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CHAPTER – 7

CORROSION BEHAVIOUR OF FLY ASH PARTICLES

REINFORCED AA2024 COMPOSITES

7.1: INTRODUCTION

The meaning of the word corrosion is "the deterioration of the substance

(usually a metal) or its properties because of a reaction with its environment".

Normally it specifically applies to metals, although plastics and other non-metals such

as concrete, bricks and timber also deteriorate in natural environments. Corrosion

causes enormous losses, which rise yearly with the increased usage of metals in

industrial development.

Corrosion also means the breaking down of essential properties in a material

due to chemical reactions with its surroundings. In the most common use of the word,

this means a loss of electrons of metals reacting with water and oxygen. Weakening

of iron due to oxidation of the iron atoms is a well-known example of electrochemical

corrosion. This is commonly known as rusting. This type of damage typically

produces oxide(s) and/or salt(s) of the original metal. Corrosion can also refer to the

degradation of ceramic materials as well as the discoloration and weakening of

polymers by the sun's ultraviolet light.

7.2: TYPES OF CORROSION ON ALUMINIUM ALLOY AND COMPOSITES

Corrosion is a slow, progressive or rapid deterioration of a metal's properties

such as its appearance, its surface aspect, or its mechanical properties under the

influence of the surrounding environment: atmosphere, water, seawater, various

solutions, organic environments, etc. In the past, the term "oxidation" was frequently

used to designate what is now a day’s commonly called "corrosion". Nevertheless, the

former was the right word because corrosion also is an electrochemical reaction

during which the metal is oxidized, which usually implies its transformation into an

oxide, i.e. into the state in which it existed in the mineral. Many different corrosion

mechanisms exist. Knowledge of the most common types is well understood [1]. For

each, the process is complex, incorporates many factors, and varies according to metal

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and specific operating conditions [2]. This section briefly discusses about some forms

of corrosion with specific interest in pitting.

7.2.1. Uniform corrosion

Depiction of uniform corrosion is by corrosive attack proceeding evenly over

the entire surface area, or a large fraction of the total area. General thinning takes

place until failure. This is the most important form of corrosion in terms of tonnage

wasted. Measurement and prediction of uniform corrosion is relatively easily, making

disastrous failures rare. In many cases, it is objectionable only from an appearance

stand point. As corrosion occurs uniformly over the entire surface of the metal, it can

be controlled by cathodic protection, use of coatings or paints, or simply by

specifying a corrosion allowance. Thus, uniform corrosion is preferred from a

technical standpoint because it is predictable and thus acceptable for design.

7.2.2. Galvanic corrosion

Galvanic corrosion is an electrochemical action of two dissimilar metals in the

presence of an electrolyte and a conductive path. It occurs when dissimilar metals are

in contact. One can depict galvanic corrosion by the presence of a buildup of

corrosion at the joint between the dissimilar metals. When a galvanic couple forms,

one of the metals in the couple acts as anode and corrodes faster than it would all by

itself, while the other acts as cathode and corrodes slower than it would alone. One

can predict the relative nobility of a material by measuring its corrosion potential.

7.2.3. Crevice corrosion

Crevice corrosion is a localized form of corrosion associated with a stagnant

solution in small sheltered volumes. It occurs in localized areas such as crevices,

joints, bolted and threaded parts or under existing corrosion deposits. It is the result of

concentration of salts, acids and moisture, which results in the development of an

occluded corrosion cell in such sheltered areas. This creates a small anode in the

crevice with the remainder of the body acting as a large cathode, so corrosion at the

crevice is highly accelerated as well as concentrated. Deposit corrosion, contact

corrosion, and gasket corrosion are terms sometimes used when a nonmetallic

material forms a crevice on the metal surface.

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7.2.4. Intergranular corrosion

Intergranular corrosion is localized attack along the grain boundaries, or

immediately adjacent to grain boundaries, while the bulk of the grains remain largely

unaffected. This form of corrosion is usually associated with impurity segregation

effects or specific phases precipitated on the grain boundaries. Such precipitation can

produce zones of reduced corrosion resistance in the immediate vicinity.

7.2.5. Pitting corrosion

Pitting corrosion is a localized form of corrosion producing cavities in the

material. Pitting is more dangerous than uniform corrosion damage because it is more

difficult to detect, predict and design against. Corrosion products often cover the pits.

