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AL-AZHAR UNIVERSITY OF GAZA

Corrosion and corrosion inhibition of

Aluminum and Aluminum alloys in acid

medium

:

: 20130288

:2014-2015

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Contents 1-Introduction about Aluminum: ............................................................................................................................. - 4 -

1.1-Key Characteristics of Aluminum: .................................................................................................................... - 5 -

Low Density . ....................................................................................................................................................... - 6 -

Strength. ............................................................................................................................................................... - 7 -

High Strength-to-Weight Ratio. ........................................................................................................................... - 7 -

Corrosion Resistance............................................................................................................................................ - 7 -

The high thermal conductivity of aluminum ........................................................................................................ - 8 -

High Electrical Conductivity ............................................................................................................................... - 8 -

Reflectivity. .......................................................................................................................................................... - 8 -

Non-toxic Characteristic. ..................................................................................................................................... - 8 -

Finishability. ........................................................................................................................................................ - 9 -

1.2-Aluminum Alloys: ............................................................................................................................................. - 9 -

Classifications and Designations: ............................................................................................................................ - 9 -

Wrought Alloy Families: ....................................................................................................................................... - 10 -

1.3-Effects of Alloying Additions: ......................................................................................................................... - 11 -

2-Corrosion in Acid Solutions : ............................................................................................................................. - 13 -

2.1-Corrosion of Aluminium and Aluminium Alloys: ........................................................................................... - 13 -

2.1-Localized Corrosion: ....................................................................................................................................... - 14 -

2.1.1-Environmentally Influenced Corrosion: ................................................................................................... - 15 -

2.1.1.1 Pitting Corrosion ................................................................................................................................ - 15 -

2.1.1.2 Crevice Corrosion : ............................................................................................................................ - 18 -

2.1.1.3 Filiform Corrosion : ........................................................................................................................... - 20 -

2.1.1.4 Biological Corrosion: ......................................................................................................................... - 21 -

2.1.2 Metallurgically Influenced Corrosion: ...................................................................................................... - 22 -

2.1.2.1Galvanic Corrosion : ........................................................................................................................... - 22 -

2.1.2.2 Intergranular Corrosion : .................................................................................................................... - 24 -

2.1.3 Mechanically Assisted Degradation: ......................................................................................................... - 24 -

2.1.3.1 Erosion ............................................................................................................................................... - 24 -

2.1.3.2 Fretting Corrosion .............................................................................................................................. - 25 -

2.1.3.3 Corrosion Fatigue ............................................................................................................................... - 25 -

2.1.4 Environmentally Induced Cracking: ......................................................................................................... - 26 -

2.1.4.2 Hydrogen Embrittlement : .................................................................................................................. - 27 -

3- Corrosion Prevention: ........................................................................................................................................ - 27 -

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3.1 Alloy and Temper Selection : ...................................................................................................................... - 28 -

3.2 Design of Equipment ................................................................................................................................... - 28 -

3.3- Organic Coating[][]: ................................................................................................................................... - 29 -

3.3.1 Surface Preparation : ................................................................................................................................. - 30 -

3.4- Inhibitors : ................................................................................................................................................... - 31 -

3.4.1 Inhibitors in acid medium: .................................................................................................................... - 33 -

corrosion inhibition of aluminium by different type of inhibitors ................................................................. - 33 -

3.4.1.1- 3-alkyloxy aniline sodium sulfonate monomeric surfactants. ......................................................... - 37 -

3.4.1.2 Different Extracts of Ocimum gratissimum. ................................................................................... - 38 -

3.4.1.3- Euphorbia hirta extract..................................................................................................................... - 39 -

3.4.1.4- black mulberry. ............................................................................................................................... - 39 -

3.4.1.5- Chrysophyllum albidum fruit extract . ............................................................................................. - 40 -

3.4.1.5- extract of Garlic. ............................................................................................................................... - 40 -

3.4.1.6- anionic polyeletrolyte pectates (PEC). ............................................................................................. - 41 -

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1-Introduction about Aluminum1:

Aluminum became an economic competitor in engineering applications toward the end

of the 19th century. The reason aluminum was not used earlier was the difficulty of

extracting it from its ore. When the electrolytic reduction of aluminum oxide (Al2O3)

dissolved in molten cryolite was independently developed by Charles Martin Hall in the

United States and Paul T.Heroult in France, the aluminum industry was born.

The emergence of three important industrial developments in the late1800s and early

1900s would, by demanding material characteristics consistent with the unique qualities of

aluminum and its alloys, greatly benefit growth in the production and use of the new

metal. The first of these was the introduction of the first internal-combustion-engine-

powered vehicles. Aluminum would play a role as an automotive material of increasing

engineering value. Secondly, electrification would require immense quantities of

lightweight conductive metal for long-distance transmission and for construction of the

towers needed to support the overhead network of cables that deliver electrical energy

from sites of power generation.

Within a few decades ,a third important application area was made possible by the

invention of the airplane by the Wright brothers. This gave birth to an entirely new

industry which grew in partnership with the aluminum industry development of

structurally reliable, strong, and fracture-resistant parts for airframes, engines, and

ultimately, for missile bodies, fuel cells, and satellite components. However, .the

aluminum industry growth was not limited to these developments. The first commercial

applications of aluminum were novelty items such as mirror frames ,house(address)

numbers, and serving trays. Cooking utensils were also a major early market.

In time, aluminum applications grew in diversity to the extent that virtually every aspect

of modern life would be directly or indirectly affected by use. Today, aluminum is

surpassed only by steel in its use as a structural material.

1 J. R. Davis ;Corrosion of Aluminum and Aluminum Alloys;ASM international 1999 pp1-13

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1.1-Key Characteristics of Aluminum:

Aluminum offers a wide range of properties that can Be engineered precisely to the

demands of specific applications through the choice of alloy, temper, and fabrication

process. The properties of aluminum and its alloys which give rise to their wide spread

usage include the following:

Aluminum is light; its density is only one-third that Of steel.

Aluminum and aluminum alloys are available in a wide range of strength values-from

highly ductile low-strength commercially pure aluminum to very tough high-strength

alloys with ultimate tensile Strengths approaching690 MPa (100 ksi).

Aluminum alloys have a high strength-to-weight ratio.

Aluminum retains its strength at low temperatures and is often used for cryogenic

applications.

Aluminum has high resistance to corrosion under the majority of service conditions ,and

no colored salts are formed to stain adjacent surfaces or discolor products with which it

comes into contact.

Aluminum is an excellent conductor of heat and electricity .

Aluminum is highly reflective.

Aluminum is non-ferromagnetic, a property of importance in the electrical and

electronics industries.

Aluminum is non -pyrophoric, which is important in applications involving inflammable

or explosive materials handling or exposure.

Aluminum is nontoxic and is routinely used in containers for food and beverages.

