IndianJournalof ChemicalTechnologyVol.5, January1998, pp. 16-28

Correlation between structure and inhibition of organic compounds for acidcorrosion of transition metals

M Jayalakshmi& V S MuralidharanCentralElectrochemicalResearchInstitute,Karaikudi,630006,India

ReceivedII March1997; accepted18 August1997

In acid media, the corrosion inhibition offered by ethanol amines, benzoic acids, acetylinic alcoholsfor iron and nickel was investigated. These molecules adsorbed on the metal surface and offeredinhibition. A correlation between inhibition and electronic configuration of the molecules wasattempted. In benzoic acids, the anchoring group is the benzene ring and the electron delocalisation inthe aromatic ring played a crucial role in the surface chelate formation. In the case of amines, theinhibition takes place due to adsorption via lone pair of electrons on the nitrogen atoms. Acetylinicalcohols offered inhibition by accepting electrons from the vacant 'd 'orbitals of the transition metals.On prolonged exposure in acid medium they form thick polymeric film on the metals thus offeringinter-phase inhibition.

A large number of aliphatic and aromatic organiccompounds, mainly those containing. nitrogen,oxygen and sulphur atoms are found to cause adistinct decrease in corrosion rate of metals in acid

media.These organic inhibitors may retard thecorrosion processes either by specific adsorptionleading to two dimensional protective layers. Inorder to understand the mechanism of inhibition,several attempts are made based on the correlationbetween the structure of organic compounds andtheir corrosion inhibition efficiencyl.8. It isconsidered that the adsorption, electro nation, bondfonnation and allied mechanisms of interaction

between the inhibitor and the metal can separatelyor together occur to bring about this inhibition.Development of quantitative structure- inhibitionrelationships (QSIR) have the longest history.Initially, pure empirical methods were employedvalidating the additivity principle for functionalgroup contributions9-11• The attempt failed becauseelectronic and steric interactions among thefunctional groups were not included. Subsequently,semi-empIrical methods were employed. Initially,Bronsted acid-base correlation12-28 is quitesuccessful but steric terms are not considered. The

dominant descriptor variable is the electroniccontribution. Much more widely used QSIR is thelinear free energy relationship and a verysuccessful application of such relationship is the

Hammett equation28. log {K/Kh} = P ,where{K/Kh} is the ratio rate coefficients or equilibriumconstants for a molecule with substituent x rather

than 11; P is a constant that depends upon the typeof reaction being studied; and depends on theelectronic characteristics of the derivative vs the

parent molecule and may be calculated30.31 ordetem1ined empiricall/o.32• The most successfulsemi empirical QSIR, is the Hansch correlation33.34.

It is not so simple as that of Hammett correlationsince it includes solubility/ dispersibility terms.However, the Hansch correlation is shown to beconsistent for chemisorption of film forminginhibitors like t-cinnamaldehyde and derivativeson steel in concentrated HCI35.36. If the

lipophiliclhydrophilic character of the moleculedoes not play a dominant role or such a situationdoes not arise, the Hammett equation holds good.

The paper deals with the inhibition of ethanolamines, acetylinic alcohols, benzoic acids in acidsolutions. The inhibition of ethanol amines and

acetylinic alcohols for acid corrosion of nickel andbenzoic acids and acetylinic alcohols for acidcorrosion of iron were studied. A uniform

approach to understand the inhibition mechanismof these compounds by correlating structure withadsorption on the metal surface leading tointermediate chelate formation, which results in

inhibition of corrosion is attempted.

:1 '

JAY ALAKSHMI & MURALlDHARAN: ACID CORROSION OF TRANSITION METALS 17

Experimental Procedure

Matt:rials

Swedish Pure Iron [ 99.95% Fe, 0.0056% Mn,0.001% Si and 0.025% C] has been used. Purenickel [99.9999%] supplied by Johnson Mathey,USA was used.

Chemicals

Benzoic acids (BA), salicylic acids (SA), orthonitro benzoic acid (ONBA) and anthranilic acid[AA] of analytical grade were used in theconcentration range of 5x10-3 to 2xlO-3 M; 2­methyl-3 butyn-2-ol (MBO), propargyl alcohol(PA), 3-butyn-2-ol (BO) and 2-butyn-l,4-diol(BD) supplied by Aldrich , USA were used in theconcentration range lxlO-4 to 10-2 M. Mono-, di­and -tri-ethanol amines of analytical gradesupplied by Sarabhai Chemicals , India were usedin the concentration of 5xl 0-2 M. Analytical gradereagents were used without further purification.

Three Electrode Cell

A three electrode cell with large platinum foil asauxiliary electrode and saturated CalomelElectrode as reference electrodes were used. Theworking electrode of iron [0.785 cm2] and Nickel[0.196 cm2] were used. The outer jacket of the cellwas kept at desired temperature by circulatingwater from a thermostat. The solution under studywere deoxygenated using purified hydrogen.

Weight change Method

Iron specimens of size [ 7.5x2.5 cm] and nickelspecimens [5xl cm] were polished and degreasedwith trichloro ethylene. They were weighed andsuspended by using glass hooks without touchingeach other in 400 mL beakers kept in a thermostat[±O.l K]. After specified periods of immersion

they were removed, washed in tap water , driedand weighed. Triplicate experiments were doneand the variation in weight change was ±5%.

Electrochemical methods

Galvanostatic polarisation-A constant currentgenerator was used to apply different current steps.The electrode potential was followed with a digitalvolt meter. The steady value in potentials wasmeasured and plotted against logarithmic current.Similarly potentio static polarisation was carriedout by varying the applied potential using PARpotentio stat system.