A small, narrow pit with minimal overall metal loss can lead to the failure of an entire

engineering system. Pitting is initiated by localized chemical or mechanical damage

to the protective oxide film. Pitting corrosion is the perforation of a metal at isolated

anodic sites on the metal surface. Water chemistry factors that can cause break down

of a passive film are acidity, low dissolved oxygen concentrations and high

concentrations of chloride. Localized damage or poor application of protective

coatings is favorable site for pit initiation.

7.2.6. Stress Corrosion

This type of corrosion results from the combined action of a mechanical stress

(bending, tension) and a corrosive environment. Each of these parameters alone

would not have such a significant effect on the resistance of the metal or would have

no effect at all. Cold deformation and forming, welding, heat treatment, machining

and grinding can introduce residual stresses.

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7.2: LITERATURE REVIEW

Corrosion is the degradation of a material by electrochemical or chemical

reaction with its environment. Corrosion causes many problems. In the past three

decades, particle-reinforced aluminum matrix composites (MMCs) have received

great interest. Particulate aluminum composites have commonly been reinforced with

ceramic particles such as silicon carbide [3-8], alumina [9-12], garnet [13, 14],

graphite [15, 16], and mica [17]. However, the cost of manufacturing these

composites is high and this limits their utilization in several engineering designs.

Recently, aluminum alloy composites reinforced with fly ash, a waste by-

product of coal combustion, has been engineered [18-29] as potential substitutes to

conventional composites in several applications in order to widen the engineering

application of particulate aluminum composites. Fly ash comprises mainly small

spheres of oxides of silicon, aluminum, iron, magnesium and calcium. Its size ranges

from 1 µm to 150µm while its density varies from 1.3 to 4.8 depending on the

mineralogy [19, 22]. Currently, fly ash is disposed in landfills, ash dams and lagoons

thereby causing environmental pollution and costing thermal power plants a huge

amount of money annually [30]. Therefore, the addition of fly ash into aluminum

alloy matrix to produce composites is a value-added initiative that lowers disposal

costs, increases energy savings by reducing the quantity of aluminum produced, and

creates a healthier environment.

The resistance of particle reinforced MMCs to environmental attack is a

critical design criterion. Several studies have indicated that the corrosion resistance of

particle-reinforced composites depends on the composition of the base alloy,

reinforcing particles, and corrosive environment [3-8, 13-17, 27, 31-39]. Other factors

include the fabrication routes for the composites, volume fraction of the reinforcing

particles, and the temperature of the corrosive medium. Aylor et al. [32] reported that

pitting corrosion attack on SiC/AA2024 alloy composites was observed

predominantly at the SiC/Al interfaces. They noted that the pits on the SiC/Al

composites were greater in number, smaller in size, and shallower in depth than those

on the unreinforced aluminum alloys. Feng et al. [40] investigated the pitting

corrosion behaviour of SiCp/Al 2024 composites and attributed the intense corrosion

of the composites to pit nucleation and propagation at the SiC/Al interface. Interfacial

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reactions between liquid aluminum and SiC generated intermetallic particles which

formed microgalvanic couples with the matrix.

Zuhair et al. [41] was investigated the electrochemical behaviour of

aluminium alloy 6061/Al2O3 metal matrix composite in 3.5% NaCl aqueous solution.

Two composite compositions were examined containing 10% and 30% (by volume)

of sub-micron alumina particulates as the reinforcement phase. The composites were

fabricated via powder metallurgy processing. Cyclic polarization tests were carried

out to determine pitting potentials and repassivation potentials in deaerated 3.5%

NaCl solution. The pitting potential was 50 mV nobler for the higher-content

reinforcement composite, while the repassivation potentials were essentially identical.

Excessive pitting of the matrix alloy was observed in both composites. Pits forming in

the 6061/Al2O3composite were more numerous and more uniformly dispersed

compared to pits on the lower-content reinforcement composite which were deeper

and more localized.

Geetha Mable Pinto et al. [42] has investigated the corrosion behaviour of SiC

particulate reinforced in aluminium metal matrix composites. Silicon Carbide

particulate -reinforced aluminium (SiCp-Al) composites possess a unique

combination of high specific strength, high elastic modulus, good wear resistance and

good thermal stability than the corresponding non-reinforced matrix alloy systems.

These composites are potential structural material for aerospace and automotive

applications. The corrosion characteristics of 6061 Al/SiCp composite and the base

alloy were experimentally assessed.