Aluminum has an attractive appearance in its natural finish, which can be soft and lustrous

or bright and shiny .It can be virtually any color or texture.

Aluminum is recyclable. Aluminum has substantial scrap value and a well established

market for recycling, providing both economic and environmental benefits.

Aluminum is easily fabricated. Aluminum can be formed and fabricated by all common

metalworking and joining methods.

Table1 lists the important physical properties of pure aluminum.

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Table2 shows the characteristics of aluminum and their importance for different end uses.

Low Density .

Aluminum has a density of only 2.7g/cm3 , approximately35% that of steel (7.83g/cm3)

and 30% of copper (8.93g/cm3) or brass (8.53g/cm3).

One cubic foot of steel weighs about 222kg ;a cubic foot of aluminum weighs only

about77kg.

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Strength.

Commercially pure aluminum has a tensile strength of about90 MPa .

Thus its usefulness as a structural material in this form is somewhat limited by working

the metal, as by cold rolling, its strength can be approximately doubled. Much larger

increases in strength can be obtained by alloying aluminum with small percentages of one

or more other elements such as manganese ,silicon, copper, magnesium, or zinc. Like pure

aluminum ,the alloys are also made stronger by cold working. Some of the alloys are

further strengthened and hardened by heat treatments.

High Strength-to-Weight Ratio.

The strength to-weight ratio of aluminum is much higher than that of many common

grades of constructional steel soften double or more.

This property permits design and construction of strong, lightweight structures that are

particularly advantageous for anything that moves-space vehicles and aircraft as well as all

types of land- and water-borne vehicles.

Corrosion Resistance.

When aluminum surfaces are exposed to the atmosphere ,a thin invisible oxide skin

forms immediately, which protects the metal from further oxidation. This self-protecting

characteristic gives aluminum its high resistance to corrosion .Unless exposed to some

substance or condition that destroys this protective oxide coating, the metal remains fully

protected against corrosion. Aluminum is highly resistant to weathering, even in industrial

atmospheres that often corrode other metals .It is also corrosion resistant to many acids.

Alkalis are among the few substances that attack the oxide skin and therefore are

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Corrosive to aluminum Although the metal can safely be used in the presence of certain

mild alkalis with the aid of inhibitors, in general, direct contact with alkaline substances

should be avoided.

The high thermal conductivity of aluminum

(about 50 to 60% that of copper) came prominently into play in the very first large-scale

commercial application of the metal in cooking utensils .This characteristic is important

whenever the transfer of thermal energy from one medium to another is involved, either

Heating or cooling. Thus aluminum heat exchangers are commonly used in the food,

chemical, petroleum, aircraft, and other industries.

High Electrical Conductivity

Aluminum is one of the two common metals having an electrical conductivity high

enough for use as an electric conductor.

The conductivity of electric conductor grade (1350) is about 62%that of the International

Annealed Copper Standard (IACS). Because aluminum has less than one-third the specific

gravity of copper, however, a pound of aluminum will go about twice as far as a pound of

copper when used for this purpose.

Reflectivity.

Smooth aluminum is highly reflective of the electromagnetic spectrum, from radio waves

through visible light and on into the infrared and thermal range. It bounces away about

80%ofthe visible light and 90% of the radiant heat striking its surface.

The high reflectivity gives aluminum a decorative appearance; it also makes aluminum a

very effective barrier against thermal radiation, suitable for such applications as

automotive heat shields.

Non-toxic Characteristic.

The fact that aluminum is nontoxic was discovered in the early days of the industry. It is

this characteristic that permits the metal to be used in cooking utensils without any

harmful effect on the body. Today a great deal of aluminum equipment is used in the food

processing industry.

Non toxicity permits aluminum foil wrapping to be used safely in direct contact with food

products.

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Finishability.

For the majority of applications , aluminum needs no protective coating. Mechanical

finishes such as polishing, sandblasting, or wire brushing meet the majority of needs. In

many instances, the surface finish supplied is entirely adequate without further finishing.

Where the plain aluminum surface does not suffice or where additional protection is

required, any of a wide variety of surface finishes may be applied.

Chemical, electrochemical, and paint finishes are all used. Many colors are available in

both chemical and electrochemical finishes. If paint, lacquer, or enamel is used, any color

possible with these finishes can be applied. Vitreous enamels have been developed for

aluminum, and the metal can also be electroplated.

1.2-Aluminum Alloys2: The mechanical, physical, and chemical properties of aluminum alloys depend on

composition and microstructure. The addition of selected elements to pure aluminum

greatly enhances its properties and usefulness. Because of this, most applications for

aluminum utilize alloys having one or more elemental additions. The major alloying

additions used with aluminum are copper, manganese, silicon, magnesium,

Classifications and Designations: It is convenient to divide aluminum alloys into two major categories: wrought composition and

cast compositions. A further differentiation for each category is based on the primary mechanism

of property development. Many alloys respond to thermal treatment based on phase solubilities.

These treatments include solution heat treatment, quenching, and precipitation hardening. For

either casting or wrought alloys, such alloys are described as heat treatable. A large number of

other wrought compositions rely instead on work hardening through mechanical reduction,

usually in combination with various annealing procedures for property development. These alloys

are referred to as work hardening or non-heat-treatable. Some casting alloys are essentially not

heat treatable and are used only in as-east or in thermally modified conditions unrelated to

solutions or precipitation effects.

Cast and wrought alloy nomenclatures have been developed. The Aluminum Association system

is most widely recognized in the United States. Their alloy identification system employs

different nomenclatures for wrought and cast alloys but divides alloys into families for

simplification.

2 Aluminum andAluminumAlloys,Metals Handbook Desk Edition,2nd ed.,J.R. Davis,Ed,ASM International, 1998,p 417-505

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Wrought Alloy Families:

For wrought alloys, a four-digit system is used to produce a list of wrought

Composition families as follows:

lxxx: Controlled unalloyed (pure) composition,

Used primarily in the electrical and chemical industries

2xxx: Alloys in which copper is the principal alloying element, although other elements,

notably magnesium, can be specified. 2xxxseries alloys are widely used in aircraft where

their high strengths (yield strengths as high as 455 MPa, or 66 ksi) are valued.

3xxx: Alloys in which manganese is the principal alloying element, used as general-

purpose alloys for architectural application sand various products

4xxx: Alloys in which silicon is the principal alloying element, used in welding rod sand

brazing sheet

5xxx: Alloys in which magnesium is the principal alloying element, used in boat hulls,

gangplanks, and other products exposed to marine environments

6xxx: Alloys in which magnesium and silicon are the principal alloying elements,

commonly used for architectural extrusions.

7xxx: Alloys in which zinc is the principal alloying element (although other elements,

such as copper, magnesium, chromium, and zirconium, can be specified), used in aircraft

structural components and other high-strength applications. The7xxxseries are the

strongest aluminum alloys, with yield strengths 500MPa (73ksi) possible.