Faradic impedance method-Sinusoidal currentnf various amplitudes were applied at the corrosionpotential using a PAR impedance system { modelno., 5206}. Impedance measurements were madein the frequency range of 0.1 to I kHz..

Small amplitude cyclic voltammetry (SACV)­The SACV experiments were carried out using ,CORROVIT' [ Tacussel make] in conjunction withX-Y-t recorder.

Results and DiscussionCorrosion inhibition for iron

Benzoic acids-In the case of benzoic andsalicylic acids, the inhibition efficiency generallydecreased with temperature while for orthonitrobenzoic acid and anthranilic acids, it increasedwith temperature. Antharanilic acid showed thehighest efficiency in the range of 70-80% at thetemperature of 363K (Table 1). Maximumefficiency was obtained within 30 min. There is amarginal decrease in efficiency with increase inimmersion time. Concentration changes also hadonly marginal influence. In the case of orthonitrobenzoic and antharanilic acids, the inhibitionefficiency tends to decrease with concentration.

Table I-Percentage inhibition efficiency for pure iron in 1.0 N sulphuric acid containing 0.01 M inhibitor

Inhibitor concentration0.01 M

303 K 323 K 363 K

A B C D A B C D A B C D

335970

16175670

1518

5573

1526

46

70

14

203060

231518

7671

3022

3340556

4040459

2439960

3848

966

12446

81...

A - 30 min; B -.~O m~n; C - 90 min; D - 120 min; BA - Be~,~ ~c acid; ONBA - Orthonitrobenzoic acid;AA-AnthllTflmhcaCld '~'i>. Ae"C!p:A ..' __' ·-.r ,,· ,,_,,:,(~

ri( {~ (6~~'1J~:~~\\..- l 0. ~ 3 . l(_ O\...J ':: /,••\'j.."L : f- ) "~fjX \. ~ "'I)~ •••.'--- _..I. L-

BASAONBA

c, ~._"..-_.~, •• ,c " ' - ,-, ".'11"- •• " . -" " I

18 INDIAN J. CHEM. TECHNOL., JANUARY ]998

= Corg/55.4 exp( -t\Gads / RT)

where Corg is the concentration of inhibitor in thebulk of the solution37-40. The Eqs (2) and (3)represent the Flory' Huggins isotherm and BockerisSwinkles isotherm respectively. A plot of LHS ofthe above equations against concentration gives a

The average activation energy for corrosionobtained from the calculated activation energiesfor 30 and 60 min of immersion time data,

revealed that ortho nitro benzoic acid , Salicylic.

acid and antharanilic acid increased the energy ofactivation. The adsorption of benzoic acid, orthonitro benzoic acid and salicylic acid is found to

obey LangmUIr Isotherm [Fig. 1].

B/(l-B)=ACexp [-t\G/R1] ... (1)At higher temperatures, this isotherm was found

to be not applicable. Increase of temperatureaffected the rate of adsorption/desorption and if therate of corrosion is faster than the rate of

adsorption, there may be a decrease in inhibitionwith temperature. In the case of antharanilic acid,commonly used isotherms were found to be notapplicable. The adsorbed unprotonated amino

benzoic acid may lie flat on the corroding iron.This organic may displace some solvent moleculesand the themlOdynamics of it depends on therelative size of the inhibitor and solvent molecule.Other isotherms are:

straight line passing through origin for the mostprobable x value. Figs 2 and 3 give the best straightline for x=3. The free energy of adsorptionobtained for antharanilic acid from Flory-Higginsand Bockris -Swinkle isotherms are -5.9 kcallmol

and -1.19 kcal/ mol respectively. ONBA showedpoor inhibition efficiency in comparison withothers. When compared with the values obtained inuninhibited sulphuric acid solution, the opencircuit potentials for all the inhibitors remained inthe noble direction. The shift in the noble directionindicated the interference of these inhibitors with

the anodic process. There is no quantitativerelation between % inhibition and shift of potentialin noble direction. ONBA exhibited very highpositive potentials compared to others (Table 2).

In the SACY, the E-I curves exhib;ted hysteresis

between the forward and reverse sweep withexponential type decay. At the start of both thesweep directions, the degree of hysteresis issensitive to sweep rate (v) and only at the lowestsweep rate, it is considered to be negligible41• Thegradient Ih vs v plot at v = 0 is

0.Q30.01 0.0 2

CORG , M

- 5.0

X ,---,[k+

C>L----.J

mlX~

1.0

... (2)

... (3)

B

x(1- BY: = Corg/55.4 exp( -LlGads / RT)

~ = (B+X(1-B»)(1-B)X XX

Fig. 2-Bockris-Swinkles adsorption isotherm for adsorptionof anthranilic acid on iron from I.ON H2S04 solution atdifferent x-values

2.0

1·0

cDI

0.5

0.30·003 0.005 0·01 0.02

CONCENTRATION,M

15,----

I ~10

<D ...:

x5

001 0.02

C ORG ' M

x = 3

0.03 ..

