine dye on the

corrosion of Cu in HN03 was studied'". The inhibition effect is more pronounced in

the case of Cu coated by a thin film of dye than that obtained by its addition to the

corrosion medium.

The inhibition efficiency of triphenylmethane dyes in the corrosion of B26S A1

alloy in 4 M HzSO4acid is in the order14': methyl violet < fuchsine base < victoria

blue < fuchsine acid < light green < crystal violet < malachite green < fast green. The

inhibitors function through general adsorption following the Langmuir adsorption

isotherm.

The effect of 2-(2-hydroxystyryl)quinolinium- 1 -Et iodide and 4-(2-

hydroxystyry1)quinolinium- 1 -Et iodide cyanine dyes on the corrosion behavior of

copper in nitric acid solution has been studied'42. The inhibition effect for two

cyanine dyes is more pronounced in case of the addition of dye to the corrosion

medium than that obtained for copper coated by the dye thin film previously treated

in dye solutions before immersion in the corrosion medium.

1.3.25. Toluidine

The corrosion of A1-Cu in HC1 and its inhibition by toluidines were st~died'~ ' .

The efficiency of the inhibitor (1 .O %) increases as: p-toluidine < o-toluidine <m-

toluidine in different concentrations (0.1N-0.3N) of HC1.

1.3.26. Boric acid

Boric acid and/or its salt prevent discoloration of Cu and Cu alloys in contact with

gas144. The boric acid is preferably metaboric acid. The inhibitors are not volatilized

and oxidized at 260-280 OC, maintain their effectiveness at temperatures, and are

useful for solar energy systems utilizing Cu in contact with water or water and

propylene glycol.

1.3.27. Quinine and strychnine

The effect quinine and strychnine on the corrosion of Cu in H2S04 was

investigated'"'. The maximum corrosion inhibition (99%) was observed with 1 O-3M

strychnine. The adsorption followed the Bockris-Swinkels adsorption isotherm.

42

1.3.28. Fatty acids

A corrosion inhibitor for steel and Cu parts of petroleum distillation columns

consists of a soap of a C18-20 fatty acid146. The soap also contains imidazoline or a

mixture of imidazoline, amide, amine, amido-imidazoline (all obtained from the

reaction of diethylenetriamine with C18 fatty acids). The corrosion of the steel and

Cu parts of the column was decreased by 60%.

Ethoxylated fatty acids from soybean oils having the general formula R-

COO(CH2-CH2-O),-H7 where R is mainly a mixture of C17H29, C17H31 and C17H33

and (n) is the number of ethylene oxide per mol (n=10, 16 and 30) have been used as

corrosion inhibitors for copper in 2 M ~ ~ 0 3 ' ~ ~ .

1.3.29. Oxalic acid

Raman spectroscopic studies of the interaction of oxalic acid and sodium oxalate

used as corrosion inhibitors for copper showed the formation of a salt complex of the

inhibitor molecules and the Cu ions'". This suggested that this chemisorbed surface

species produced the protective layer.

1.3.30. Haloacetic acids

The corrosion behavior of Cu in HNO3 containing chloro-, dichloro-, trichloro-,

bromo- and iodoacetic acids was studied'49. A considerable decrease in corrosion

rate is observed in the inhibited HNO3.

1.3.3 1 Carboxylic acids

1,2,3,4-Butanetetracarboxylic acids CO~RICH~CH(CO~R~)CH(CO~R~)CH~-

(C02R4) (Rl-4 = C1-18 alkyl) or their partial esters are used as heat-resistant

corrosion inhibitors for Cu and Cu alloys'''. The treated Cu showed a low tarnish

after 4 hr at 170 OC.

A corrosion inhibitor for Cu alloy pipes used for heat exchanger in desalination of

seawater contains ferrous sulphate and citric acid"'.

Electrochemical. methods were used in a systematic study of the abilities of the

homologous straight chain mono-and a, a-dicarboxylates to inhibit corrosion of Cu

in aerated, mildly saline, and near-neutral aqueous solutions'j2. Performance of both

compound types is critically dependent upon their chain length, the metal, and the

43

number of carboxylate groups. For dicarboxylates, with the possible exception of

mild steel, longer chain lengths were found advantageous. This was not true for

monocarboxylates, which showed abrupt decreases in inhibitor ability outside the

optimal range. The dramatic variations in inhibitor efficiencies probably resulted

from competing reactions, such as adsorption and complexation at the metal

(hydroxide) surface and micelle formation.

1.3.32. Schiff bases

The inhibiting action of some Schiff bases on the corrosion of Cu and its alloys in

HC1 and H2S04 was studiedL5'. The Schiff bases were synthesised by reacting

salicylaldehyde with aliphatic or aromatic amines. The Schiff bases prepared from

aliphatic amines stimulate Cu corrosion, whereas those obtained from aromatic

amines inhibit the corrosion. In both acid solutions considered, N-2-

thiophenylsalicylidenimine was particularly efficient. The inhibiting action is mainly

exerted on the cathodic reduction reaction. The effect of the various Schiff bases is

explained on the basis of their stability and by considering the effect of the different

substituents in the aromatic rings.

1.3.33 Organic P compounds

Some triaryl and trialkyl phosphites were investigated as corrosion inhibitors for

cu in HNO~ so~ut ions '~~ .

1.3.34. Aldehydes

Mixtures of RCHO and RlNH2 , where R and R1 are Cl-19 alkyls, C5-12

cycloalkyl, C7-13 aralkyl, C6, C10, or C14 aryl or substituted aryl, or monocyclic

heterocyclic remainder with 3 5 ring C atoms; or R is optionally -(CH2),CHO, and

R1 is -(CH& NH2; n = 1-6; 157 were used as inhibitors for copper in lubricants,

hydraulic fluids, machining fluids, or cooling media

1.3.35. Benzoylhydrazide

The effect of benzoylhydrazide derivatives on the corrosion of Cu in 3M HN03

solutions was studied1j8. The derivatives are adsorbed on the Cu surface according to

the Langmuir adsorption isotherm.

44

1.3.36. Polymeric compounds

Cu or Cu alloy fins for automotive radiators are coated with the corrosion

inhibitor containing poly(hydroxystyrene) compound and polyepoxy compound'59.

1.4. Background to this work

As can be seen from the preceding literature review, many organic compounds

containing N, 0 and S have been studied as inhibitors for copper corrosion in various

environments. However, benzotriazole and its derivatives remain the most widely

used compounds in industrial applications (e.g. cooling waters) due to their low cost,

availability and high effectiveness particularly in near neutral and alkaline waters. In

general, introducing substituents into either the benzene or benzotriazole ring of

BTAH (mostly some active functional group or a long chain alkyl group) increases

inhibition efficiency. This leads to the formation of some type of protective layer on

the copper surface most probably by chemisorption through the lone pair of electrons

of a N atom and physical adsorption by the alkyl group.

The nature of benzotriazole and its function as an inhibitor for copper corrosion

has been widely studied but there still remain contradictory opinions as to the nature

of the interaction of benzotriazole with the copper surface. One of the disadvantages

of benzotriazole as an inhibitor for copper is its loss of efficiency in acidic

environments. The performance of benzotriazole in acidic solutions can be improved

by adding additives such as KI (synergistic effect with 1- ions), but this so far has

received little practical application in industry. Some authors have tried to modify the

BTAH molecule and it had been suggested that the steric hindrance of the

substituents on the benzene part of benzotriazole leads to an increase in the inhibition

efficiency of this compound. However, the inhibition mechanism in acidic solutions

still remain controversial.

The work in this thesis is concerned mainly with the study of the inhibitive

effect of derivatives of BTAH (4-and 5 -carboxybenzotriazole and their alkyl

derivatives) for copper corrosion in aqueous environments. The compounds were

manufactured commercially by Johoku Ltd, Japan, and preliminary tests showed that

particularly the long alkyl chain compounds had very good inhibition efficiencies. It

can be hypothesised that the size of the alkyl chain in the ester substituent can

essentially reinforce the interaction of the benzotriazole portion of the molecule with

the copper surface and hence the inhibitive effect. It can also be expected that these

45

novel compounds will retain the general advantages of benzotriazole (reasonable

cost, availability, solubility). The present work aims to the above hypothesis by

studying the inhibitive effect of a series of alkyl esters in an acidic environment. In

addition the corrosion kinetics and nature of the species formed by interaction with

the copper surface will also be examined. Understanding the function of these

compounds as inhibitors of copper will contribute to the elucidation of the behaviour

of benzotriazole derivatives in general, and this should also lay the foundation for the

synthesis of more effective inhibitors. Molecular modelling will also be employed as

an aid to understanding the nature of the molecular interaction with the copper

surface and for the prediction of the inhibition efficiencies of compounds that are

structurally similar.