8xxx: Alloys characterizing miscellaneous compositions. The 8xxxseries alloy scan

contain appreciable amounts of tin, lithium, and/or iron.

The 5xxx and 6xxx series aluminum alloys are commonly used in marine applications

where low density materials, good mechanical properties and better resistance tocorrosion

are desired3.

3 H. Ezuber et al. / Materials and Design 29 (2008) p801

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1.3-Effects of Alloying Additions4: A brief summary of the effects of the principal alloying additions on aluminum is given

here. Emphasis is placed on their influence on strength and response to heat treatment.

Copper is one of the most important additions to aluminum. It has appreciable solubility

and a substantial strengthening effect through the age-hardening characteristics it imparts

to aluminum. Many alloys contain copper either as the major addition(2xxx or

2xx.xseries) or as an additional alloying element, in concentrations of 1 to 10%.

Manganese has limited solid solubility in aluminum but in concentrations of about 1%

forms an important series of non-heat-treatable wrought aluminum alloys(3.xxxseries).It is

employed widely as a supplementary addition in both heat treatable and non-heat treatable

alloys and provides substantial strengthening.

Silicon lowers the melting point and increases the fluidity (improves casting

characteristics) of aluminum A moderate increase in strength is also provided by silicon

additions.

Magnesium provides substantial strengthening and improvement of the work-hardening

characteristics of aluminum It has a relatively high solubility in solid aluminum, but AI-

Mg alloys containing less than 7% Mg (5xxxseries) do not show appreciable heat

treatment characteristics. Magnesium is also added in combination with other elements,

notably

copper and zinc, for even greater improvements in strength.

Zinc is employed in casting alloys and in conjunction with magnesium in wrought alloys

to produce heat treatable alloys (7:xxx series) having the highest strength among

aluminum alloys.

Copper and silicon are used together in the commonly used3xx.xseries casting alloys.

Desirable ranges of characteristic sand properties are obtained in both heat treatable and

non-heat-treatable alloys.

Magnesium and silicon are added in appropriate proportions to form Mg2Si, which is a

basis for age hardening in both wrought and(6xxxseries) and casting(3xx.xseries) alloys.

Tin improves the antifriction characteristic of aluminum, and cast Al-Sn

alloys(8xx.xseries) are used for bearings.

4 F.King, Aluminum and its Alloys, Ellis Horwood Limited,1987

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Lithium is added to some alloys in concentrations approaching 3 wt % to decrease density

and increase the elastic modulus. Examples include Al-Cu-Li alloys (e.g., 2091)

containing1.7 to 2.3% Li and Al-Li-Cu-Mg alloys (e.g., 8090) containing 2.2 to 2.7% Li.

Table 3: Strength ranges of various wrought aluminum alloys:-

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2-Corrosion in Acid Solutions 5:

Corrosion of a metal (such as M) occurs in acid solutions due to the simultaneous metal

oxidation and proton reduction :

M M+2 + 2e- (1)

2H++ 2e- H2 (2)

The overall reaction is a spontaneous reaction :

M + 2H+ M+2 + H2

because protons are an oxidizing reagent.

Because electrons are set free by the anodic reaction and absorbed by the cathodic reaction

the current flowing into the cathodic reaction must be equal (and opposite in sign) to the

current flowing out of the anodic reaction.

In the case of general corrosion the area of the cathodic and anodic sites is the same and

thus the current densities are equal.

2.1-Corrosion of Aluminium and Aluminium Alloys:

Aluminium is a thermodynamically reactive metal but it owes its excellent corrosion

resistance to the natural formation of a thin but very stable oxide film.

In neutral aqueous solutions (4 < pH < 9) a 50 thick oxide film protects the metal

(passivation). Only in a very acid solution aluminium is homogeneously corroded by

forming Al+3and in alkaline solution with formation of aluminates (AlO2-). The resistance

and stability of the oxide layer is a function of the environment and alloy composition and

of the microstructure of the metal (influenced by heat treatments).

5 J. Vereecken, Vrije ;Corrosion Control of Aluminium -Forms of Corrosion and Prevention; Universiteit Brussels;p3

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In order to maintain the excellent corrosion resistance of aluminium alloys it is necessary

to take into account a certain number of precautions. Some kinds of localized corrosion

can occur if the metal is used in un-favorable conditions.

2.1-Localized Corrosion:

Environmentally influenced corrosion :

Pitting corrosion

Crevice corrosion

Filiform corrosion

Biological corrosion

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Metallurgically influenced corrosion :

Galvanic corrosion

Intergranular corrosion

Mechanically assisted degradation:

Erosion

Fretting corrosion

Corrosion fatigue

Environmentally induced cracking:

Stress corrosion cracking

Hydrogen embrittlement.

2.1.1-Environmentally Influenced Corrosion:

2.1.1.1 Pitting Corrosion

In aerated aqueous solutions and in the presence of Cl- ions random formation of pits can

be observed at defects in the protected oxide film. Pitting develops only at potentials more

cathodic than the pitting potential Ep(Figure 2). The intersection of the anodic curve for

aluminium (solid line) with a curve for the applicable cathodic reaction (one of the

representative dashed lines) determines the potential to which the aluminium is polarized,

either by cathodic reaction on the aluminium itself or on another metal electrically

connected to it. The potential to which the aluminium is polarized by a specific cathode

reaction determines corrosion current density and corrosion rate.

Figure 2:Typical anodic polarization curve for aluminium (solid line)

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The potential above which pits will initiate (Ep) decreases with an increase of the Cl-

concentration. However, only when cathodic reactions can occur (high enough

concentration of O2, low overpotential) pitting corrosion of aluminium starts.

the pit growth on the aluminium surface can be stimulated

by different reactions :

within the corrosion pit and preventing re-passivation :

enrichment of Cl- ions;

generation of an acid solution;

limited O2supply;

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in the pit mouth :

formation of crust;

around the pit :

passivation;

deposition of more noble metals.

The first step in the pitting process is the adsorption of chloride on the oxidecovered

surface. When an ion, such as chloride, interacts with an ionic surface, such as an oxide,

the attractive forces consist of (i) coulombic forces, (ii) induction of the adsorbent by the

approaching ion, (iii) electrostatic polarization of the ion, and (iv) non-polar van der Waals

forces.

Table 3: Forces involved in the interaction of an anion with an Al-oxide surface.[6]

Of these attractive forces, the largest are the first two interactions, which are ionic, as

shown in Table 3. As stated above, in neutral solutions the oxide film on aluminum will

have a positive surface charge. Thus, adsorption of chloride ions is most favored on a

positively charged oxide surface for which the ionion forces are attractive in nature.