Fig. 1-log6/(I-6) vs loge plot for benzoic orthonitrobenzoicand salicyclic acids at 303 K

Fig. 3-Flory-Huggins adsorption isotherm for adsorption ofanthranilic acid on iron from I .ON H2S04 at different x values

I -III WI '" '~I""'III "" "

",_, __ C'~_'_'__-- _

ACL' ~,- 7-·;1\.".JAY ALAKSHMI & MURALIDHARAN: ACID CORROSION OF TRANSITION METALS 19

Table 2-Corrosion potentials (vs SCE) and percentage inj1ibition for pure iron at differentconcentrations of inhibitors in 1,0 N sulphuric acid solution 303 K

Concentration of BAONBASAAA

inhibitorMEcorr%InhEcorr%InhEcor%InhEcorr%Inh

0.005

-50041-4607-49041-48074

0.01-53038-2009-49048-47066

0.02-47060-35015-48049-46052

Ewrr for pure iron in 1.0 N H2S04 is -530 mV vs SCE% Inhibitor values correspond to 30 min valuesEcorrvalues are taken as soon as the sr.ecimen is immersed

AA

20.020.0

20.20

SA

10.010.013.3

Table 3-Rp -= (dll/di)T]~o in ohms obtained from SACVmethod for pure iron into H2S04 containing concentration of

inhibitors at 303 KONBA BAConcentration of

inhibitor of, M0.005 2.8 10.00,01 3.0 J 0.00.02 3.0 16.6

Ro for pure iron in 1.0 N I-IzS04 = 3.8 n

... (4)

... (5)

... (6)

The electrode was polarised in the anodic directionupto 12.5mV and in the cathodic direction upto-12.5m V at different scan rates. The polarisationresistance values for ONBA are slightly less thanin 1.0N uninhibited acid suggesting that ONBA is

not an inhibitor. The Rp values for other acids

suggests that th,e order of inhibition isAA»SA»BA [Table 3]. At the corrodingelectrode surface, these acids may perform thefollowing functions: (i) Chemisorbing on thesurface, (ii) Forming a more or less stable complexwith a corrosion intermediates from the dissolution

sequence, (iii) Formation of a complex with a finaloxidative propensity The electron distributionthrough out the molecule will have some effect inthe above three functions. A molecule with a greattendency for desorption may have small tendencyfor the formation of a surface complex. Amolecule with a great affinity for lattice ion but anegligible tendency for long lived adsorption willfunction as an accelerator to corrosion. The

scheme may be as follows:

Fe + OH-- B FeOHads+e ... (7)FeOH B FeOH++e " (8)FeOH+ B Fe2++ e .. , (9)

The dissolution phenomena would further lead tosurface chelation with inhibitors as

... (10)

FeOH In --+ (FeOH In)ads .. , (11)

(FeOH ~)ads~ FeOH I+n+e (12)FeOH rn-)- FeIn2+ + OH-- (13)

The inhibitor may interact or displace thesolvent molecule or specifically adsorbed ion.I + S~Iads + S ... (14)If step (10) is the dominant mechanism for theproduction of chelate, then the formation of chelatewould be enhanced by increased coverage ofadsorbed inhibitor. If the surface coverage ofFeOH leads to the subsequent formation of FeOH+,then the chelation (step 10) must be faster, thanstep (8) or the adsorbed inhibitor concentration isto be more. If chelation forms by step (4) and theequilibrium are shifted to the right relative to eachother and thestiochiometry of (8) i.e., whether n>1will determine whether the inhibition will increaseor decrease or remain constant.

Inhibition or adsorption is determined by theelectron distribution in the anchoring group of theadsorbate. Benzene ring remains to be anchoringgroup in the case of benzoic acid. The anchoringability is further increased by the hydroxy groupsubstitution in SA and which improvesdelocalisation of electrons in the aromatic ring.Increasing inhibition with concentration suggestthat chelation may take place with (FeOH)ads' The

observed adsorption pseudo capacitance 157.5

j..JF cm-2 compared with uninhibited values

20 INDIAN 1. CHEM. TECHNOL., JANUARY 1998

.;;..

Table 4-Effect of immersion time on the

percentage inhibition efficiency in Hel containing10-2 M inhibitor

suggests the roughing of the surface taking placeduring corrosion and the true area of the surfaceincreased with time.

In the case of ONBA, the nitrogen groupreduced the electron density in the aromatic ringand hence the tendency for adsorption. At 303 K,the inhibition offered at all concentrations is

negligible. At higher temperatures and at allconcentrations an inhibition efficiency around 50%and the formation of a black film on the surface

suggest the inhibition by a non-porous productwhich is not observed at 303K. The reduction of

ONBA to amino benzoic acid is expected atconcentration >O.OIM. A shift in corrosion

potential to -200 mY and -350 mY compared with-460 mY (uninhibited acid) suggests that thiscompound reduced on iron. At highertemperatures, the reduction product adhered to thesurface and brought about enhanced inhibition.

The presence of amino group in AA stabilisesthe electron density in the benzene ring. It is seenfrom the weight loss data that increase ofconcentration decreased the efficiency whiletemperature increased it. The energy of activationfor corrosion was found to be high in presence ofAA. It displaced three water molecules asevidenced by isotherms considered. The smallernegative values of free energy of adsorptionsuggested weak Yan der Waal type of forces. Theadsorption might be due to the six membered ringand nitrogen atom of the amino group. Theadsorption ofring substituted anilines41 on mercurywas attached to the surface by 1t-bonds of thearomatic ring. Therefore, the electron density atnitrogen was directly coupled to the density at thearomatic ring. In this case adsorption was due tothe interaction of 1t-electrons as evident by

Inhibitor Time of immersion, min

increasing inhibitor efficiency compared with BA.The greater inhibition was achieved only by theadsorption occurring parallel to the surface. Thetendency to adsorb was greater than the formationof surface complex (step 14). The inhibitionefficiency remaining constant with time suggeststhat adsorption was favoured. If chelation wasfavoured, the inhibition efficiency would increasewith time of immersion. An inhibition efficiencyof 60 to 80% suggests that the coverage by

FeOHads is low. Due to better inhibition, surfaceroughing cannot occur; hence a high value ofadsorption pseudo capacitance of 145.8 /IF cm-2

needs to be understood in terms of protonatedamine or a proton. The presence of protonatedamine or proton may affect the dielectric constantof the medium. The inhibition efficiency was dueto unprotonated amino acids. The presence of(FeOHadJ affected the area available for protondischarge in the cathodic reaction.