1.5. Objective of this work

The detailed objectives of this are as follows: .

1. To evaluate the inhibition efficiency of 4-and 5-carboxybenzotriazole and a

series of their alkyl ester derivatives for copper corrosion in aqueous

environments by standard weight loss measurement at different temperatures, pH,

concentration of inhibitor and time of exposure.

2. To establish the effect on the kinetics of the corrosion reactions of each

compound by potentiodynamic polarisation.

3. To evaluate the interaction of each compound with the metal surface by

electrochemical impedance spectroscopy (EIS).

4. To examine the nature of the species adsorbed on the copper surface by surface

enhanced Raman scattering (SERS) spectroscopy.

5. To establish the inhibitive effects of a coating formed on copper by a series of

alkyl esters dissolved in different solvents. This will include:

+ Polarisation study of the coating in different test environments

*:* Study of the anti-tarnishing properties of each ester in a supplied

environment

*3 Investigation of the nature and orientation of the species adsorbed on the

metal

6. To investigate molecular modelling as a tool for inhibitor design.

46

1.6. An account of scientific papers contributing to the objective & aims of the

thesis

The diagram in Fig. 1.1 can be used as a useful illustrative tool for summarizing

the research of this thesis.

Study on inhibition efficiency of 4- and 5-carboxybenzotriazole for copper corrosion (Chapter 3)

Study on the octyl esters of carboxybenzotriazole (Chapter 4)

& Study on other esters with different alkyl chain length: methyl, butyl, hexyl (Chapter 5)

Study on inhibitive effect of the films

Computer simulation to design new -b inhibitor (Chapter 7)

Fig. 1.1. A summary of the progress linking the scientific papers

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137. J.D. Talati, J.M. Daraji, J. Electrochem. Soc. India 35 (1986) 175

138. J.D. Talati, D.K. Gandhi, Indian J. Technol. 20 (1982) 312

139. J.D. Talati, D.K. Gandhi, Indian J. Technol. 23 (1985) 232

140. M.T. Makhlouf, Z.H. Khalil, J. Chem. Technol. Biotechnol. 38 (1987) 89

141. J.D. Talati, J.M. Daraji, J. Indian Chem. Soc. 65 (1988) 94

53

142. M.T. Makhlouf, Z.H. Khalil, Collect. Czech. Chem. Commun. 58 (1993)

2003

143. R.B. Patel, J.M. Pandya, K. Lal, Trans. SAEST (1982), 17(4), 321-4

144. Kurita Water Industries, Ltd.. Japan; Japanese Patent 82-5 1390 82033 1

145. N.C. Subramanyam, B.S. Sheshadri, S.M. Mayanna, Br. Corros. J. 19

(1984) 177

146. E. Cristoloveanu, T.I. Decean, A. Dumitm, V. Manescu, Romanian

Patent 82-106943 820318; CA 103:74665

147. S.M. El-Haleem, M. Abdallah, A.I. Mead, Ann. Univ. Ferrara, Sez. 5

Suppl. (1995), 10 (8th European Symposium on Corrosion Inhibitors,

1995, Vol. 2), 963-73

148. H. Jeziorowski; B. Moser, Chem. Phys. Lett. 120 (1985) 41

149. N.C. Subramanyam, S.M. Mayanna, Indian J. Technol. 24 (1986) 17

150. A. Maeda; Chiyoda Kagaku Kenkyusho, Japan; Japanese Patent 86-

115876 860519; CA 108:172180

15 1. A. Sakanishi, K. Ueda, M. Sakimura; Mitsubishi Heavy Industries, Ltd.,

Japan; Japanese Patent 86-29 1596 86 1209; CA 1 10:2793 1 152. G.T. Hefter, N.A. North, S.H. Tan, Corrosion 53 (1997) 657

153. I.H. Omar, F. Zucchi, G. Trabanelli, Surf. Coat. Technol. 29 (1986) 141

154. A.S. Fouda, S.A. Gomah, M.N. Moussa, Qatar Univ. Sci. J. (1992), 12,

64-8

155. L. Komora, E. Komorova, Czech Patent 84-8196 841029; CA

109:26436

156. C.M. Mustafa, S.M. Shahinoor Islam Dulal, Corrosion 52 (1996) 16

157. W. Neagle; Ciba-Geigy A.-G., Switz.;German Patent 87- 10 195 870429

158. M.M. El-Tagoury, G.E. Bekheit, Bull. Soc. Chim. Fr. (1991), (Nov.-

Dec.), 827-3 1

159. T. Suzuki; Shikoku Chemicals Corp., Japan; Japanese Patent 89-324685

891213; CA 116:260574

54

CHAPTER 2

THEORY OF CORRROSION AND EXPERIMENTAL METHODS EMPLOYED

55

56

2.1. Electrochemical nature of corrosion 1-10

In aqueous environments the majority of metallic corrosion processes are

electrochemical. The reaction at the less stable anodic sites (A) on metal M (e.g.,

where there are dislocations, imperfections) can be explained by a simplified

equation:

M(s) -+ M"+(aq) + ne- (s) (2.1)

The corresponding cathodic reaction occurs at the cathodic sites (C) at the

metal/solution interface:

R(aq) +ne-(s) -+ Rn- (aq) (2.2) The species R is an oxidising agent in solution that can receive electrons from the

metal . In the corrosion process, all the electrons produced by the anodic reaction move

through the remaining sound metal to the cathodic site and are accepted by the

oxidising agent. The overall reaction of the corrosion process can be given by:

M(s) + R(aq) -+ M"+ (aq) + R"(aq)

Metal M

ne- ec - 3 '

A

Water

R + Rn-

M 9 M"+

Fig 2.1 Metal dissolution in water containing an oxidising agent R

On the metal surface many short-circuited galvanic cells (as shown in Fig. 2.1) are

set up. The metal will continue to dissolve in the presence of excess oxidant and this

process is spontaneous (negative AG) as long as the equilibrium (reversible) potential

for M(s) I Mn+(aq) is more negative than that of R(aq) I Rn-(aq).

For copper the main anodic reaction is copper dissolution:

CU(S> -+ Cu2+(aq> + 2e (E? =+0.337V) (2.4) Several cathodic reactions can occur in aqueous solution corresponding to the

copper dissolution reaction. In acidic solutions containing oxygen the main cathodic

reaction is:

57

Oz(aq) + 4Hf(aq) + 4e-(s) -+ 2 HlO(1) (l? = +1.229V) (2.5) In basic environments where there is sufficient oxygen, the following cathodic

reaction predominates:

Oz(aq) +2Hz0(1) +4e-(s) -+ 4OH-(aq) (l? = +0.401V) (2.6)

2.2. Corrosion kinetics

Once it is established that corrosion will occur, the next practical step is to

determine how fast is the corrosion rate. This is the objective of corrosion kinetics.

Consider reduction at a cathodic site. The rate of reduction depends essentially on the

rate at which the oxidant, e.g., Hf is transported in solution up to the cathodic site

and also on the rate of electronation (similar consideration applies to the oxidation

process). Slow rate of electronation and transport can now have an effect on the

reversible potential : the cathode potential no longer remains at its equilibrium value.

It is said to be polarised and the change in potential is called the overpotential. More

specifically slow electronation gives rise to the activation overpotential whilst slow

transport results in a concentration overpotential. Both forms are additive.

2.2.1 ..Activation polarization and the Butler - Volmer equation

To predict the corrosion rate (mm yr-l) of a pure metal M by the electrochemical

method, it is necessary to determine the corrosion current density, icon:

dW Micorr corrosion rate = - = - dt nF

Where:

M = atomic mass of metal

n = number of electrons transferred in corrosion process

F = Faraday's constant

i = corrosion current density A m-2

(Kg mol-')

(96,450 C mol-')

The corrosion current density is not directly measurable because a corroding

metal does not show any net current flow. However, it may be possible to determine

the corrosion current density by using the Butler-Volmer equation or its

approximation the Tafel equation.