When the oxide surface is negatively charged, ionion forces are not attractive in nature so

that adsorption

of Cl onto the negatively charged oxide surface is much less favored but can proceed

through the operation of van der Waals (dispersion) forces.There is considerable

experimental evidence which shows that chloride ions are adsorbed on the oxide-covered

aluminum surface. Studies involving radiotracer techniques7.

6 J.H. de Boer, in: H. Mark, E.J.W. Verwey (Eds.), Advances in Colloid Science, vol. III, Interscience

Publishers, New York, 1950, p. 1 7 J.O.M. Bockris, Y. Kang, J. Solid State Electrochem. 1 (1997) 17.

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adsorption from solution onto powders8, X-ray photoelectron spectroscopy (XPS)9, X-ray

absorption spectroscopy (XAS)10, and Auger spectroscopy11 , all show that Cl is adsorbed

onto the oxide film on aluminum prior to the onset of pitting.

There is a growing body of evidence that chloride ions penetrate the passive film on

aluminum. In an XPS study on the uptake of chloride by oxide films on aluminum,

Natishan et al.12

Very high purity aluminium (1099) has excellent resistance to pitting. Among commercial

alloys, the aluminium-magnesium alloys (5xxx) have the lowest pitting probability and

penetration rates. With low (< 0.04 %) copper content aluminium-manganese (3xxx)

alloys show comparable pitting behaviour. In aluminium-magnesium-silicon (6xxx) alloys

pitting is combined with intergranular corrosion. Aluminium-copper (2xxx) and

aluminium-zinc-magnesium-copper (7xxx) alloys are normally clad to protect against

pitting13.

2.1.1.2 Crevice Corrosion14 :

A very general method to improve the corrosion resistance of metals is to avoid the

presence of narrow openings or spaces between metal to metal or non-metal to metal

components. Localized corrosion at these sites will start due to the formation of an oxygen

differential cell. The corrosion in the crevice will be accelerated by an acidification due to

hydrolysis.

The factors affecting crevice corrosion are :

Geometrical factors:

type of crevice (metal-metal, metal-non metal)

crevice tightness

crevice depth

8 T.H. Nguyen, R.T. Foley, J. Electrochem. Soc. 127 (1980) 2563.

9 J. Augustynski, J. Painot, J. Electrochem. Soc. 123 (1976) 841

10 S. Yu, W.E. OGrady, D.E. Ramaker, P.M. Natishan, J. Electrochem. Soc. 147 (2000) 2952

11 L.D. Atanasoska, D.M. Drazic, A.R. Despic, A. Zalar, J. Electroanal. Chem. 182 (1985) 179.

12 P.M. Natishan, W.E. OGrady, E. McCafferty, D.E. Ramaker, K. Pandya, A. Russell, J. Electrochem.

Soc. 146 (1999) 1737. 13

J. Vereecken, Vrije ;Corrosion Control of Aluminium -Forms of Corrosion and Prevention; Universiteit Brussels;p8 14

J. Vereecken, Vrije ;Corrosion Control of Aluminium -Forms of Corrosion and Prevention; Universiteit Brussels;p9

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exterior-interior surface area ratio

Environmental factors:

bulk solution (O2, pH, Cl-)

mass transport

crevice solution (hydrolysis equilibria)

biological factors

Electrochemical reactions :

metal dissolution

O2 reduction

H2 evolution

Metallurgical factors:

alloys composition

passive film characteristics.

To prevent crevice corrosion some precautions must be taken15 :

When crevice corrosion is considered possible, the crevice should be sealed with a non

hardening elastomer to prevent the entry of moisture. Some sealants become hard and

crack on aging, allowing moisture to enter. The elastomeric requirement is essential for

joints in equipment that work in service, such as in all types of vehicles - road transport,

ships, and airplanes. There are two general types of sealant:

(1) one-component systems such as butyls or silicones, and

(2) two-component systems such as polysulfides and epoxies.

To prevent poultice corrosion which is a special case of crevice corrosion, the contact of

the bare aluminium surface with moisture-absorbing materials such as paper, cloth, wood, asbestos, and non-cellular foams should be avoided.

15

ASM(Ed.): "Metals Handbook", Vol. 13 "Corrosion", p.583

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2.1.1.3 Filiform Corrosion 16:

Filiform corrosion is another case of localized corrosion that may occur on an aluminium

surface under an organic coating. It takes the form of randomly distributed thread-like

filaments, and is sometimes called vermiform or warm track corrosion.

Aluminium is susceptible to filiform corrosion in a relative humidity range of 75 to 90%

with temperatures between 20 to 40C. Typical filament growth rates average about 0.1

mm/d. Filament width varies with increasing humidity from 0.3 to 3 mm. The depth of

penetration in aluminium can be as deep as 15. Numerous coating systems used on

aluminium are susceptible to filiform corrosion, including nitrocellulose epoxy,

polyurethane, alkyd, phenoxy and vinyls. Condensates containing chloride, bromide,

sulphate, carbonate and nitrate ions stimulate filiform corrosion.

Filiform corrosion is an oxygen concentration cell in which the anodic active area is the

head of the filament and the cathode is the area surrounding it, including the tail

(Figure 4). At the head pH values as low as 1.5 to 2.5 have been reported.

Fig 4 process of filiform corrosion in aluminum :

16

Joseph R. Davis; Corrosion of Aluminum and Aluminum Alloys;ASM;p54

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Anodic reaction produces Al+3 which react to form insoluble precipitates with the

hydroxyl (OH-) ions produced in the oxygen reduction reaction occurring in the tail.

Phosphate coatings or chromium containing conversion coatings applied to the metal

surface prior to organic coating are widely used to protect against filiform corrosion but

they are not always completely successful.

2.1.1.4 Biological Corrosion17:

Biological organisms are present in virtually all natural aqueous environments and can

attack and grow on the surface of structural materials, resulting in the formation of a

biological film or biofilm.

The presence of a biological film does not introduce some new type of corrosion, but it

influences the occurrence and/or the rate of known types of corrosion, e.g.:

-increase or decrease of the corrosion rate due to oxygen reduction;

-production of different aeration or chemical concentration cells;

-production of organic and inorganic acids as metabolic by-products;

17

Yang SS, Chen CY, Wei CB, Lin YT. Microbial corrosion of aluminum alloy. Zhonghua Min Guo Wei Sheng Wu Ji Mian Yi Xue Za Zhi. 1996 Nov;29(4):185-96. PubMed PMID: 10592801.

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-production of sulfides under anaerobic conditions.

For example, pitting corrosion of integral wing aluminum fuel tanks in aircraft that use

kerosene-base fuels has been known to occur. The attack proceeds under microbial

deposits in the water phase and at the fuel/ water interface. The organisms grow either in

continuous mats or sludges, or in volcanolike tubercules with gas bubbling from the center

The organisms commonly held responsible are Pseudomonas, Cladosporium and

Desulfovibrio. Cladosporium resinae produces a variety of organic acids (pH 3-4) and

metabolizes certain fuel constituents. These organisms may also act with the slime

forming Pseudomonads to produce oxygen concentration cells under the deposit.