Acetylinic alcohols-Immersion time increasedthe inhibition efficiency and for a givenconcentration of inhibitor, BO offered highestinhibition followed by PA, BD = MBO (Table 4).Fig. 4 presents typical galvanostatic polarisationcurves for pure iron in three different Helsolutions. The linear segments of the anodic andcathodic polarisation curves were extrapolated tocorrosion potentials to obtain corrosion current.The anodic and cathodic Tafel slopes were found

_200

-300

w

0=i _400lJl>>E~_500w

-600

Fig. 4- Typical galvanostatic polarisastion curves for iron inthree different hydrochloric acid solutions

'If;

30 45 60 90PA 93 95 97 98

MBO 75 86 90 98BO 98 95 96 98SO 78 86 90 87

PA: Propagyl alcohol; MBO: 2 Methyl 3 Sutyn­

2-01; BD: 2-Butyn- 1:4 diol; BO: 3-Butyl-2-o1

-700

-8ceI

L.2

I10

---1-_J20 3J

11'1 1'1' I I' 'I~IIII 'I'I '" ,I

JAY ALAKSHMI & MURALIDHARAN: ACID CORROSION OF TRANSITION METALS 21

... (19)At steady state, the W and FeX produced mustdiffuse away continuously. Also the current is

Fe2++ m H20+ nCl-- ~[Fe(OH)m(CI)J2-n-m+mH+ ... (18)

therefore, the over all reaction can be expressed byFe2++ m H20+ n CI-- BFeX + nH+ + 2e

to be 80 to 120 mV decade-t respectively. These proportional to the diffusion rates of W and FeX.alcohols retard th-e rate of corrosion by increasing The relation can be expressed bycathodic polarisation (Fig. 5). Acid concentration - 2FD +increased the inhibitor efficiency (Tabel 5) i= -2FDFex [OOFe/dx]x..o- H [ooH+I dx]x=o

sug~e~ting ,that ~e, ~o~h H+ ion and Cl- i~ns m ... (20)p~cI~a.te In, the InhIbItion proc~ss. Concentration Application of diffusion layer simplifies the

of InhIbItor Increased the effiCIency for a given' 'tu ti' d ( )' d t b h'd 'PA f~ d ., , .. Sl a on an a + -0 IS assume 0 e mucaCI concentration. 0 lere more InhIbItIon In H x-

1.0N acid while MBO offered highest inhibition in large~ than the activity of W ion in the bulk of the5N and 8N acids; BD offered the least inhibition. solution.

Adsorption of CI- ion on the iron surface and i = [2FDH+ (aH+)x=o] =.[2FDpcx (apcx)x=o]subsequent complex formation was envisaged42-4S, a m8 8

In sulphate solutions, the formation of iron (21)complex had been suggested46. In similar lines, the ...anodic Tafel slope of 80 mV Idecade under steady where 0 is the diffusion layer thickness; Insertion

state polarisation may be due to the formation of of (aH+)x=Othus obtained gives,hydroxo chIoro iron complex. The Bockris 2FK D +mechanism for iron dissolution46 is as follows i; = H H exp[(1 + fJ)F/j.'a I RT] ... (22)

Fe + OH-- B(FeOH)ads + e (15) ,m8(FeOH) ~ FeOH+ + e [rds] (16) That IS

FeOH+ B Fe2++OH-- (17) {2FK D + }1I2In addition, the reaction for the formation of the ia = a H exp(l + fJ)F/j.'a I RT ... (23)

1· d b 'l'b . m8comp ex ISassume to e at equI I numTherefore

{M,/d In ia}= 2RTI(1+P)F ... (24)Since the metal surface is covered by adsorbed

Chlorocomplex, the positively charged alcoholmay react as:

ORI I I I I I[-C - C - C = CR] + W B[-C-C-C = CW2]

" I I I ... (25)-100

-200

Table 5-Influence ofHCI concentration of the. percentageinhibition efficiency for nickel in various concentration of

inhibitors

-300

InhibitorConcentration% Inhibition

M

l.ON5,ON8.0NUJ ~-400

PA10-4316668III

10-3316667> >10-27884

E",-500

88

1&I

010-4

~

0BD 125065

-600

4410-328506544 10-2

536990

-700~

---0...._.....•.•..I MBO10-4256767

-eool

'0.I10-33772

II001

10-.25887891

2 102030

l)mABO10-4425959

Fig, 5- Typical galvanostatic polarisation curves for pure iron

10-3426565

in different concentrations ofBD

10-2507070

22 INDIAN 1. CHEM. TECHNOL., JANUARY 1998

MEA: Monoethanol amine, DEA: Diethanol amine, TEA:Triethanol amine

Table 6----Energy of activation and percentage inhibition forcorrosion of nickel

Corrosion inhibitors for nickel

Ethanol amines-Potentiostatic experimentswere carried out after immersing the electrode inthe solution under study and the open circuit

potential reached a steady value. Corrosioncurrents were obtained by the extrapolation ofanodic and cathodic Tafel lines to the corrosion

potential. In all experiments, the anodic Tafel lineextrapolation gave a higher corrosion currentcompared with the cathodic Tafel line

0.5 1.0

100

INHIBITOR CONC,M

Fig. 6----Typical plot of log(GIl-G» vs loge for allethanolamines for Nickel in I.ON H2S04 at 303 K

extrapolation method. The corrosion currentsobtained by anodic Tafel line extrapolation wereused for analysis. The anodic and cathodic Tafelslopes were unaltered in presence of inhibitors inthe concentration studied.