58

Suppose metal M is in equilibrium with a solution of its own ions (Mn+ (aq)) and

its reversible or equilibrium potential is then changed to some other value (e.g. by

connecting it to another electrode to form a galvanic cell or to a potentiostat). The

resultant net current density (anodic or cathodic) is given by the Butler-Volmer

equation:

where:

inet = net current density (I/area of corroding metal) (A m-2>

io= equilibrium exchange current density (A m-2>

a = transfer coefficient ( - 0.5)

77 = activation overpotential (V)

T = absolute temperature (K)

R = gas constant (8.314 J K ' mol)

n = number of electrons in process

The Butler-Volmer equation relates the net current density (the electrode is not at

equilibrium) to the change in potential experienced by the electrode. The equilibrium

exchange current density io is the current density in each direction when the electrode

reactions (for example (2.1) and (2.2)) are at equilibrium. The potential change is the

potential by which the electrode' is polarised away from the equilibrium potential and

its value depends on the reaction rate of electron transfer. The slower this step the

greater is 7. The transfer coefficient a is related to an energy barrier which the

reacting species must overcome for the electron transfer to occur.

The Butler-Volmer equation has simplified forms:

(a) When 77 is small (+ 5 mV):

Equation (2.8) reduces to:

i.e. ia

RT io F

q a = - (2.10)

59

relatively low [02]. If the cathodic charge transfer step is very fast (i.e. there is no

activation polarisation) the concentration of oxygen in the interfacial region adjacent

to the cathodic site (CI) will drop to a value which is less than that in the bulk solution

(cg). This is because in this situation the rate of diffusion of 0 2 from the bulk of the

solution is not fast enough to replace those molecules reduced. As a result of the drop

in [ 0 2 ] at the interface the overall rate of reduction process will be decreased. A

limiting condition can be reached when the concentration of the reducible species at

the interface is reduced to zero and under these conditions the rate of oxygen

reduction, expressed as the cathodic current density, becomes fixed at a maximum

achievable value, i.e., the limiting value. The relationship between the cathodic

current density i, and the concentration is given by:

(2.16) nFD ( ~ i -ce)

6 ic =

where i, = cathodic current density ( A m-2)' .

n = F =

D =

CI =

number of electrons involved in the reduction reaction

Faraday constant (96 487 C mol-I)

diffusion coefficient of the reducible species (m2 s- ' )

concentration of reducible species at the interface (cathodic site)

(mol m-3>

6 = distance from metal surface over which concentration gradient

occurs (m) For the limiting situation where CI = 0 equation (2.16) becomes:

(2.17) nFDcs 6

iL = -

where iL = limiting or maximum current density that can flow.

The slow rate of diffusion of the oxygen molecule will have an effect on the

cathode potential. The reversible electrode potential is calculated using the Nernst

equation, assuming that the activity of the electroactive species at the interface is the

same as for the bulk solution, i.e., at a distance well removed from the metal. This is

not the case under the conditions described above and the electrode will exhibit a

potential different to its reversible potential, i.e. it has become concentration

polarised. The difference in potential is called the concetration overpotential q,C .

61

The relationship between the cathodic current density i, and the concentration

overpotential (no activation polarisation) is given by:

i, =i, [ l-exp- '2'1 (2.18)

where ic =

iL =

F=

N =

R =

T =

r," =

cathodic current density ( A m-2)

limiting or maximum current density (A m-2)

Faraday constant (96487 C mol-')

number of electrons involved in the cathodic reaction

8.314 J K-' mol-'

temperature (K)

concentration overpotential for the cathodic reaction (E actual- E,,,)

Equation (2.19) can be arranged to give:

r," - - E l o g [ l - k ] nF (2.19)

2.2.3. Combined activation and concentration polarisation

Both activation and concentration polarisation may occur at an electrode and are

additive. For a single cathodic process equations (2.15) and (2.19) may be added to

give:

- 2.3RT log? i + --log[l 2.3RT - (2.20) nF 7 7 , O k l l - - -

aF 10

This equation can be rearranged to give the following approximate expression for

the cathodic current density

(2.21) . I , =

i, exp[- aqF 1 R T ] i, exp(-aqF 1 R T ]

1,' if.

or (2.22) i, i,

I , =- I, + i c

For anodic processes concentration polarisation is usually not important.

1 2 4 6 2.3. Polarisation methods and corrosion rate estimation ' ' I

62

Polarisation methods such as potentiodynamic polarisation, are often used for

laboratory corrosion testing. The technique has the potential to provide useful

information regarding the corrosion mechanisms, corrosion rate and susceptibility of

specific materials to corrosion in designated environments. Potentiodynamic

polarisation is a technique where the potential of the electrode is varied at a selected

rate by application of a current through the electrolyte. The polarisation can be

carried out by using a potentiostat. Three electrodes are required: the working

electrode (WE)(that is the metal/alloy under investigation), the reference electrode

(the potential of the WE is measured relative to this potential), and counter or

auxiliary electrode (that the majority of the current passes through). The counter

electrode is required to prevent any resistive potential drop (ohmic drop) across the

reference electrode.

The potential of a corroding metal (WE) is varied (polarised) from its equilibrium

value (Ecorr) firstly in the negative and then in the positive direction and the current

response to the applied potential is recorded. The voltage/current density data pairs

produced from the polarisation of the, WE can then be used to construct a polarisation

diagram similar to Fig. 2.3

+

E

-

log i

Fig. 2.3 Comparison of experimental and theoretical polarisation curves for reaction

M+2H+ + M2+ +H2

In Fig 2.3 typical experimental polarisation curves (with both the anodic and

cathodic reactions exhibiting activation polarisation only) are overlaid with the Tafel

lines for the dissolution of metal M in oxygen-free acid. At the corrosion potential

63

(E,,,,) there is, by definition, no net current flowing in the cell. At a more negative

potential from E,,, (less than approx. k 50 mV), the current flowing will consist of

both the anodic and cathodic components and the plot is not linear. Only at higher

overpotentials (negative or positive) are the experimental curves in agreement with

the linear Tafel relationships. For cases similar to the above the corrosion current

density (therefore the corrosion rate) can be estimated from the intersection of the

two Tafel lines, i.e., the linear portion of the experimental curves. In some cases one

or both of the reactions may not exhibit activation polarisation only, i.e. be under

activation control, and this makes the estimation of the corrosion current density

either more difficult or impossible. When only one reaction is under activation

control, the intersection of the linear Tafel region for this reaction with the corrosion

potential may suffice for the calculation of corrosion rate. If the cathodic reaction is

under complete diffusion control at E,,, the limiting current density will represent

the corrosion current density. If both anodic and cathodic curves are non-linear any

arbitrary point more positive than E,,, (e.g. 100 mV) can be used as an estimate of

the rate of anodic dissolution. In this study the accurate determination of corrosion

rate in the absence and presence of inhibitors required the use of coupons (see

section 2.4). Relative corrosion rates only were obtained from polarisation curves.

I Anodic inhibitor I Cathodic inhibitor

log i log i

Fig. 2.4 Effect of cathodic and anodic inhibitors on E,,,

A corrosion inhibitor is referred to as being either cathodic or anodic according to

the reaction that it retards. If it retards both reactions, it is called a mixed inhibitor.

64

This behaviour can be identified from the polarisation curves and the movement of

E,,, from its value in the uninhibited system (Fig. 2.4)

1,2,5,9 2.4. Weight loss method to measure corrosion rate and inhibition efficiency

The simplest and most accurate method of estimating the corrosion rate is weight

loss analysis. A weighed sample (coupon) of the metal or alloy under consideration

is introduced into the corrosive environment, and later removed after a reasonable

time interval. The coupon is then cleaned of all corrosion product(s) and is

reweighed. The weight loss can also be determined by the amount of metal dissolved

into the solution and as corrosion product using instrumental techniques such as

atomic absorption spectroscopy (AAS) or induced coupled plasma - atomic

emission spectroscopy (ICP-AES). The weight loss is converted to an average

corrosion rate (R) , as follows: ,

where:

?+'I = initial coupon weight (mg)

W2 = final coupon weight (mg)

A = coupon area (dm2)

D = exposure time (days)

R = corrosion rate (mg dm-2 day-')

The technique requires no complex equipment or procedures, merely an

appropriately shaped coupon, a carrier for the coupon (coupon holder, if necessary),

and a reliable means of removing corrosion product without disruption of the metal

substrate. The weight loss measurement is still the most widely used means of

determining corrosion loss, despite being the oldest method currently in use. Weight

loss determination has a number of attractive features that account for its sustained

popularity:

0

0

Simple - no sophisticated instrumentation is required to obtain a result.

Direct - a direct measurement is obtained, with no theoretical assumptions or

approximations.

65

Versatile - it is applicable to all corrosive environments, and gives

information on all forms of corrosion.

The method is commonly used as a calibration standard for other means of

corrosion monitoring, such as linear polarisation and electrical resistance. In

instances where metal wasted is slow and averaged data are acceptable, weight loss

monitoring is the preferred technique.

The percent inhibition efficiency (IE%) was calculated for the compond under

investigation according to the following equation:

where ws and wb are the coupon weight losses in solution with and without inhibitor.