Figure 5 :biological corrosion microbial deposit in the form of volcanolike tubercles18:

Control of this type of attack has usually focused on a combination of reducing the water content of fuel tanks, coating and using biocides and fuel additives.

2.1.2 Metallurgically Influenced Corrosion19:

2.1.2.1Galvanic Corrosion :

Due to the fact that different metals must be used very often electrically coupled in an

integrated structure a corrosion cell can occur resulting in acceleration of the corrosion

process in less resistant metals.

18

J. Vereecken, Vrije ;Corrosion Control of Aluminium -Forms of Corrosion and Prevention; Universiteit Brussels;p11 19

Edward Ghali; Corrosion Resistance of Aluminum and Magnesium Alloys; John Wiley & Sons;2010;pp215-241

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the galvanic series of aluminium alloys and other metals in a NaCl solution. This galvanic

series, however, is not necessarily valid in non-saline solutions. For example aluminium

is anodic to zinc in an aqueous 1 M sodium chromate (Na2CrO4) solution and cathodic to

iron in an aqueous 1 M sodium sulfate solution (Na2SO4). Under most environmental

conditions, aluminium and its alloys are the anodes in galvanic cells with most other

metals, protecting them by corroding sacrificially.

Contact of aluminium with more cathodic metals results in an increase of the potential of

aluminium; this must be avoided in any environment in which aluminium itself is subject

to pitting corrosion.

To prevent the galvanic corrosion some precautions must be taken:

Low potential difference (< 50 mV)

High ratio area anode-cathode; e.g. stainless steel bolts in bare aluminium structures

Low conductivity of the corrosion medium

Slow kinetic of cathodic reduction, e.g. by inhibitors in closed loop circuits Of course,

without aqueous environment and without oxydants (O2) no galvanic corrosion can start.

The galvanic corrosion can occur after deposition of heavy metals on aluminium.

Reduction of only a small amount of these ions can lead to severe localized corrosion. The

influence of Cu, Pb, Hg, Ni and Sn is very important in acidic solutions.

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A Cu2+concentration of 0.02-0.05 ppm in neutral or acidic solutions is generally

considered to be the threshold value for initiation of Al pitting. This value is a function of

the pH value and

the concentration of Cl-, HCO3 -and Ca2+.

2.1.2.2 Intergranular Corrosion :

Galvanic corrosion can occur on the macroscopic but also on microscopic level.

Intergranular corrosion is a form of localized surface attack in which a narrow path is

corroded preferentially along the grain boundaries of a metal. The driving force is a

difference in corrosion potential that develops between a thin grain boundary zone and the

bulk of the immediately adjacent grains.

In the 2 xxx series alloys the anodic path is a narrow band on either side of the boundary

that is depleted in copper; in the 5xxx series alloys MgAl3is anodic to aluminium and is

preferentially dissolved when the constituent forms a continuous path along grain

boundaries; copper free 7xxx series alloys are Zn and Mg - bearing constituents on the

grain boundary and generally considered to be the anodic. In the copper bearing 7xxx

series alloys, it appears to be the copper depleted bands along the grain boundaries, which

cause intergranular corrosion. The 6xxx series alloys generally resist this type of

corrosion, if the Si-content is kept at values near the stoichiometric Mg2Si composition or

below.

2.1.3 Mechanically Assisted Degradation20:

2.1.3.1 Erosion

In noncorrosive environments, such as high purity water, the stronger aluminum alloys

have the greatest resistance to erosion-corrosion because resistance is controlled almost

entirely by the mechanical components of the system. In a corrosive environment, such as

seawater, the corrosion component becomes the controlling factor; thus, resistance may be

20

J. Vereecken, Vrije ;Corrosion Control of Aluminium -Forms of Corrosion and Prevention; Universiteit Brussels;pp13-14

- 25 -

greater for the more corrosion-resistant alloys even though they are lower in strength.

Corrosion inhibitors and cathodic protection have been used to minimize erosioncorrosion,

impingement, and cavitation on aluminum alloys.

Fig. Turbulent Eddy mechanism for growth of erosion corrosion pit.

2.1.3.2 Fretting Corrosion

Fretting corrosion is a combined wear and corrosion process in which material is removed

from the contacting surface when motion between the surfaces is restricted to very small

amplitude oscillation. Factors affecting fretting are:

-contact load -amplitude -frequency -number of cycles

-relative humidity temperature .

2.1.3.3 Corrosion Fatigue

Fatigue strengths of aluminum alloys are lower in such corrosive environments as

seawater and other salt solutions than in air, especially when evaluated by low-stress

longduration tests.

Such corrosive environments cause smaller reductions in fatigue strength in the more

corrosion-resistant alloys, such as the 5xxx and 6xxx series, than in less resistant alloys,

such as the 2xxx and 7xxx series.

Like SCC of aluminum alloys, corrosion fatigue requires the presence of water. In contrast

- 26 -

to SCC, however, corrosion fatigue is not appreciably affected by test direction with

respect to the rolling, forging or extrusion direction, because the fracture that results from

this type of attack is predominantly transgranular.

2.1.4 Environmentally Induced Cracking21:

2.1.4.1 Stress Corrosion Cracking (SCC)

SCC is a complex mechanism involving metallurgical, mechanical and environmental

parameters. SCC in aluminium alloys is characteristically intergranular.

According to the electrochemical theory, this requires a condition along the grain

boundaries that makes them anodic to the rest of the microstructure so that corrosion

propagates selectively along them. This theory is confirmed by the fact that cathodic

protection retards or eliminates SCC.

Parameters :

magnitude and duration of tensile strength acting at the surface;

residual stresses during quenching;

grain structure and stress direction (resistance in short transverse direction

controls applications of products);

environment : Cl- and a decrease of the pH-value accelerate the attack.

The SCC of high-strength aluminium alloys such as 2024, 7075 and 7079 is often caused

by sustained residual or assembly tension stresses acting in the short transverse direction.

The stresses developed by service loads are usually intermittent and are designed to

operate in a favorable direction (longitudinal or long transverse) relative to the grain

structure.

The following guidelines should be considered by the designer to minimize SCC :

select alloys and tempers that are resistant to SCC;

use stress-relieved parts;

perform forming and straightening on freshly quenched material, W-temper, to ensure

less severe effects;

machine exterior surfaces before heat treating, because quenching causes more

desirable compressive surface stresses;

machine internal surfaces after heat treating to partially remove internal stresses;

21

Hatch, E.:"Aluminium : Properties and Physical Metallurgy", ASM, 1984, p. 242

- 27 -

avoid fitup stresses by careful attention to tolerance. Poorly fitted parts and misaligned

parts should not be forced into place.