The inhibitor efficiency has been calculated for

monoethanolamine[ MEA] diethanolamine [ DEA]and triethanol amine [TEA] from the corrosioncurrents.

%efficiency = {Icorr-I'corr} /Icorrx 100

where loom l~orr are the corrosion rates in absenceand presence of the inhibitors respectively.

MEA exhibited more inhibition than DEA andTEA at 303K and above. All three raised the

energy .of activation and MEA .was found to raise itmore when compared with DEA and TEA[Table 6]. This suggests that these amines mayeither participate in' the electrode process or maychange the potential difference of themetal/solution interface by adsorption. Since thereis no change in the anodic and cathodic Tafelslopes in presence of these amines, inhibition maybe due to the adsorption, Langmuirian type, which

changes the interfacial potential difference (Fig. 6).Temperature rise caused an increase in corrosionbut there is no definite relation between inhibition

and concentration at temperature >303K.Adsorption depends upon molecular size, shape,

charge distribution, deform ability of the surface

-5.5-9.1-6.9-7.1

Mkcal/mol

73

40

33

% Inhibitionat 303 K

1.0 N H2S04

1.0 N H2S04 + 0.05 M MEA

1.0 N H2S04 + 0.05 M DEA

1.0 N H2S04 + 0.05 M TEA

Solution

o1H

! i I +

FeOHads+[-C-C-C = CH 2] ~

r I I OH

I 1 I[FeOH-C-C-C= C H\] ... (26)

OIH I II I i

[FeOH-C-C-C-CH2+]+OH-- -+Fe2++OH-

I I I OHI I +

+[-C-C-C=CH2] .•• (27)I IThe formation of Chlorohydroxy iron complex isnot favoured as the production of Fe2+ ions ishindered .Tne observed Tafel slope of 80 mV/decade confim1s this. This monomeric inhibitor

molecule can undergo polymerisation so that a

polymeric two dimenSIOnal system is formedwhich lightly adheres to the metal. In the case ofBO and PA, the polarisation of the triple bond isstabilised first by a non-classical carbonium ionand then by a keto contiguration which is known tocomplex with FeOHads' The terminal carbon ­carbon triple bond stabilises 1t-electron interaction.In the case of BD, the non-terminal carbon-carbon

triple bond .does not favour 1t-bond interactionbecause of steric hinderance.

I III 1'1' I 'I '~I'" 'III'" " "'I "1' I' ,. I'll''''' "I! ,,,.,,' I" 'I "II '1'1 " '

JAY ALAKSHMI & MURALIDHARAN: ACID CORROSION OF TRANSITION METALS 23

_12 -8 -4 0 .4 .8 .12FIG:7

TYPICAL SACV FOR NICKELin 1N HCl

Fig. 7- Typical SI\CV for the corrosion of nickel in I .ON HCIat different sweep rates

active groups as well as charge on the metalundergoing corrosion43• Owing to the acidity of themedium, the amines may exist in the protonatedform. The stability of the protonated formincreased with the electron density on the nitrogenatom. MEA exhibits higher inhibition as itenhances the formation of the protonated species.In DEA and TEA, due to the inductive effect theelectron density on the nitrogen atom has beendecreased and the formation of the protonatedspecies has been slowed down. Since Ethanolamines have more than one centre viz., nitrogenand oxygen for electron donation, the participationof the lone pair of oxygen is also possible. But allthe lone pairs of electrons on oxygen atoms maynot come closer to the metal, as the size of themolecule hinders adsorption. Though DEA andTEA have more oxygen atoms , their participationis restricted due to stenc hindrance.

Acetylinic a/coho/s-Galvanostatic and potentio­static methods gave nearly the same ·value in I.ON

and IO.ON acid solutions. There is very littledeviation in 2N and 5N acids. Inhibition

.i. 25mV/s

5 mV Is

63mV/s

8 mV/s

12.5 m VIs

25mV/s

l00L/lA4 mY

E

Table 7---Comparison of percentage inhibition efficiencies obtained from different methods

Weight loss

ImpedanceGalvanostaticPotentiostaticSALV

method

methodmethodmethodfrom

1.0 N Acid + 0.01 MinhPA

8395959550

MBO

7596878653

BO

5098919253

BD

6587868627

2.0N Acid+O.Ol MinhPA

85-979743

MBO

--909636

BO

97-929120

BD

72-9221

3.0 N Acid + 0.01 MirihPA

9295929476

MBO

9780769681

BO

8587938880

BD

--899677

4.0 N Acid + 0.01 MinhPA

10098898769

MBO

6097939741

BO

9894879041

BD

9066939046

24 INDIAN J. CHEM. TECHNOL., JANUARY 1998

(34)

(35)

(37)

(38)

OH

OR

I IIf [NiC1-C-C-C=CHz] is denoted by I , thenI I I

R

as,Ni + cr- H NiClads+e

OH ORI I ii' I

[-C-C-C = CH] + H+H [-C-C-C = CH'z]I I I I IOR

t I INiClads+[-C-C-C = CH+z]HI I

ORI I I

[NiCl-C-C-C = CH+] ... (36)I I

hydrogen ions would result in a decrease in thecorrosion rate of the two reactions. Hydrogen

evolution reaction has a higher Tafel slope and cancontrol corrosion by its low exchange currentdensity.