The choice of technique for initial preparation of the coupon surface, and for

cleaning the coupon after use, is critical in obtaining useful data. In this study, copper

coupons (3cm x lcm x 0.02 cm) were cut from AR copper sheet, polished with

PI200 grade S i c abrasive paper, degreased with acetone, etched with HNO3 1: 1 and

washed thoroughly with distilled water. The coupons were then immersed -

immediately in the test solution. After the test, any corrosion product(s) on the

coupons were removed by HC1 (3:l). The solutions were combined and the

concentration of copper was determined by ICP-AES.

2.5. Electrochemical impedance spectroscopy - corrosion mechanism study and

estimation of polarisation resistance 1,2,10-14

Electrical resistance is the ability of a circuit element to resist the flow of

electrical current. Ohm’s law (Equation 2-26) defines resistance in terms of the ratio

between voltage E and current I:

E I

R = - (2.26)

Ohm’s law is limited to only one circuit element, i.e., the ideal resistor. An ideal

resistor has several simplifying properties:

It follows Ohm’s Law at all current and voltage levels

0 Its resistance value is independent of frequency

0 AC current and voltage signals through a resistor are in phase with each other

66

The real world, however, contains circuit elements that exhibit much more complex

behavior and in place of resistance we use impedance, which is a more general

circuit parameter. Impedance is also a measure of the ability of a circuit to resist the

flow of electrical current. Unlike resistance, it is not limited by the simplifying

properties listed above.

Electrochemical impedance is usually measured by applying an AC potential

(AEsinot) to an electrochemical cell and measuring the current through the cell.

Suppose one applies a sinusoidal potential excitation. The response to this potential

is an AC current signal (Akin(cut + q)), containing the excitation frequency and its

harmonics (2~0, 3 0 etc.). The impedance 2 has a magnitude AE/AI and phase q and

hence is a vector quantity.

Electrochemical impedance is normally measured using a small excitation signal.

This is done so that the cell's response is pseudo-linear. It is possible to express the

impedance as a complex function. The potential is described as,

E(t) = Eoexpuot) (2.27)

and the current response as,

Z(t) = Ioexp(j0o-j @) (2.28)

The impedance is then represented as a complex number,

E Z

2 = - = 2, exp(j 4) = 2, (cos 4 + j sin 4) ( 2-29)

AC impedance can provide kinetic and mechanistic information when applied to

the study of electrochemical systems. For this reason the technique (normally called

electrochemical impedance spectroscopy or EIS) is applied to an increasing extent

for the study of corrosion processes in solution. The following are some areas of

corrosion where AC impedance has been applied successfully:

0 Rate determination

0 Inhibitor performance

0 Coating performance

0 Passive layer characteristic

The advantages of AC impedance over DC techniques are:

67

AC impedance techniques use very small excitation amplitudes, usually in the

range of 5-10 mV peak-to-peak. Excitation amplitudes of this order perturb the

electrochemical test system to a minimum extent, thus reducing errors caused by

the measurement technique itself.

Since AC impedance measurements provide data on both electrode capacitance

and charge transfer kinetics, the technique can provide valuable mechanistic

information.

The AC impedance technique does not involve potential scan and hence can be

applied to low conductivity solutions where DC techniques are subject to serious

potential control errors.

Although AC impedance measurements offer a great deal of information,

sophisticated techniques are required to interpret the data and extract meaningful

results. In this study, a frequency response analyser (FRA) in conjunction with a

potentiostat allowed a cell to be stimulated with an AC signal and the response in

terms of the cell’potential and current was measured. Using the transfer function

facility of the FRA, the impedance or admittance can be determined directly. A plot

of impedance against frequency or the magnitude and phase against frequency allows

the establishment of the characteristics of the system

1

Figure 2.5 Nyquist plot with

impedance vector

Figure 2.6 Bode plot with one time

constant

There are two ways to present the EIS data. If the real part is plotted on the x-axis

and the imaginary part on the y-axis of a chart, we get a Nyquist plot (Figure 2.5). In

this plot the y-axis is negative and each point on the Nyquist plot is the impedance at

68

one frequency. On the Nyquist plot the impedance can be represented as a vector of

length 14. The angle between this vector and the x-axis is @, where @ = arg(2).

Another popular presentation method is the Bode plot (Fig. 2.6). The impedance is

plotted with log frequency on the x-axis and both the absolute value of the

impedance (14 =ZO ) and phase-shift on the y-axis. Unlike the Nyquist plot, the Bode

plot explicitly shows frequency information

EIS data is commonly analysed by fitting it to an equivalent electrical circuit

model. Most of the circuit elements in the model are common electrical elements

such as resistors, capacitors, and inductors. To be useful, the elements in the model

should have a basis in the physical electrochemistry of the system.

2.5.1 Physical electrochemistry and common equivalent circuit models

2.5.1.1 A purely capacitive coating

A metal covered with an undamaged coating generally has a very high impedance.

The equivalent circuit for such a situation is in Figure 2.7.

jAMCIIl-

Fig 2.7 Purely capacitive

coating

Fig 2.8 Randles cell schematic diagram

2.5.1.2. Randles cell

The Randles cell is one of the simplest and most common cell models. It includes

a solution resistance, a double layer capacitor and a charge transfer or polarisation

resistance. In addition to being a useful model in its own right, the Randles cell

model is often the starting point for other more complex models.

The equivalent circuit for the Randles cell is shown in Figure 2.8. The double

layer capacity is in parallel with the impedance due to the charge transfer reaction.

2.5.1.3. Mixed kinetic and diffusion control

69

Fig 2.9 Equivalent circuit with mixed kinetic and charge transfer control

This circuit (Fig 2.9) models a cell where polarisation is due to a combination of

kinetic and diffusion processes (W: Warburg impedance)

2.5.1.4. Coated metal

Most coatings degrade with time, resulting in more complex behavior. After a

certain amount of time, water penetrates into the coating and forms a new

liquid/metal interface under the coating. Corrosion phenomena can occur at this new

interface. The interpretation of impedance data from failed coatings can be very

complicated. A simple equivalent circuit was shown in Fig 2.10.

Fig 2.10 Equivalent circuit for a failed coating

2.5.2. Extracting model parameters from data

EIS data is generally analysed in terms of an equivalent circuit model. The analyst

tries to find a model whose impedance matches the measured data.

The type of electrical components in the model and their interconnections controls

the shape of the model's impedance spectrum. The model's parameters (i.e. the

resistance value of a resistor) controls the size of each feature in the spectrum. Both

these factors effect the degree to which the model's impedance spectrum matches a

measured EIS spectrum. In a physical model, each of the model's components is

70

postulated to come from a physical process in the electrochemical cell. The choice of

which physical model applies to a given cell is made from knowledge of the cell's

physical characteristics.

Modern EIS analysis uses a computer to find the model parameters that cause the

best agreement between a model's impedance spectrum and a measured spectrum.

For most EIS data analysis software, a non-linear least squares fitting (NLLS)

Levenberg-Marquardt algorithm is used.

NLLS starts with initial estimates for all the model's parameters. Starting from this

initial point, the algorithm makes changes in several or all of the parameter values

and evaluates the resulting fit. If the change improves the fit, the new parameter

value is accepted. If the change worsens the fit, the old parameter value is retained.

Next a different parameter value is changed and the test is repeated. Each trial with

new values is called an iteration. Iterations continue until the goodness of fit exceeds

an acceptance criterion, or until the number of iterations reaches a limit.

2.6. Coulometry"

A metal surface, when pre-conditioned in an inhibitor solution, can show some

anti-tarnishing property. The amount of corrosion product, and hence the

effectiveness of the inhibitor as an anti-tarnishing agent, can be determined by

coulometry . In this work, constant-current coulometry was used. The method employed an

electrolysis cell operated at constant current. A schematic circuit is shown in Figure

2.1 1

Fig. 2.1 1 Constant current source based on an operational amplifier

71

The current is supplied by the operational amplifier. The potential across the

resistor R must be equal to the battery potential A@, and the current through R must

be i = AWR. Since the input impedance of the operational amplifier is high, virtually

all the current through R flows through the cell, independent of the cell’s internal

resistance and potential. Thus, provided that R is much less than the input impedance

of the operational amplifier and i(Rce!l+R) is within the range of the operational

amplifier voltage, the circuit will supply a constant current. With the addition of a

reference electrode, the working electrode potential can be monitored. The current

has to be very small so that mass transport can keep up with consumption at the

electrode.