B. Bobic et al. [43] studied the corrosion behaviour of MMCs with aluminium

alloy matrix. The corrosion characteristics of boron, graphite, silicon carbide, alumina

and mica reinforced aluminium MMCs were reviewed. The reinforcing phase

influence on MMCs corrosion rate as well as on various corrosion forms (galvanic,

pitting, stress corrosion cracking, corrosion fatigue) was discussed. Some corrosion

protection methods of aluminium based MMCs were described.

H.J. Greene and F. Mansfeld [44], Corrosion protection of aluminium metal-

matrix composites (MMC) by anodizing treatments was investigated. Electrochemical

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behaviour of MMC without protection also was investigated. Electrochemical

impedance spectroscopy (EIS) and potentiodynamic polarization measurements were

used to characterize the properties of protective surface layers. Material studied was

Al 6061/SiC. The MMC had similar corrosion (Ecorr) and pitting (Epit) potentials as

the matrix alloy. The cathodic current density for oxygen reduction in 0.5% N sodium

chloride (NaCl) increased for Al 6061/SiC MMC with reinforcement concentration,

which was attributed to electrochemically active interfaces between the matrix and the

reinforcement particles. Anodizing and hot-water sealing were less effective for

MMC than for the matrix aluminium alloys.

J. Bienia et al. [45] studied the microstructural characteristics of aluminium

matrix AK12 composites containing of fly ash particles, obtained by gravity and

squeeze casting techniques. Pitting corrosion behaviour and corrosion kinetics are

presented and discussed. It was found that: (1) in comparison with gravity casting,

squeeze casting technology is advantageous for obtaining higher structural

homogeneity with minimum possible porosity levels, good interfacial bonding and

quite a uniform distribution of reinforcement, (2) fly ash particles lead to an enhanced

pitting corrosion of the AK12/9.0% fly ash (75-100µm fraction) composite in

comparison with unreinforced matrix (AK12 alloy), and (3) the presence of nobler

second phase of fly ash particles, cast defects like pores, and higher silicon content

formed as a result of reaction between aluminium and silica in AK12 alloy and

aluminium fly ash composite determine the pitting corrosion behaviour and the

properties of oxide film forming on the corroding surface.

J. Zhu, L.H. Hihara [46] studied the corrosion performance of a continuous

alumina-fiber reinforced metal–matrix composite (MMC) and its monolithic matrix

alloy (Al–2%Cu–T6) in 3.15 wt. % sodium chloride solution. Corrosion initiation

sites, mapping of corrosion current density and pH at corrosion sites, mass loss

resulting from immersion, and polarization behaviour were studied. Results show that

the MMC exhibited inferior corrosion resistance as compared to its monolithic matrix

alloy. Corrosion of the MMC initiated preferentially along the fiber/matrix interface

or in regions of plastic deformation. The build-up of acidity at localized corrosion

sites on the MMC was enhanced by the formation of micro-crevices caused by fibers

left in relief as a result of corrosion.

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Although many researchers have studied the corrosion behaviour of aluminum

alloys and conventional aluminum alloy-based composites, there is dearth of

information on the corrosion of AA2024 alloy and the effect of fly ash addition on its

corrosion behaviour. The studies by Bienias et al. [27] and Remachandra et al. [28]

reported an increase in pitting corrosion of aluminum-silicon alloys reinforced with

fly ash. Bienias [27] reported that the addition of fly ash to an aluminum-silicon

(4xxx) alloy in 3.5 wt. % NaCl solution increased the susceptibility of the alloy to

pitting corrosion. Si crystals that precipitated from the reaction between SiO2

and

liquid aluminum formed microgalvanic couple with the matrix. Since the matrix was

anodic to Si, it dissolved. The addition of fly ash to the base alloy also generated sites

for particle-matrix decohesion such as pores and voids. Corrosion attack at these sites

increased the severity of pitting corrosion of the composites.

Therefore, the present investigation makes an attempt to synthesize the AA

2024 alloy–(2 to 10 wt. %) fly ash composites by stir casting route and focused to

study the corrosion behaviour of AA2024 alloy and the effect of fly ash addition on

its corrosion resistance. The corrosion resistance experiments were carried out by

accelerated electrochemical studies using the potentiodynamic method. The

morphology of the corroded specimens was examined using Optical Microscopy.