In presence of alcohols the observed Tafel slopeof 40 mV suggests that the rate determining step ischanged. Cathodic Tafel slopes remained same orenhanced in presence of inhibitors suggesting thathydrogen evolution reaction is not affectedconsiderably. In 10.ON acid solutions, the presenceof alcohols had no effect except in the case of BOand BD (Table 9). This needs discussion. Since themetal surface is covered by adsorbed complex, theprotonated positively charged alcohols may react

I I I[NiCl-C-C-C = CH\]+Cl- ~ [NiClz]I I

H ORI I I

OR +[-C-C-C = CH\]+eI I I I I

[-C-C-C = CH+z] ~ [-C=CH-CR=CH], I I IRSystem Weight loss method

1.0 NHCI

5.0NHCI10.0 NHCI

Pure acid

-19.84-14.72-10.46PA

-28.8-22.80-43.42MBO

-30.0--25.72

BO-21.72--23-41

BD-28.14-15.20-26.72

efficiencies from weight loss method showedeither the same or nearly 10% of theelectrochemical methods. Impedancemeasurements (Fig. 7) gave nearly the same valueas obtained by steady state electrochemicalmethods (Table 7).

The activation energy obtained for corrosionincreased by acid concentration suggesting thatboth hydrogen and chloride ions favour c0l10sion.In presence of these alcohols, these increased(Table 8).

Table 8--ActI, vation energies for corrosion of nickel(kcal/Mol) obtained with different concentration ofHCI and

0.0 I M concentration of inhibitors

The dissolution of Nickel takes place as follows:Ni + Ct - H NiClads+e (28)NiClads+H+~ NiCl H+ (29)NiCl H+ +cr- H Niz++2Cl-+H+ (30)with NiClads obeying Langmuir isotherm. Theobserved Tafel Slope of 60± 10m V suggested that

NiClads+H+~ NiCl H+ ... (31)is the slow step. The rate expression may bewritten as

ia= K. (H+) (Cr )(exp) -FAr/laIRT ... (32)where Ar/Jo is the interfacial potential difference. Inthe hydrogen evolution reaction, the ratedetermining step isH++e ~ H ... (33)corresponding to a Tafel slope of 135 mY. Boththe rate determining steps involve the participationof [H+] ions. A decrease of both nickel and

Table 9-lnfluence of inhibitors on the tafel slopes values obtained from galvanostatic measurement

Inhibitor

1.0 NHCI2.0NHCl5_0NHCI1O.0NHCI.rO.OIM

AnodicCathodicAnodicCathodicAnodicCathodicAnodicCathodic

Acid

7014070150601308510030

120301104014085130,.

PAMBO

40110301004014080100BO

4014040120401404090BD

4012040120401207090

"'IMII'"'' 1'11'"'' ''''11'1'11' I" 'I "II ' '1'1 " I

JAY ALAKSHMI & MURALlDHARAN: ACID CORROSION OF TRANSITION METALS

~ _ ~ U",,",' _

25

10-5 M

10-4 M

10-3M1O-2M

()= iOR-iORiOR

This suggests an anodic Tafel slope of 4OmV. the outer Helmholtz plane ('II-potential) of the

The formation of Chloro complex is not favoured double layer that exists on a metal-solutionin presence of alcohols. interface and the electric charge on certain

Alkyl chain and hydro phobicity-Adsorption of inhibitor molecules. This adsorption depends onalcohols on the metal may be as follows: the zero charge potential of the metal,

(i) Chemisorption either by donation of polarisability of anions and the electricalelectrons from the triple bond to the vacant d properties of the organic surface active molecules.orbitals, or (ii) accepting electrons from the d The stability of the protonated form of ethanolorbitals to anti bonding orbitals of the triple bond, amines increased with the electron density on theor (iii) both by electron and back electron Nitrogen atom. Monoethanolamine exhibiteddonation. higher inhibition as it enhanced the formation of

In strong acid solutions, hydroxyl group of these the protonated species. In di-tri e~hanol amines,alkynols can be thought to act just as electron due to the inductive effect, the electron density onwithdrawing group in the molecule because it the nitrogen atom has been decreased and thecombines with proton in the acid forming formation of the protonated species has beenOxonium ions. Therefore, hydroxyl groups slowed doWn. The lone pairs of electrons onattached to alpha carbon of the triple bond should oxygen atoms may not come closer to the metal ashave stronger electron withdrawing property. than the size of the molecule hinders adsorption.that attached to carbon atom reducing electron Though di- and tri-ethanol amines have moredensity of the triple bond within the molecule. oxygen atoms, their participation is restricted dueGreater is the decrease of electron density more to steric hindrance.adsorption by back donation on the surface and it Chemisorption of an organic molecule on ais expected that adsorption by back donation metal in a corrosion medium involves the

inhibits mainly the cathodic reaction on Ni~J<:e1. displacement of water molecules from the metalThe alcohols offer inhibition by ad~orption, surface and the eventual charge sharing or even

forming positively charged species, interaction of actual charge transfer or change sharing. Thethese species with adsorbed NiCI to form a molecular structure, mode of adsorption andpositively charged barrier layer. These positively chemical nature of the "anchoring' inorganic groupbarrier layer prevents the approach of H+ions near determines the efficiency of inhibition. Thethe surface reduces the exchange current density of chemical bonding was discussed in terms of theh.e.r. This reduction in exchange current density is Lewis acid base theory, the charge transfer theory,responsible for the reduction in corrosion. the extent of 1t-character of the electrons. TheUnified mechanism of corrosion inhibition formation of quasi surface compounds or surface