The copper electrode (working electrode) was exposed to a tarnishing

environment, Na2S solution. In the cell, the tarnished product (containing mainly

Cu2S and Cu20) was then reduced. On a ‘potential - time curve, several arrests will

be observed, each arrest corresponding to the reduction of a species. The reduction

time (or the amount of charge) for each potential arrest is proportional to the amount

of tarnish product formed on the surface of the working electrode. For the copper

electrode, pre-conditioned in a solution of inhibitor, the reduction time can show the

effectiveness of that inhibitor as an anti-tarnishing agent.

2.7. Surface-enhanced Raman scattering (SERS) techniqueI6

Surface enhanced Raman scattering is a very useful tool in the investigation of

adsorption-mediated corrosion inhibition, because it has high intensity and is easily

applicable to the in situ observation of the behaviour of inhibitors on metal surfaces

in corrosive media.

2.7.1. The Raman effect and normal Raman scattering.

When light is scattered from a molecule, most photons are elastically scattered.

The scattered photons have the same energy (frequency) and, therefore, wavelength,

as the incident photons. However, a small fraction of light (approximately 1 in 10’

photons) is scattered at optical frequencies different from, and usually lower than, the

frequency of the incident photons. The process leading to this inelastic scattering is

termed the Raman effect. Raman scattering can occur with a change in vibrational,

72

rotational or electronic energy of a molecule. Chemists are concerned primarily with

the vibrational Raman effect. The difference in energy between the incident photon

and the Raman scattered photon is equal to the energy of a vibration of the scattering

molecule. A plot of intensity of scattered light versus energy difference is a Raman

spectrum.

2.7.1.1. The scattering process.

The Raman effect arises when a photon is incident on a molecule and interacts

with the electric dipole of the molecule. It is a form of electronic (more accurately,

vibronic) spectroscopy, although the spectrum contains vibrational frequencies. In

classical terms, the interaction can be viewed as a perturbation of the molecule’s

electric field. In quantum mechanics the scattering is described as an excitation to a

virtual state lower in energy than a real electronic transition with nearly coincident

de-excitation and a change in vibrational energy. The scattering event occurs in 1 0-14

seconds or less. The virtual state description of scattering is shown in Figure 2.12(a).

The energy difference between the incident and scattered photons is represented

by the arrows of different lengths in Figure 2.12(a). -Numerically, the energy

difference between the initial and final vibrational levels, v , or Raman shift in wave

numbers (cm-’), is calculated through equation (2.30):

(2.30)

in which h incident and h scattered are the wave number (cm-*) of the incident and Raman

scattered photons, respectively. The vibrational energy is ultimately dissipated as

heat. Because of the low intensity of Raman scattering, the heat dissipation does not

cause a measurable temperature rise in a material.

At room temperature the thermal population of vibrational excited states is low,

although not zero. Therefore, the initial state is the ground state, and the scattered

photon will have lower energy (longer wavelength) than the exciting photon. This

Stokes shifted scatter is what is usually observed in Raman spectroscopy. Figure 2.12

(a) depicts Raman Stokes scattering.

A small fraction of the molecules are in vibrationally excited states. Raman

scattering from vibrationally excited molecules leaves the molecule in the ground

state. The scattered photon appears at higher energy, as shown in Figure 2.12(b).

73

Energy Incident

Photon

- - - - - - -

Incident Photon I

1

- - - - - - - -

Anti - S toke s Scatter

Inidd

I Fiad

Figure 2.12. Energy level diagram for Raman scattering; (a) Stokes Raman scattering

(b) anti-Stokes Raman scattering

This anti-Stokes-shifted Raman spectrum is always weaker than the Stokes-

shifted spectrum, but at room temperature it is strong enough to be useful for

vibrational frequencies less than about 1500 cm-l. The Stokes and anti-Stokes spectra

contain the same frequency information. The ratio of anti-Stokes to Stokes intensity

at any vibrational frequency is a measure of temperature. Anti-Stokes Raman

scattering is used for contactless thermometry. The anti-Stokes spectrum is also used

when the Stokes spectrum is not directly observable, for example because of poor

detector response or spectrograph efficiency.

2.7.1.2. Vibrational energies.

The energy of a vibrational mode depends on molecular structure and environment.

Atomic mass, bond order, molecular substituents, molecular geometry and hydrogen

bonding all effect the vibrational force constant which, in turn dictates the vibrational

energy. For example, the stretching frequency of a phosphorus-phosphorus bond

ranges from 460 to 610 to 775 cm-' for the single, double and triple bonded moieties,

respectively. Much effort has been devoted to estimation or measurement of force

constants. For small molecules, and even for some extended structures such as

peptides, reasonably accurate calculations of vibrational frequencies are possible

with commercially available software.

Vibrational Raman spectroscopy is not limited to intramolecular vibrations.

Crystal lattice vibrations and other motions of extended solids are Raman-active.

74

Their spectra are important in such fields as polymers and semiconductors. In the gas

phase, rotational structure is resolvable on vibrational transitions. The resulting

vibration/rotation spectra are widely used to study combustion and gas phase

reactions generally. Vibrational Raman spectroscopy in this broad sense is an

extraordinarily versatile probe into a wide range of phenomena ranging across

disciplines from physical biochemistry to materials science.

2.7.1.3. Raman selection rules and intensities.

A simple classical electromagnetic field description of Raman spectroscopy can

be used to explain many of the important features of Raman band intensities. The

dipole moment, P, induced in a molecule by an external electric field, E, is

proportional to the field as shown in equation 2.

P = a E (2.3 1)

The proportionality constant a is the polarizability of the molecule. The polarizability

measures the ease with which the electron cloud around a molecule can be distorted.

The induced dipole emits or scatters light at the optical frequency of the incident

light wave.

Raman scattering occurs because a molecular vibration can change the polarizability.

The change is described by the polarizability derivative, where S d S Q is the

normal coordinate of the vibration. The selection rule for a Raman-active vibration,

that there be a change in polarizability during the vibration, is given in equation

(2.3 2).

(2.32) aa - * o SQ

The Raman selection rule is analogous to the more familiar selection rule for an

infrared-active vibration, which states that there must be a net change in permanent

dipole moment during the vibration. From group theory it is straightforward to show

that if a molecule has a centre of symmetry, vibrations which are Raman-active will

be silent in the infrared, and vice versa.

Scattering intensity is proportional to the square of the induced dipole moment,

that is to the square of the polarizability derivative (8~1 l8Q)~

If a vibration does not greatly change the polarisability, then the polarisability

derivative will be near zero, and the intensity of the Raman band will be low. The

vibrations of a highly polar moiety, such as the 0 -H bond, are usually weak. An

75

external electric field can not induce a large change in the dipole moment and

stretching or bending the bond does not change this.

Typical strong Raman scatterers are moieties with distributed electron clouds,

such as carbon-carbon double bonds. The pi-electron cloud of the double bond is

easily distorted in an external electric field. Bending or stretching the bond changes

the distribution of electron density substantially, and causes a large change in

induced dipole moment.

Chemists generally prefer a quantum-mechanical approach to Raman scattering

theory, which relates scattering frequencies and intensities to vibrational and

electronic energy states of the molecule. The standard perturbation theory treatment

assumes that the frequency of the incident light is low compared to the frequency of

the first electronic excited state. The small changes in the ground state wave function

are described in terms of the sum of all possible excited vibronic states of the

molecule.

2.7.1.4. Polarisation effect.

Raman scatter is partially polarised, even for molecules in a gas or liquid, where

the individual molecules are randomly oriented. The effect is most easily seen with

an exciting source which is plane polarised. In isotropic media polarisation arises

because the induced electric dipole has components which vary spatially with respect

to the coordinates of the molecule. Raman scatter from totally symmetric vibrations

will be strongly polarised parallel to the plane of polarisation of the incident light.

The scattered intensity from non-totally symmetric vibrations is 3/4 as strong in the

plane perpendicular to the plane of polarisation of the incident light as in the plane

parallel to it.

The situation is more complicated in a crystalline material. In that case the

orientation of the crystal is fixed in the optical system. The polarisation components

depend on the orientation of the crystal axes with respect to the plane of polarisation

of the input light, as well as on the relative polarisation of the input and the observing

polariser.

2.7.2. Surface-enhanced Raman scattering

The Raman scattering from a compound (or ion) adsorbed on or even within a few

Angstroms of a structured metal surface can be 103-106 X greater than in solution.

This surface-enhanced Raman scattering is strongest on silver, but is also observable

76

on gold and copper as well. At practical excitation wavelengths, enhancement on

other metals is unimportant. Surface-enhanced Raman scattering (SERS) arises from

two mechanisms. The first is an enhanced electromagnetic field produced at the

surface of the metal. When the wavelength of the incident light is close to the plasma

wavelength of the metal, conduction electrons in the metal surface are excited into an

extended surface electronic excited state called a surface plasmon resonance.