Where built-in surface tensile stresses cannot be avoided, techniques such as shot

peening and surface rolling, or thermal stress relief from second stage ageing, can be

utilized to reduce the undesired stresses.

postweld heat treat weldments.

2.1.4.2 Hydrogen Embrittlement :

Hydrogen embrittlement is a form of environmentally assisted failure that results most

often from the combined action of hydrogen and residual or applied tensile stress.

Only recently it has been found that hydrogen embrittles aluminium. For many years, all

environmental cracking of aluminium and its alloys was represented as SCC. Hydrogen

damage in aluminium alloys may take the form of intergranular or transgranular cracking

or blistering. Hydrogen diffuses into the aluminium lattice and collects at internal defects

(e.g. during annealing of solution treating in air furnaces prior to age hardening).

Dry hydrogen gas is not detrimental to aluminium alloys; however, with the addition of

water vapor, subcritical crack growth increases dramatically.

The threshold stress intensity of cracking of aluminium also decreases significantly in the

presence of humid hydrogen gas at ambient temperature.

Hydrogen embrittlement of the 7000 series has been more intensively studied.

3- Corrosion Prevention:

They are a number of corrosion preventives measures, special to specific types of

aluminium

corrosion, the main methods of preventing corrosion of aluminium equipment :

alloy and temper selection

design of equipment

organic coating (and sealants)

inhibitors

cathodic protection

surface treatment

- 28 -

modification of the environment

3.1 Alloy and Temper Selection :

The choice of an aluminium alloy for a given use is often based on strength, formability,

ease of welding, or product availability. However, corrosion resistance must be included

when making the choice.

In general, aluminium-magnesium alloys (5xxx) have the best corrosion resistance,

followed by commercial-purity alloys (1xxx), aluminium-manganese alloys (3xxx), and

aluminium-magnesium-silicon alloys (6xxx) in that order, with only small differences

within families. These alloy families are normally used without protection although they

are sometimes painted (sidings for buildings) or anodized (window frames) for aesthetic

reasons.

The aluminium-copper-magnesium alloys (2xxx) and the medium- and highstrength

aluminium-zinc-magnesium-copper alloys (7xxx) are usually given a protective measure

such as cladding or painting.

The importance of temper was mentioned previously. The H116 temper for 5083, 5086

and 5456 gives better resistance to intergranular and exfoliation corrosion. In 7xxx alloys,

the T7x temper provides improved resistance to SCC, for example, the T73 and T76

tempers of 7075 alloy.

These tempers are a compromise, because strength is somewhat lower than that of the T6

temper. The lower strength of these tempers in 7075 has been offset by the development of

stress-corrosion resistant tempers in 7049, 7050 and 7010 alloys.

3.2 Design of Equipment

The design of equipment can have an important influence on the corrosion behaviour,

even in environments in which aluminium is normally resistant.

Some guidelines can help the designer to minimize corrosion of aluminium in service :

avoid contacts with dissimilar metals, but if they must be used, apply suitable

protection;

avoid crevices, but if they must be present, and if thin sections are involved,

prevent ingress of moisture by application of sealants;

join by continuous welding, rather than by skip welding or riveting;

- 29 -

provide for complete draining and easy cleaning;

avoid contact of bare aluminium surfaces with water-absorptive materials, but

if they must be used together, apply suitable protection;

avoid sharp bends in piping systems;

avoid heat transfer hot spots;

avoid direct impingement by fluid streams;

avoid excessive mechanical stress concentrations;

when locating equipment, choose the least corrosive environment possible;

eliminate sharp edges in equipment that is to be painted.

3.3- Organic Coating[22][23]:

The main components of paint and lacquers are binder, pigment and volatile solvents and

thinners.

The binder is responsible for the coherence within the paint film and for its adherence to

the substrate. Sometimes, if the binder is brittle a plasticizer is added.

The pigment is primarily responsible for colour and hiding power but it plays an important

role for the physical properties of the film. In anti corrosion primers the pigments have

inhibitive or rust preventing properties. In top paints the pigments are flake-like particles

in order to increase the oxygen diffusion path.

A solvent is used if the binder is a solid substance at ambient temperature. The solvent is

of importance not only for the applicability of the paint but affects also adherence and

other properties.

Clearly the organic coating assists in limiting environmental ingress to the aluminium

substrate; further, inhibitive pigments within the primer coat reduce corrosion or loss of

adhesion through interfacial degradation and also provide protection at damaged regions.

The different methods of applying organic coatings are:

1. Brushing

2. Dip coating

3. Roller coating

4. Flow coating

5. Fluidized bed coating

6. Spray coating :

7. Electrophoresis.

22

Munger, C.G.: "Corrosion Prevention by Protective Coatings", NACE, Houston 1984. 23

"Metals Handbook" , ASM, Vol. 13 "Corrosion", p. 399.

- 30 -

The performance of organic coating systems can be maximized by following the specific

recommendations of suppliers regarding surface preparation, pretreatment, selection of

compatible conversion coat, primer and topcoat, application and curing. If continuing

maximum corrosion protection is required, the organic coating systems must be

maintained periodically.

3.3.1 Surface Preparation 24:

The most important factor in obtaining good paint adhesion is the preparation of the

Surface.

the aluminium alloy surface is relatively complex and is difficult to characterize precisely.

Thus, for example, hydrocarbons present on the surface may shield the substrate giving

rise to non-uniform behaviour; aesthetic reasons may also dictate removal of such

contamination. Consequently, an initial reason for surface treatment is to remove such

contamination and detritus. Degreasing in organic solvents or vapours has been an initial

approach, but a subsequent etching treatment in alkali usually follows. Appropriate rinsing

schedules must also be adopted to remove residual solution or solvent and to limit cross

contamination. It should also be appreciated that electrochemical processes, such as

alkaline etching, leave a residual film on the aluminium surface. A so-called desmut in

nitric acid follows, which removes contaminants, but the alumina film developed in the

alkali, perhaps reinforced through immersion in the acid, largely remains. In addition, such

film formation over the macroscopic surface leaves a more uniform surface, with the nitric

acid immersion serving to remove some residual metal impurity segregates. Such

segregates may result from dissolution of second phase material, with appropriate cations

relocated by deposition on the aluminium surface.

Consequently, a surface with reduced local activity ensues, with a reduced driving force

for corrosion in particular environments.

24

S. Wernick, R. Pinner and P. G. Sheasby: The Surface Treatment and Finishing of Aluminium and its Alloys, Fifth Edition, Volume 1, ASM International Finishing Publications Ltd, England (1987)

- 31 -

They are Various pretreatment systems but a chemical conversion coatings are the most

widely used form of pretreatment when painting of aluminium.