Organic chemical compounds inhibit corrosion complexes is subjected to the spatial arrangement

by adsorbing at the metal solution interface. The but it is evident they' should. be only one laye~principal types are electrostatic adsorption, thick. Inhibition was found to mcrease when multI

chemisorption and adsorption resulting 1t-bond site association with the metal occurs...interaction with the metal. In order to understand the role' of acetyhmc

Electrostatic adsorption results from the alcohols in affecting the hydrogen evol~tioncoulombic interactions between the electric field at reaction, the cathodic polarisation curves obtamed

Table lo-Hydrogen evolution reaction - comparison of reduction in exchange current densityfor hydrogen evolution and inhibition effici~ncy in 10.0 N HCl

System FA BO MBO BDe ~% e ~% e ~% e L%

0.89 74 0.86 74 0.97 86 0.76 860.92 78 0.90 76 0.98 88 - 850.95 84 0.89 74 0.98 93 0.83 900.97 90 0.89 90 0.99 98 0.89 93

26 INDIAN 1. CHEM. TECHNOL., JANUARY 1998

in ION Hel solutions were extrapolated to thereversible hydrogen evolution potential in thesesolutions (Table 10). The exchange current densitywas found to be 1.0x I0-4 Alcm2 and it was reduced

by these alcohols.

8=[Io,H - I 'o,HI IO,H=reduction in exchange current

density for h.e.r where IO,H, I' O.Hare the exchangecurrent densities in presence and in absence ofinhibitors. Increase of concentration of alcohols inIO.ON HCI solutions increased e and % inhibition.

There is no linear relationship betweenconcentration and inhibition. Even 10M concen­

tration offered 76% of the surface coverage andh.e.r may take place on the uncovered surface.Enhanced concentration help in the formation ofpositively charged protonated species.

A discussion on the structure of acetylinicalcohols and the inhibition efficiencies is presentedbelow:

Prop argyl alcohol has an hydroxy group in thealpha position to the acetylinic function. Thisfavours the formation of non-classical carboniumion intermediate and further to keto double bond

configuration (Fig. 8). BO has both hydroxy and

methyl group in the alpha position. The formationof carbonium ion is less favoured in BO compared

to PA. MBO has one hydroxy and two methylgroups in the alpha position. BD is a symmetricalmolecule with non-terminal triple bond. Thestabilisation of 1t- electrons in the carbon-carbon

triple bond is favoured as MBO> BO. PA>BD.Intra-molecular protonation is favoured in theorder BO>P A>BD>MBO. MBO is a methylsubstituted PA and increase in the alkyl chain

imparts greater hydrophobicity to the interfaciallayer, sequeezing out water molecules from theinterface. These water molecules considerablyreduce the protons near the interface. Thesubsitituted 1t-electrons of the terminal bond cancome closer to the metal.

While correlating the structure with inhibltionthe following facts are to be considered.

(i) The 1t-e1ectron density on the C=C triplebond, (ii) Protonation at the lone pair of electronson the oxygen atom, (iii) Keto- enol tautomerism,(iv) Intra molecular protonation through Hbonding.

In the case of benzene molecule, the 1t-bonds

~ ,

5600

0.03c/s

10000

2000 I 4000Z, ohms

5000IZ, ohms

a

.....

<:) ®

.~Ul

E~ 500

::,: 25.1c/5 .O~C/S2.31e Is 750 1500 2250 3000,

Z,ohms

14000

2 .Oc /s

@5.ge/5

SOOO I 10000Z , ohms

5000 10000I

Z ~ ohms

~2:[~

UlE~2:::~ V'2. leiS

N

0

""26

~E-a4•.. ~N2

Fig. 8-lmpedance diagram for the corrosion of nickel. in I.ON HCI containing 1O-2M alkynols at 303K: (a) No Inhibitor (b) PA(c) MBO (d) BO (e) BO

1'111 q 1 'I tl I~'-I ' 'I

JAYALAKSHMI & MURALIDHARAN:ACID CORROSIONOF TRANSITION METALS 27

result also from SP, hybridization. Because ofresonance, the main contribution to the bondcharacter of benzene comes from bonds on Ortho

form (Kekule form); the para form (Dewar form) ismuch weaker because the exchange energybetween electron pairs, decrease rapidly withdistance. Once a molecule gets Chemisorbed onthe corroding surface, the kinetics of adsorption /desorption decides the formation of surfacechelation .A molecule with a great affinity forlattice ion but a negligible tendency for long livedadsorption will favour dissolution. Theprecipitation of the solution soluble metal complexat the interface usually offer barrier protection. SAand ONBA chelated with FeOHads intermediatefavoured dissolution at their higher concentrations.

In the case of AA, the tendency to adsorb isgreater than the formation of surface complex. Theunprotonated amino group of the acid offeredinhibition; however the inhibition is decided by thedynamic equilibrium between unprotonated andprotonated amino acids. In the case of acetylinicmolecule, an SP hybridisation produces two bondsat 180 and hence a linear molecule. Alkynols arechemisorbed on the metal surface either bydonation of 1t-electronsof the triple bond to vacantd orbitals of the transition metal or by acceptingthe electrons from the 1t-antibonding orbitals or byboth. The hydrogen evolution on nickel wasinhibited by these alcohols (80 to 99%). Thesesuggest that these alkynols in strong acid solutionsfavour stronger electron withdrawing propertyfrom the 1t-antibonding orbitals of nickel. Inpresence of the higher surface coverage of thesealkynols, the adsorbed hydrogen metal bond isstrengthened. These impede both hydrogenevolution reaction and its permeation to the metal.Quantum chemical calculations on the adsorptionof PA on iron confirmed that inhibitor acts as a

strong electron acceptor from iron and thefavourable mode of adsorption is via C=C49 triplebond.