Molecules adsorbed or in close proximity to the surface experience an exceptionally

large electromagnetic field. Thus if an alkyl group is close to the surface the

intensities of the C-H stretching mode are enhanced. However, in aqueous solution

these bands may be obscured by water molecules. Vibrational modes normal to the

surface are most strongly enhanced.

The second mode of enhancement is by the formation of a charge-transfer complex

between the surface and analyte molecule. The electronic transitions of many charge

transfer complexes are in the visible, so that resonance enhancement occurs.

Molecules with lone pair electrons or pi clouds show the strongest SERS. The

effect was first discovered with pyridine. Other aromatic nitrogen or oxygen

containing compounds, such as aromatic amines or phenols, are strongly SERS

active. The effect can also been seen with other electron-rich functionalities such as

carboxylic acids.

The intensity of the surface plasmon resonance is dependent on many factors

including the wavelength of the incident light and the morphology of the metal

surface. The wavelength should match the plasma wavelength of the metal. This is

about 382 nm for a 5 pm silver particle, but can be as high as 600 nm for larger

ellipsoidal silver particles. The plasma wavelength is to the red of 650 nm for copper

and gold, the other two metals which show SERS at wavelengths in the 350-1000 nm

region. The best morphology for surface plasmon resonance excitation is a small

(-4 00 nm) particle or an atomically rough surface.

SERS is used to study monolayers of materials adsorbed on metals, including

electrodes. Other vibrational spectroscopy methods, e.g., infrared reflection

measurements, have been employed for the identification of chemical species in the

protective film formed on metal surfaces in the reaction with the inhibitors and for

the determination of film thickness. Since most of the infrared measurements have

been performed ex situ, direct information about adsorption-mediated corrosion

inhibition has scarcely been obtained. Surface enhanced Raman scattering is more

77

useful than other vibrational spectroscopy methods in the investigation of adsorption-

mediated corrosion inhibition, because it has high sensitivity and easily applicable to

the in situ observation of the behaviour of inhibitors on metal surfaces in corrosive

media.

Benzotriazole (BTAH), a common inhibitor for copper and copper based alloys,

has been the subject of SERS observation. In this work, by means of SERS, the

relation between the molecular structure of compounds structurally related to BTAH

and their inhibitive action against the corrosion of copper has been investigated.

2.8. Molecular rn~delling'~

Molecular modeling can be used to study the adsorption of inhibitor species on

the metal surface. If a model of the interactions that control the adsorption can be set

up, a measure of the binding energy of the molecule with the metal can be calculated.

Comparison of the relative binding energy of a series of structurally similar ligands

may lead to the prediction of other related compounds with potentially more

effective inhibiting actions.

2.8.1. Empirical force field models - Molecular mechanics

Quantum mechanical theory permits the calculation of molecular energy and the

structures with lowest energy should be predominant according to Boltzmann

statistics. Ab initio quantum mechanics also permits calculation of inter-molecular

interaction energy. However, the massive amount of integrals to be calculated for

large molecules makes these methods very time-consuming and almost impossible in

some cases. Therefore, in our study empirical force fields models are used for faster

energy calculations. The molecules are assumed to consist of balls (the atoms)

connected by springs (the bonds). The internal energy, Epot is expressed in terms of a

set of molecular coordinates (bond length energy, bond angle energy, torsion energy,

van der Waals' energy of interaction, charge-charge energy of interaction, and

miscellaneous or cross energy terms). Thus for a consistent valence force field

(CVFF) Epot is given by

Epot = Ebonds + 2 Eangles Etorsions 4- E van der Waals + c Echarge c Ecrosst

78

The forces on the atoms enable solving Newton's laws of motion, which leads to

simulations of time dependent phenomena.

Some features of the molecular mechanics force field models are:

The force fields are primarily design to reproduce structural properties but they

can also be used to predict other properties.

The functional form and parameters are transferable. Transferability means that

the same set of parameters can be used to model a series of related molecules,

rather having to define a new set of parameters for each invidual molecules.

Force fields are empirical. The functional forms employed in molecular

mechanics force field are often a compromise between accuracy and

computational efficiency; the most accurate functional form may often be

unsatisfactory for efficient computation. As the performance of computers

increases, it becomes possible to incorporate more sophisticated models.

For a force field it is usually necessary to assign an atom type to each atom in the

system. The atom type is more than just the atomic number of an atom; it usually

contains information about its hybridization state and .sometimes the local

environment.

2.8.2. Energy minimisation and related methods for exploring of the energy surface

Except for the very simplest systems, the potential energy is usually a

complicated, 'multi-dimensional function of the coordinates. For a system of N atoms

the energy is a function of 3N-6 or 3N Cartesian coordinates. It is therefore

impossible to visualise the entire energy surface except for some simple cases where

the energy is just a function of one or two coordinates.

In molecular modelling we are especially interested in minimum points on the

energy surface. Minimum energy arrangements of the atoms correspond to stable

states of the system; any movement away from the minimum gives a configuration

with a higher energy. There may be a very large number of minima on the surface.

The minimum with the very lowest energy is known as the global energy minimum.

To identify those geometries of the system that correspond to minimum points on the

energy surface we used a minimisation algorithm. Vast amounts of literature on such

methods are available and are divided into two groups: those which use derivatives

of the energy with respect to the coordinates and those which do not. Derivatives can

79

be useful because they provide information about the shape of the energy surface

and, if used properly, they can significantly enhance the efficiency with which the

minimum is located.

There are many factors that must be taken into account when choosing the most

appropriate algorithm (or combination of algorithms) for a given problem; the ideal

minimisation algorithm is the one that provides the answer as quickly as possible

using the least amount of memory. No single minimisation method has yet proven to

be the best for all molecular modeling problems and so most software packages offer

a choice of methods.

Most minimisation algorithms can only go downhill on the energy surface and so

they can only locate the minimum that is nearest to the starting point. Fig 2.13 shows

a schematic energy surface and the minima that would be obtained starting from

three points A, B and C. To locate more than one minimum or to locate the global

energy minimum we therefore usually require a means of generating different

starting points, each of which is then minimised. Some specialised minimisation

methods can make uphill moves to seek out minima lower in energy than the nearest

ones, but no algorithm has yet proved capable of locating the global energy

minimum from and arbitrary starting position. The shape of the energy surface may

be important if one wishes to calculate the

x

5

I

Confornational parameter

Fig. 2.13 A schematic one-dimensional energy surface

relative populations of various minimum energy structures. For example, a deep and

narrow minimum may be less highly populated then a broad minimum that is higher

in energy as the vibrational energy levels will be more widely spaced in the deeper

minimum and so less accessible. For this reason, the global energy minimum may

not be the most populated minimum. In any case, the ‘active’ structure (e.g. the

80

biologically active conformation of a drug molecule) may not correspond to the

global minimum, or to the most highly populated conformation, or even to a

minimum energy structure at all.

The input to a minimisation program consists of a set of initial coordinates for the

system. The initial coordinates may come from a variety of sources. They may be

obtained from an experimental technique, such as X-ray crystallography or NMR. In

other cases a theoretical method is employed, such as a conformational search

algorithm. A combination of experimental and theoretical approaches may also be

used.

Energy minimisation is very widely used in molecular modelling and is an

integral part of techniques such as conformational search procedures. Energy

minimisation is also used to prepare a system for other types of calculations. For

examples, energy minimisation may be used prior to a molecular dynamics or Monte

Carlo simulation in order to relieve any unfavourable interactions in the initial

configuration of the system. This is especially recommended for simulations of

complex systems such as macromolecules or large molecule assemblies.

2.8.3. Conformational search and computer simulation methods

The objective of conformational search is to identify the preferred confirmation

of a molecule: those conformations that determine its behaviour. This usually

requires us to locate conformations that are at minimum points on the energy surface.

An important feature of methods for performing energy minimisation is they move to

minimum point that is closest to the starting structure. For this reason, it is necessary

to have a separate algorithm which generates the initial starting structure for

subsequent minimisation. Several techniques can be used for this purpose, in this

study only two methods were employed to explore the conformational space of

molecules: molecular dynamics and Monte Carlo methods.

2.8.3.1. Molecular dynamics method

In molecular dynamics, successive configurations of the system are generated by

integrating Newton's law of motion. The result is a trajectory that specifies how the

position and velocity of the particles in the system vary with time. A common

strategy is to perform the simulation at a very high temperature. The additional

kinetic energy enhances the ability of the system to explore the energy surface and

prevent the molecule getting stuck in a localised region of conformational space. A

81

molecular dynamics run is typically over 1 x 10-l' seconds and a large number of

structures with their attendant energies are generated over this period. Representative

structures are then sampled at random and used for subsequent minimisation.