Aluminium can be pretreated with

amorphous phosphates

zinc phosphates

chromating

phosphochromating

phosphofluozirconium coating.

3.4- Inhibitors 25:

Inhibitors such as chromates that reduce the anodic corrosion reaction are termed anodic

inhibitors, whereas those (e.g. polyphosphates) reducing cathodic corrosion reaction are

termed cathodic inhibitors.

If anodic inhibitors are used in an insufficient amount, they tend to increase pitting.

25

Hollingsworth, E.H., Hunsicker, H.Y. and Schweitzer, P.A.: Corrosion and Corrosion Protection Handbook, Pt. Aluminium Alloys, pp.153-186, Marcel Dekker Inc., New York and Basel, 1989

- 32 -

Cathodic inhibitors are safer in this respect.

Mixed anodic and cathodic inhibitor systems are also used.

Phosphates, silicates, nitrates, fluorides, benzoates, soluble oils and certain other

chemicals alone or in combination have been recommended for use with aluminium in

some services.

In mildly alkaline solutions the corrosion of aluminium can be inhibited by additions of

sodium silicate. Silicates with a high ratio of silicate to soda are widely used in alkaline

cleaning solutions, carbonates and phosphates. In mixed-metal, water-handling systems,

such as an automobile cooling system, inhibitors mixtures have been developed to prevent

corrosion of all metals in the system, including aluminium.

Inhibitors may affect the anodic corrosion reaction, in which case they are termed anodic

inhibitors, or they may inhibit the cathodic corrosion reaction in which case they are

termed cathodic inhibitors.

Anodic inhibitors can be dangerous if not added in sufficient amount, since while they

may reduce the effective anode area, attack at the remaining sites will be more severe than

in the absence of the inhibitor.

Cathodic inhibitores are safer since a partial reduction of the effective cathode area will

reduce corrosion at the anode. They are, however, usually less effective than anodic

inhibitors.

Chromate (in the form of sodium or potassium chromate or dichromate) is the most

commonly used inhibitor with aluminium and is an anodic type inhibitor. To prevent the

pitting of aluminium in aggressive water, the addition of 500 ppm. of sodium chromate or

dichromate with a pH adjusted to 8.5 is effective.

Phosphate, silicate, nitrate, nitrite, benzoate, soluble oils and other chemicals have also

been recommended alone or in combination to inhibit the corrosion of aluminium by

aggressive waters.

Inhibition of water is usually economic only in a recirculated, closed loop system. In a

mixed metal system including, for example aluminium and copper it is important to design

a good inhibitor system and maintain the pH above 8.0-8.5 to prevent copper dissolution

and their subsequent deposition on the aluminium. surface.

The complexity of the inhibition tends to make it difficult for a general engineer to

- 33 -

develop a satisfactory water treatment without the help of a specialist.

Frequently, laboratory testing on location is necessary to establish the best treatment.

3.4.1 Inhibitors in acid medium:

Deepa Prabhu et-al was investigated the corrosion behaviour of aluminium and 6063

aluminium alloy in phosphoric acid of different concentrations at different temperatures.

The study was done by electrochemical method, using Tafel polarization and

electrochemical impedance spectroscopy (EIS) techniques.

The rate of corrosion of 6063 aluminium alloy was found to be higher than that

of pure aluminium.

The corrosion rate increase with increase in the concentration of phosphoric acid.

The corrosion rate increases with increase in temperature in both metals26.

Most of the effective inhibitor have hetero atom such as O, N, S containing multiple bonds

in their molecules through which they can adsorb on the metal surface. The sites of these

elements have higher electron density, making them the reaction centers. An articles on

the

corrosion inhibition of aluminium by different type of inhibitors are summarized in Table

(4)

corrosion inhibition of aluminium by different type of inhibitors

Table (4) :corrosion inhibition of aluminium by different type of inhibitors are27 :

26

eepa Prabhuetal /Int.J.ChemTech Res.2013,5(6) 27

M. Aliofkhazraei; Developments in Corrosion Protection;CH20; InTech pp471-472

- 34 -

- 35 -

- 36 -

- 37 -

3.4.1.1- 3-alkyloxy aniline sodium sulfonate monomeric surfactants.

S.M. Sayyah et al. studied of 3-alkyloxy aniline sodium sulfonate monomeric surfactants

and their analogues polymers as mixedtype inhibitors for the corrosion of aluminium in

0.5M HCl. and investigated it via weight loss and potentiodynamic polarization

techniques.

They find that Monomeric surfactants 3-(6-sodium sulphonate hexayloxy) aniline

(MC6), 3-(10-sodium sulphonate decyloxy) aniline (MC10) and 3-(12-sodiumsulphonate

- 38 -

dodecyloxy) aniline (MC12) and and their analogues polymeric surfactants poly 3-

(hexayloxy sulphonic acid) aniline (PC6), poly 3-(decyloxy sulphonic acid) aniline (PC10)

and poly 3-(dodecyloxy sulphonic acid) aniline (PC12).

The inhibition efficiency of the polymeric surfactant was higher than that of the

monomeric surfactant, and the inhibition efficiency increased with increasing inhibitor

concentration but decreased with increasing temperature.

The inhibition efficiency of PC11R is higher than that of PC12. This may be assigned to the

elimination of the terminal -SO3H groups in PC12. The presence of terminal CH3 groups

stabilizes the inhibition efficiency. Also, the presence of terminal CH3 in the alkyl chain

in

PC11R molecules may reduce repulsion between anionic head with similar charge as in

PC12.

This allows a closed layer to form more easily and hence higher inhibition efficiency28.

3.4.1.2 Different Extracts of Ocimum gratissimum.

I. J. Alinnor and P. M. Ejikeme studied Corrosion Inhibition of Aluminium in Acidic

Medium by Different Extracts of Ocimum gratissimum .

Ocimum gratissimum leaf contains alkaloid, saponins, flavonoids and tannins.

This study indicates that extract of Ocimum gratissimum inhibits Al surface in presence of

1M HCl. The corrosion process is inhibited by adsorption of extracts on aluminium

surface.

Inhibition efficiency increase with increase in inhibitor concentrations .

The result of the analysis shows that inhibition efficiency and degree of surface

coverage decreases as temperature increases. Activation energies of different extracts

increase as concentration of inhibitor increases. The negative values of Gads

shows that adsorption of inhibitor on surface of aluminium is spontaneous.

The trend of inhibition efficiency of different extracts was in order:

Distilled H2O > C2H5OH>1M HCl 29.

28

S.M. Sayyah, S.S.Abd El-Rehim, M.M. El-Deeb andS.M. Mohamed; The Corrosion Inhibition of Aluminium by Some of 3-alkyloxyaniline Monomeric Surfactants and TheirAnalogues Polymers in 0.5 M HCl Solution;intech;p502 29

I. J. Alinnor, P. M. Ejikeme; Corrosion Inhibition of Aluminium in Acidic Medium by Different Extracts of Ocimum gratissimum;ACS,2012,2(4)pp122-135

- 39 -

3.4.1.3- Euphorbia hirta extract.