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7.4 EXPERIMENTAL DETAILS

The corrosion resistance experiments were carried out by accelerated

electrochemical studies using the potentiodynamic method on AA 2024 alloy and AA

2024 alloy–(2 to 10 wt.%) fly ash composites. The potentiodynamic measurements

were made in the corroding media of aerated solution of 3.5 wt% NaCl and 4 to 5

pellets (3–4 g in wt.) of potassium hydroxide (KOH) aqueous solution of pH 10.0 at

25 0

C. Saturated calomel electrode (SCE) and carbon electrode were used as reference

and auxiliary electrodes, respectively. Polarization testing was carried out using a

system (Model: ACM Instruments Gill AC – 1130, UK) well linked to a PC and

controlled by commercial software (model: Gil AC). The experimental set up was

shown in figure 7.1.

Cylindrical samples of 12mm diameter and 10mm height were cut from the

alloy and composite castings. The samples were mechanically polished with SiC as

abrasive medium to get the mirror finish and thoroughly rinsed with distilled water

and placed, when wet, in the measurement vessel. The potential scan was carried out

at 0.166 mV s-1 with the initial potential of - 0.25V (OC) SCE to the final pitting

potential. The testing procedure was followed as per the ASTM G 5-94 standards. It

was made sure that the samples have an active surface area of 1 cm2. The potential at

which current increased drastically was considered to be the critical pitting corrosion

Epit. Before starting the experiments, the Ecorr value was measured for

approximately 30 min. Specimens exhibiting relatively more passive potential (or less

negative potentials) were considered to have better pitting corrosion resistance. The

samples were taken out and rinsed thoroughly with distilled water and dried. The

metallographic studies were carried out by an optical microscope (Model: Olympus,

C-5060 - G - 4 - Japan).

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(a)

(b)

Figure 7.1: (a) Experimental set up of ACM Pitting Corrosion tester (Model: ACM

Instruments Gill AC – 1130- UK) (b) Closer view of the corrosion cell

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7.5: RESULTS AND DISCUSSION

Pitting corrosion appears mainly on metals and alloys in the passive state as a

result of disarrangement of passive layer by aggressive environment elements

(frequently Cl- ions) on the heterogeneities of metals [47]. In the case of composites

introduction of considerable amounts of alloy additions and reinforcing elements to

the matrix releases intermetallic phases in the structure, which lead to the formation of

galvanic couples favorable to corrosion. The use of Al–Cu alloys to fabricate MMCs

by liquid metal infiltration can lead to the formation of intermetallics, such as CuAl2,

which increase some mechanical properties of the alloy and can act as local anodes or

cathodes, inducing a high susceptibility to localized types of corrosion like pitting,

intergranular attack, etc. Localized pitting in the vicinity of copper aluminide (CuAl2)

precipitates within the 2000 series of aluminum alloys was observed [48]. Moreover,

factors influencing corrosion of the composites include porosity, segregation of

alloying elements to the reinforcement/matrix interface, presence of an interfacial

reaction product, high dislocation density around the reinforcement phase, voids at the

reinforcement/matrix interface and electrical conductivity of the reinforcements [49,

50].

The corrosion characteristics are represented by corrosion potential (Ecorr)

and pitting potential (Epit) and these were obtained from the electrochemical studies

and analysis of anodic polarization curves. Figure 7.2 represents the polarization

curves of the AA 2024 alloy and AA 2024–fly ash composites (2–10 wt. % fly ash) in

3.5% aqueous NaCl solution. The values of the potentials of Ecorr, Epit and ∆E are

listed in Table 7.1. The ∆E is a measure of the width of the passive region on the

polarization curve and provides an indication of the susceptibility to pitting.

From the results of C.K. Fang et al [51] the polarization curves for monolithic

Al and the MMCs are similar in shape, but different in corrosion potential and passive

current density. Each curve shows a passive region. A stable passive film exists at the

corrosion potential since no active/passive peak is observed. The MMC with a higher

volume fraction of reinforcement results in a more active corrosion potential and a

higher passive current density. From figure 7.2, the nature of the potentiodynamic

polarization curves reveals the alloy and composites was undergone spontaneous

passivation; and the shape of potentiodynamic curves of alloy and composites under

study show similar behaviur. The values of Ecorr for the AA 2024 alloy and the AA

2024 alloy–10% fly ash composite were -1347 mV and -1124.1 mV, respectively; and

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Epit of the alloy and 10% composite were -576.75 mV and - 625.79 mV, respectively;

which indicates that the composite is more active (higher potential) than alloy (lower

potential). The ∆E values for alloy and 10% composite were 770.65 and 498.31 mV,

respectively. The slope of the linear sections of the anodic curves in relation to the x-

axis above the Epit point, together with a sudden increase in the current density,

indicates rapid development of corrosion pits.