2.8.3.2 Monte Carlo simulation method

In general, a Monte Carlo simulation generates configurations of a system by

making random changes to the position of the species present, together with their

orientations and conformations where appropriate. For conformational searching, a

random search can explore conformational space by changing either the atomic

Cartesan coordinate or the torsion angles of rotatable bonds. At each iteration, a

random change is made to the current conformation. The new structure is the refined

using energy minimisation. Each newly generated structure (after energy

minimisation) is accepted as the starting point for the next iteration if it is lower in

energy than the previous structure or if the Boltzmann factor of energy difference is

larger than a random number between 0 and 1. If not, the previous structure is

retained for the next iteration. The procedure continues until a given number of

iterations have been performed or until it is decided that no new conformation can be

found.

2.8.4. Application of molecular modelling in inhibition study

In aqueous metallic corrosion, to be effective an inhibitor must displace water

from the metal's surface, interact with anodic and cathodic sites to retard the

oxidation and reduction corrosion reactions, and prevent transportation of water and

corrosion-active species to the surface. Therefore, it is necessary to design a ligand

that adsorbs strongly with the metal. Molecular modelling has the potential to study

this adsorption and possibly identify structurally similar compounds worthy of

synthesis and subsequent corrosion testing. If a molecular mechanics model of the

interactions that control the adsorption of a minimised structure can be set up, a

measure of the binding energy of the molecule with the metal can be calculated.

Comparing the relative binding energy of a series of structurally similar ligand may

lead to the prediction of other related compounds with potentially more effective

inhibiting action.

82

1.

2.

3.

4.

5 .

6.

7.

8.

9.

REFERENCES

D.A. Jones, “ Principles and Prevention of Corrosion”, Prentice-Hall Inc., New

Jersey, 1996

V.S. Sastri, “Corrosion Inhibitors: Principles and Application”, John

Wiley&Sons, West Sussex, 1998

M. Pourbaix, .‘ Lectures on Electrochemical Corrosion”, Plenum Press, 1973

J.C. Scully, “The Fundamentals of Corrosion”, 3‘d edition, Pergamon Press, 1990

M.G. Fontana, “ Corrosion Engineering”, 3 rd edition, McGraw-Hill Book

Company, 1987.

H.H.Uhlig, “Corrosion and Corrosion Control”, 2 nd edition, John Wiley and

Sons Inc., 1971

P.W. Atkins, “Physical Chemistry”, 4 th edition, Oxford University Press, 1990

D. P. Schweinsberg, “ Dynamic Electrochemistry and Metallic Corrosion”, QUT

Press, 1992

C.C. Nathan, “Corrosion Inhibitors”, 2nd edition, NACA, Houston, Texas, 1974

10. I.L. Rozenfeld, “Corrosion Inhibitors”, IS‘ edition, McGraw-Hill, 1981 .

1 1. J.R. Macdonald, “Impedance Spectroscopy; Emphasizing Solid Materials and

Systems” , Wiley Interscience Publications, 1987.

12. A.J. Bard, L.R. Faulkner, “Electrochemical Methods; Fundamentals and

Applications, Wiley Interscience Publications, 1 980.

13. J.R. Scully, D.C. Silverrnan, and M.W. Kendig, editors, “Electrochemical

Impedance: Analysis and Interpretation”, ASTM, 1993.

14. C. Gabrielle, “Identification of Electrochemical Processes by Frequency

Response Analysis”, Solartron Instrumentation Group, 1980.

15. P.H. Reiger, “Electrochemistry”, 2nd edition, Chapman&Hall Inc., 1994

83

16. W. Suetaka, J.T. Yates, Jr. “ Surface Infrared and Raman Spectroscopy :

Methods and Applications”, Plenum Press, New York, 1995

17. A.R. Leach, “Molecular Modelling : Principles and Applications”, 2”d edition,

Pearson Education Ltd, 200 I

84

CHAPTER 3

INHIBITIVE EFFECT OF 4- AND 5-

CARBOXYBENZOTRIAZOLE ON COPPER CORROSION IN ACIDIC SULPHATE AND HYDROGEN SULPHIDE SOLUTION

V. Otieno-Alego“, N. Huynha, T. Notoyab, S.E. Bottlea, D.P. Schweinsberg”

Centre for Instrumental and Developmental Chemistry, Queensland University of a

Technology, Brisbane, Queeensland 4000, Australia

Graduate School of Engineering, Hokkaido University, Sapporo, 060, Japan

Current Address:Faculty of Science, University of Canberra, Canberra,ACT 26 16,

Australia

85

STATEMENT OF JOINT AUTHORSHIP

Title

acidic sulphate and hydrogen sulphide solution

Inhibitive effect of 4- and 5-carboxybenzotriazole on copper corrosion in

Authors V. Otieno-Alego, N. Huynh, T. Notoya, S.E. Bottle, D.P. Schweinsberg

V. Otieno-Alego

Developed experimental design and scientific method; conducted measurement;

analysed and interpreted data; wrote manuscript

N. HUYNH (Candidate)

Contributed to experimental design and scientific method; conducted measurements;

analysed and interpreted data; assisted with manuscript

N. Notoya

Contributed to data interpretation and manuscript

S.E. Bottle

Contributed to data interpretation and manuscript

D.P. Schweinsberg

Contributed to experimental design and scientific method; contributed to data

interpretation and manuscript

86

halla

This journal article is not available online. Please consult the hardcopy thesis available from the QUT Library

CHAPTER 4

INHIBITIVE ACTION OF THE OCTYL ESTERS OF 4- and 5-

CARBOXYBENZOTRIAZOLE FOR COPPER CORROSION IN

SULPHATE SOLUTIONS

N.HUYNH, S.E. BOTTLE, T. NOTOYA* and D.P. SCHWEINSBERG'

Centre for Instrumental and Developmental Chemistry, Queensland University of

Technology, Brisbane. Queensland 4000, Australia

*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan

' Corresponding author

101

STATEMENT OF JOINT OWNERSHIP

Title Inhibitive action of the octyl esters of 4- and 5-carboxybenzotriazole for copper

corrosion in sulphate solutions

Authors N. Huynh, S.E. Bottle, T. Notoya and D.P. Schweinsberg

N. HUYNH (Candidate)

Developed experimental design and scientific method; conducted measurements;

analysed and interpreted data; wrote manuscript

N. Notoya

Contributed to data interpretation and manuscript

. S.E. Bottle

. Contributed to data interpretation and manuscript

D.P. Schweinsberg

Contributed to experimental design and scientific method; contributed to data

interpretation and manuscript

102

halla

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CHAPTER 5

STUDIES OF ALKYL ESTERS OF

CARBOXYBENZOTRIAZOLE AS INHIBITORS FOR COPPER

CORROSION

N. Huynha, S.E. Bottlea, T. Notoyab

Schweinsberg”*

A. Trueman‘ B. Hinton‘ and D.P.

Centre for Instrumental and Developmental Chemistry, Queensland University of

Technology, Brisbane, Queensland 4000, Australia

. bGraduate School of Engineering, Hokkaido University, Sapporo 060 Japan

Aeronautical and Maritime Research Laboratory, Melbourne, Victoria, Australia

* Corresponding author

a

C

119

STATEMENT OF JOINT OWNERSHIP

Title

corrosion

Studies of alkyl esters of carboxybenzotriazole as inhibitors for copper

Authors N.Huynha, S.E. Bottle", T. Notoyab , A. Trueman' , B. Hinton' and D.P.