Conclusion

Understanding the mechanism of corrosioninhibition offered by organic qmines, acetylinicalcohols and benzoic acids by correlating theirstructure and electron delocalisation with the

observed inhibition tendencies proves to be atechnologically viable exercise. To screen theorganic compounds that can be used as corrosioninhibitors such understanding is essential andworth while.

ReferencesI Hackerman N & Macrides A C, Ind Eng Chem, 46,

(1954)523.2 Donhaue F M & Ken Nobe, J Electrochem Soc, 114

(1967) 1012.3 Hackerman N, Eur symp on Met Corr Ferrara Campt,

Rend Univ, Ferrara, 89 (1961) II.4 Grigorev V P & Osipov 0 A, Third International

Conference on Metallic Corrosion, Moscow, 158 (1966)1.5 Grigorev V P & Kuzetsov V V, Zashch Met, 3, (1967)

178.

6 Grigorev V P & Ekilit W, Zashch Met, 4 (1968) 31, 582.7 Donhaue F M & ken Nobe, J Electrochem Soc, 112 (1965)

886.

8 Donhaue F M, Akiyama & Ken Nobe, J Electrochem Soc,114 (1967) 1006.

9 Klopman S, JAm Chem Soc, 106 (1984) 7315.10 Free S M & Wilson J W, J Med chern7 (1964) 395.II Dupin P, De Savignac A & Lattes, Werkst Korros, 33

(1982) 203.12 Homer L & Pliefke E, Werkst Korros, 33 (1982) 98.13 Aromaki K, Boshoku Gijutsu, 33 (1983) 144, 33 (1984)

431,35 (1986) 574, 36 (1987) 445.14 Aramaki K & Nishihara H, J Electrochem Soc, 134 (1987)

1059.15 Aramaki K, Corros Eng, 36 (1987) 409.16 Aramaki K, Hagiwara M & Nishihara H, J Electrochem

Soc, 103 (1987) 1896.17 Aramaki K, Mochizuki T & Nishihara H, J Electrochem

Soc, 135 (1988) 2427; 138 (1991) 394.18 Aramaki K, Node Y & Nishihara H, J Electrochem Soc,

137 (1990) 1354.19 Walters F H, J Chem Educ, 68 (1991) 29.20 Pearson R G, J Chem Educ 45 (1968) 581, 643.21 Pearson R G, JAm Chem Soc, 85 (1963) 3533.22 Homer L, WerkstKorros, 24 (1973) 860.23 Homer L & Schodel D, WerkstKorros, 25 (1974) 711.24 Spinelli P, Fra tesi R & Roventi G, Werks Korros, 34 .

(1983) 161.25 Yamaguchi M, Nishikara H & Aramaki K, Corros Sci, 36

(1983) 161.26 Aramaki K, Tomihari M, Furuya S, Yamaguchi M &

Nishikara H,Corros Sci, 36 (1994) 1133 .

27 Szklarska -Smia lowska Z, Proc 7th Eur Symo oncorrosion inhibitors, Universita degli Studi di Ferrara VolI, (1991) 976.

28 Stair PC,J Am Chem Soc, 104 (1982) 4044..29 Hine J, Physical Organic Chemistry (Mc Graw HIli, New

York),1956,69.30 VostaJ, Eliasek J & Knizek P, Corrosion, 32 (1976) 183.31 Reynolds W F et aI.:, JAm Chem Soc 105 (1983) 378.

28 INDIAN 1. CHEM. TECHNOL., JANUARY 1998

32 Singh I, Corrosion, 49 ( 1993) 473.

33 Hansch C & FUJita,.!Am Chern Soc, 86 (1964) 1616.

34 Hansch C, Ann Rep Med Chern ( USA), 33 (1968) 348.35 Growcock F B, Corrosion, 45 ( 1989) 1003.36 Growcock FB, Frenier WW & Andereozzi, Corrosion, 45

(1989) 1007.37 Flory P J, J Chern Phy, 10 (1942) 5 I.

38 Higgins M L. Ann N Y Acad Sci, 4 (1942) 31.39 Dhar H P, Conway B E & Joshi, Electrochem Acta, 18

(1973) 789.40 Bockris J OM & Swinkle D A J, J E/ectrochem Soc, III

(1964) 736.41 Macdonald D D,J Electro Chern Soc, 125 ( 1978) 1443.

42 Blomgren E & Bockris J 0 M, J Phy Chern, 63 (1959)475.

43 Tedeschi R D & More G L, I & EC Product Res Dev, 9

(1970)83.44 Monroe R F, Acetylinic derivatives as hydrochloric acid

inhibitors, AAAS Meeting Dec 28,1957.45 Frenier WW, Lopp VR & Grancock, Proc European

Symposium on corrosion inhibitors, Ann University,Ferrara, Italy, 1985.

46 Kurbanov F K & allaborgenov N 0, Zasch Met, 15 (1979)472"

47 Bengali A & KenNobe, J Electrochem Soc. 126 ( 1979)1118.

48 Mirgaeva Z F, Kurbanov F K, Ikramov & KhamrabaevaKh A, J Appl Chern (USSR), 57 (1985) 2429.

49 Katej P, Vosta J, Pancir J & Hackerman N, J E/ectrochem

Soc, IO ( 1965) 679 .

II!I 'l "!'" " '~I!,,''1'1 '" ,."'I" I'"'' '1'11""' 'II" ,1'1' I" 'I "11'1 'I

.,