L. A. Nnanna et al. studied of corrosion inhibition of aluminium alloy of type AA3003 in

acidic and alkaline media by Euphorbia hirta extract (asthma-plant) the extract, which

include tannins, alkaloids, and essential oil.

The inhibitive properties of tannins have been attributed to the reaction of the

polyphenolic fraction of the tannins moieties,which ensures effective protection of the

metal surfaces.

and they were found the extract from the studied leaf material to be effective green

inhibitor of aluminium alloys corrosion in both acidic and alkaline environments of 0.5 M

HCl and

0.25 M NaOH, respectively. The corrosion process in acidic medium was inhibited by

adsorption of the extract organic matter on the aluminium alloys surface and the

blocking of active sites.

The extract inhibited the corrosion of aluminium alloys in acidic media by means of

hindering both cathodic and anodic electrode processes, because the greater the number of

bonds in the extracts, the higher the inhibition efficiency. It is found that the

inhibitive action was basically controlled by temperature, exposure time and

concentration of the inhibitor 30.

3.4.1.4- black mulberry.

A. I. Ali and N. Foaud studied Inhibition of Aluminum corrosion in 2M hydrochloric

acid solution using black mulberry ,active compound is Gallic acid.

extract has been studied by weight loss, electrochemical polarization technique and

hydrogen evolution measurement. It was found that the berry extract acts as a good

inhibitor for aluminum corrosion in the acid solution. The inhibition action of the extract

in was discussed in view of the adsorption of its components on aluminum surface. It

was found also that the adsorption is a spontaneous process and follows Langmuir

adsorption isotherm. The inhibition efficiency (IE) increases as the extract concentration

is increased. Moreover, the effect of temperature on the IE was studied. The IE decreases

with increased the temperature. It was found that the presence of extract increases the

activation energy of the corrosion reaction. Moreover, the thermodynamic parameters

of the adsorption process were calculated. It was found also that the extract

provides a good protection to aluminum against pitting corrosion in chloride ion

containing solutions31.

30

L. A. Nnanna, I. U. Anozie, A. G. I. Avoaja, C. S. Akoma and E. P. Eti; Comparative study of corrosion inhibition of aluminium alloy of type AA3003 in acidic and alkaline media by Euphorbia hirta extract; African Journal of Pure and Applied Chemistry Vol. 5(8), pp. 265-271 31

A. I. Ali* and N. Foaud;J. Mater. Environ. Sci. 3 (5) (2012) 917-924

- 40 -

3.4.1.5- Chrysophyllum albidum fruit extract .

I. C. Madufor, U. E. Itodoh and M. U. Obidiegwu, M. S. Nwakaudu studied Inhibition of

aluminium corrosion in 1.5M H2SO4 solution in the absence and presence of

Chrysophyllum albidum fruit extract (CAFE) .

Which contain ascrobic acid, tannins, tannins, and phytic acid.

at temperature range of 30 60oC was studied using weight loss and thermometric

techniques. The fruit extract acts as an inhibitor in the acid environment. The

inhibition efficiency increased with increase in inhibitor concentration but decreased with

increase in temperature. The inhibiting effect of the Chrysophyllum albidum fruit

extract could be attributed to the presence of some phytochemical constituents in

the fruit extract which is adsorbed on the surface of the aluminium. The

Chrysophyllum albidum fruit extract was found to obey Temkin adsorption isotherm at all

the concentrations and temperatures studied. Thermodynamic parameters reveal that the

adsorption process is spontaneous32.

3.4.1.5- extract of Garlic.

Saedah R. Al-Mhyawi studied the inhibition efficiency of extract of Garlic which contain

at least 33 sulfur compounds like aliin, allicin, ajoene, allylpropl,diallyl, trisulfide,

sallylcysteine, vinyldithiines, Sallylmercaptocystein, and others.

Besides sulfure compounds garlic contains 17 amino acids and their glycosides, arginine

and others. Minerals such as selenium and enzymes like allinase, peroxidases, myrosinase

Garlic contains a higher concentration of sulfur compounds.

the inhibition efficiency of extract of Garlic on aluminium in hydrochloric acid solutions

evaluated by weight loss techniques.

Values of inhibition efficiency obtained are dependent upon the concentration of inhibitor

and temperature. Generally, inhibition was found to increase with inhibitor concentration,

half-life, activation energy but decrease with temperature and firstorder rate constant at the

temperatures studied. Physical adsorption mechanism has been proposed for the inhibition

and Langmuir , Temkin adsorption isotherm was obeyed. Garlic is an inhibitor of

aluminium corrosion in 0.5 M hydrochloric acid solution.

The values of standard free energy of adsorption suggest that the adsorption of inhibitor on

aluminium surface occurred by physisorption mechanism. the negative sign of the Free

Energy of adsorption indicates that the adsorption of the inhibitors on the aluminum

surface was a spontaneous process.the negative values of enthalpy of

32

I. C. Madufor, U. E. Itodoh, M. U. Obidiegwu, M. S. Nwakaudu; Journal of Engineering (IOSRJEN) Volume 2, Issue 9;pp 16-23

- 41 -

adsorption (H) suggest that the chemical reaction involved in the adsorption of the

inhibitors on the metal surface is an exothermic process, hence increase in the reaction

temperature of the medium will decrease the inhibition efficiency33.

3.4.1.6- anionic polyeletrolyte pectates (PEC).

Refat M. Hassan and Ishaq A. Zaafarany studied Corrosion inhibition of aluminum (Al)

in hydrochloric acid by anionic polyeletrolyte pectates (PEC) as a water-soluble natural

polymer polysaccharide using both gasometric and weight loss techniques. The

results drawn from these two techniques are comparable and exhibit negligible

differences.

The inhibition efficiency was found to increase with increasing inhibitor concentration and

decrease with increasing temperature.

The inhibition action of PEC on Al metal surface was found to obey the

Freundlich isotherm. Factors such as the concentration and geometrical structure of

the inhibitor, concentration of the corrosive medium, and temperature affecting the

corrosion

rates were examined. The kinetic parameters were evaluated and a suitable

corrosion

mechanism consistent with the kinetic results is discussed in the paper.

Anionic polyelectrolyte pectates as a natural polymer may be considered as a safe

and effective inhibitor for decreasing the corrosion of Al in acidic medium. The

geometrical configuration and functional groups within the inhibitor molecule are

the two main important factors to influence the inhibition efficiency34.

33

AL-MHYAWI, Orient. J. Chem., Vol. 30(2), 541-552 (2014) 34

Refat M. Hassan,Ishaq A. Zaafarany; Materials 2013, 6, 2436-2451