Sc hweinsberg"

N. HUYNH (Candidate)

Developed experimental design and scientific method; conducted measurements;

analysed and interpreted data; wrote manuscript

N. Notoya

Contributed to data interpretation and manuscript

S.E. Battle

Contributed to data interpretation and manuscript

D.P. Schweinsberg

Contributed to experimental design and scientific method; contributed to data

interpretation and manuscript

A. Trueman

Contribution to EIS measurement and interpretation

B .Hin t on

Contribution to EIS measurement and interpretation

120

halla

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CHAPTER 6

STUDY ON THE INHIBITIVE EFFECT OF THE FILMS OF

ALKYL ESTERS OF CARBOXYBENZOTRIAZOLE ON

COPPER CORROSION IN SULPHATE AND SULPHIDE

ENVIRONMENTS

N.Huynh, S.E. Bottle, T. Notoya" and D.P. Schweinsberg#

Centre for Instrumental and Developmental Chemistry, Queensland University of

Technology, Brisbane, Queensland 4000, Australia

*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan

# Corresponding author

STATEMENT OF JOINT OWNERSHIP

Title Study on the inhibitive effect of the films of alkyl esters of

carboxybenzotriazole on copper corrosion in sulphats and sulphide environments

Authors N. Huynh, S.E. Bottle. T. Notoya and D.P. Schweinsberg

N. HUYNH (Candidate)

Developed experimental design and scientific method; conducted measurements;

analysed and interpreted data; wrote manuscript

N. Notoya

Contributed to data interpretation and manuscript

S.E. Bottle

Contributed to data interpretation and manuscript

D.P. Schweinsberg

Contributed to experimental design and scientific method; contributed to data

interpretation and manuscript

142

halla

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CHAPTER 7

COMPUTER SIMULATION OF THE COROSION INHIBITION

OF COPPER IN ACIDIC SOLUTION BY ALKYL ESTERS OF 5-

CARBOXYBENZOTRIAZOLE

J. BARTLEY, N.HUYNH, S.E. BOTTLE, H. FLITT,T. NOTOYA* and D.P.

SCHWEJNSBERG#

Centre for Instrumental and Developmental Chemistry, Queensland University of

Technology, Brisbane, Queensland 4000, Australia

*Graduate School of Engineering, Hokkaido University, Sapporo 060 Japan

Corresponding author

.

#

157

STATEMENT OF JOINT OWNERSHIP

Title Computer simulation of the corrosion inhibition of copper in acidic solution

by alkyl esters of 5-carboxybenzotriazole

Authors J. Bartley, N.Huynh, S.E. Bottle. H. Flitt, T. Notoya and D.P.

Schweinsberg

J. Bartley

Developed scientific method; analysed and interpreted data; wrote manuscript

N. HUYNH (Candidate)

Contributed to experimental design and scientific method; conducted measurements;

analysed and interpreted data; assisted with manuscript

H. Flitt

Contributed to experimental design and scientific method; contributed to data

interpretation and wrote manuscript

N. Notoya

Contributed to data interpretation and manuscript

S.E. Bottle

Contributed to data interpretation and manuscript

D.P. Schweinsberg

Contributed to experimental design and scientific method; contributed to data

interpretation and wrote manuscript

158

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CHAPTER 8

GENERAL CONCLUSIONS AND FUTURE WORK

175

176

8.1. General conclusions

The objective of this work was to study the inhibitive effect of soluble

benzotriazole (BTAH) derivatives for copper corrosion in aqueous environments.

Whilst BTAH is an excellent inhibitor in alkaline solution its efficiency drops off

markedly as the pH decreases. It was hypothesized that a possible way to increase

surface adsorption and subsequent better inhibition over a wide pH range might be

through the preparation of derivatives, particularly carboxybenzotriazoles and alkyl

esters of these compounds. To achieve this objective the following techniques:

weight loss measurements, potentiodynamic polarisation, SERS spectroscopy,

electrochemical impedance spectroscopy and coulometry were employed. Molecular

modelling was also investigated as a tool for inhibitor design.

Studies on individual isomer of 4- and 5- carboxybenzotriazole (CBT) showed

that the inhibition efficiency for copper corrosion in aerated acidic sulphate solution

of each isomer was pH, concentration and time dependant. At lower pH the 5-isomer

is the better inhibitor and this behaviour continues at higher pH where 4-CBT

promotes corrosion. The inhibition efficiency of the 5-isomer increased with

exposure time; this was attributed to the increase in the thickness of the underlying

oxide layer. The anti-tarnishing test showed that whilst both isomers exhibited these

properties, 5-CBT was once again the superior inhibitor.

It was found that a commercial mixture of the octyl esters of 4- and 5-

carboxybenzotriazole inhibits copper corrosion in sulphate environments open to air.

The IE% is concentration, pH and time dependent. The inhibition efficiency of the

ester mixture at the 1 x l 0-4 M level (pH - 0) is 98% which compares very favourably

with that for BTAH (- 50%). Although the inhibition efficiency of the ester mixture

decreased gradually as the pH was raised, an IE of 75% at pH - 8 indicates that the

177

mixture is suitable for practical use at the higher pH values. SERS indicates that at

low pH inhibition is due to chemisorption of 4- and 5-CBTAH-OE molecules on the

copper surface through an azole nitrogen. At high pH inhibition results from the

formation of a polymer complex.

The improved inhibition efficiency of the esters at low pH can be attributed to

chemisorption (as for BTAH) in conjunction with physical adsorption through the

alkyl group. In addition, increased shielding of the copper surface occurred by van

der Waals’ forces of attraction between adjacent octyl groups.

With respect to other alkyl esters of 4- and 5-~arboxybezotriazole, hexyl, butyl

and methyl, it was found that all of these inhibited copper corrosion in sulphate

environments open to air. In each case the inhibition efficiency is concentration, pH

and time dependent. Both coupon tests and EIS measurements indicate that inhibition

efficiency depends on the length of the alkyl chain. At pH - 0 the inhibition

efficiency decreased in the order octyl >hexyl >butyl >methyl. At higher pH (- 8) the

order is reversed. At the 1 ~ 1 O - ~ M level (pH - 0) the inhibition efficiency of each of

the alkyl esters is equal to or better than that for BTAH, with CBTAH-OE providing

98% inhibition. At higher pH (- 8) the inhibition efficiency in each case is reduced in

comparison to BTAH, but is still good enough for practical use (2 75%)

The inhibitive behaviour of the alkyl esters at low pH can be attributed to

chemisorption through an azole nitrogen of the protonated alkyl esters. The

hydrocarbon chain is also physically adsorbed and the increase in physical adsorption

as the chain is lengthened accounts for the improved inhibition efficiency. Langmuir

treatment of the data indicates that near the optimum concentration of each inhibitor

the adsorbed molecules are adsorbed as a unimolecular layer and molecular

interaction is minimal. At high pH inhibition results from the formation of disordered

polymeric films. These conclusions are supported by polarisation and SERS

measurements.

178

Dry films formed by immersing copper in solutions of alkyl esters of

carboxybenzotriazole also inhibit copper corrosion in both strongly acidic (pH - 0)

and near neutral (pH - 8) sulphate corrodents. The inhibition efficiency depends on

the solvents used to dissolve the esters, solution temperature and immersion time.

Aqueous coating solutions furnish the most protective films. Films formed by

CBTAH-BU, CBTAH-HE and CBTAH-OE are more protective than that formed by

BTAH. The inhibition efficiency of the alkyl ester film increases as the alkyl chain is

made longer. CBTAH-OE in particular gives excellent protection in corrodents of

low and near neutral pH and is also effective inhibitor for protection against

atmospheric corrosion.

EIS spectra show that the film formed by CBTAH-OE can be stable up to 3 days

in acidic sulphate solution (pH - 0) and up to 10 days in near neutral solution @H - 8).

SERS spectra of the dry films indicate that the ring system is close to the metal

surface and reinforces the hypothesis that the alkyl ester molecules interact with the

copper surface through an azole ring nitrogen lone pair of electrons, and also

experience physical adsorption with the copper through the hydrocarbon chain.

Molecular modelling incorporating molecular mechanics and molecular

dynamics is a useful tool to simulate the adsorption from acidic solution of a single

target molecule representative of each of the esters (5-CBTAHC-R) with a clean

copper (1 10) surface. The optimum crude binding energy (Ebind) between each

protonated ester molecule and the surface varied linearly with the alkyl chain length.

The resulting linear correlation between IE% and E bind for compounds that are

structurally similar suggested that the crude binding energy of a single molecule with

copper may be used to predict the inhibition performance of other compounds

constituting a series.

8.2. Future work.

This study has shown that the inhibitive effect of the compounds studied for

copper corrosion results from chemisorption though the azole ring and in the case of

the alkyl esters by physical adsorption of the alkyl chain on the metal surface. The

study provides a basis for designing other compounds which may have higher

179

inhibition efficiency over a wider range of pH. Some of the molecular structures that

may be promising candidates for copper corrosion inhibition are as follows:

Molecules with two or multiple azole rings to provide stronger chemisorption

with the copper surface.

Molecules with different alkyl chain lengths at different positions in the benzene

ring, which can enhance both the activity of an azole ring and the physical

adsorption of the alkyl chain.

By manipulating the molecular structure in these directions, the list of novel

inhibitors is potentially very large.

In addition to the techniques outlined in this thesis, the following considerations

relating to the evaluation of new compounds as corrosion inhibitors should be

established:

cost and ease of synthesis

solubility

toxicity

stability

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