The optical micrographs of AA 2024 alloy and AA2024 alloy–10% fly ash

composite after corrosion were shown in figures 7.3 and 7.4 respectively. For the

alloy, pitting was found to occur along the grain boundary area and in the interface

between α solid solution and θ (CuMgAl2), as shown in figure 7.3(a) and (b). On the

other hand, for the ALFA composite pitting was concentrated along the grain

boundaries as well as the interfaces between a solid solution θ (CuMgAl2), and

between the fly ash particles and the matrix phase, figure 7.4 (a) and (b). These

microstructures of alloy and ALFA composite revealed an enhanced pitting corrosion

in the composite. During the formation of MMCs intermetallic precipitates and

segregation of alloying elements have been found to occur at the interface [52]. This

prevents the formation of a continuous resistive layer of aluminum oxide –

reinforcement particles across the entire surface.

The presence of fly ash particles acts as sites to initiate pits. In particular, the

interfaces between reinforcement and matrix material, where the matrix surface is

broken, will act as pit initiators. Since pits initiate at flaws, and interface of matrix

aluminum and reinforced particle is a flaw, the metal aluminum is pitted by aerated

sodium chloride solution. During the corrosion process an increase in the

concentration of aggressive Cl- anions and H

+ ions may occur in the pits and pores

due to impede replenishment of the solution. Due to increase in concentration of Cl-

and H+ ions passivation becomes difficult. Therefore, the dissolution of metal in the

pits is stimulated [47]. Rapid dissolution of aluminum occurs within a pit, while

oxygen reduction takes place on adjacent surfaces. The rapid dissolution of aluminum

within pits tends to produce an excess of positive charge in these areas, resulting in

the migration of chlorides ions to maintain electro neutrality. Thus, there will be high

concentrations of hydrogen ions in pits as result of hydrolysis, and this process

accelerates with time.

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Figure 7.2: Potentiodynamic polarization curves of the AA 2024 alloy and AA 2024

alloy – (2-10% wt) fly ash composite.

Table 7.1: Electrochemical data of AA 2024 alloy and AA 2024 alloy (2-10% fly ash

composite) from the corrosion testing

Material Ecorr(mV) Epit(mV) ∆E (mV) = Epit- Ecorr

AA2024 alloy -1347.4 -576.75 770.65

AA2024-2% fly ash composite -1377.2 -719.69 657.51

AA2024-6% fly ash composite -1273.4 -703.26 570.14

AA2024-10% Fly ash composite -1124.1 -625.79 498.31

Figure 7.3: Optical Micrographs of AA 2024 alloy contrasting between the corroded

and uncorroded areas (a) at 100X (b) at 200X

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(a)

(b)

: Optical Micrographs of AA 2024 alloy contrasting between the corroded

and uncorroded areas (a) at 100X (b) at 200X; Etchant: Keller’s reagent.

: Optical Micrographs of AA 2024 alloy contrasting between the corroded

Etchant: Keller’s reagent.

Uncorroded

area

Corroded area

Uncorroded

area

Corroded

area

200

(a)

(b)

Figure 7.4: (a) Optical microstructure of ALFA composite contrasting between the

corroded and uncorroded areas at 100X (b) Optical microstructure of

ALFA composite showing the corroded area at 200X, Etchant; Keller’s

reagent.

Uncorroded

area

Corroded

area

Corroded

area

Uncorroded

area

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7.6: CONCLUSIONS

1. Al–fly ash (ALFA) composites were produced by stir casting route

successfully. There was a uniform distribution of fly ash particles in the matrix

phase and also existing a good bonding between matrix and fly ash

reinforcements.

2. The results of the unreinforced AA2024 alloy had a lower corrosion rate than

the composites at the tested same pH value.

3. Copper enrichment and re-distribution is the root cause of pitting in Al-Cu

alloys; chloride ions which accelerate the corrosion process may be attributed

to oxide film break down or assisting the anodic reaction.

4. Increase in the percentage of fly ash content in AA 2024 alloy matrix will be

advantageous to reduce the density and increase the strength of the alloy;

however, there is a significant increase in the corrosion attack.

5. Fly ash particles lead to an enhanced pitting corrosion of the ALFA

composites in comparison with unreinforced matrix (AA 2024 alloy); this

increase was more for higher the amount of fly ash in the matrix.

6. The enhanced pitting corrosion of ALFA composite is associated with the

introduction of nobler second phase of fly ash particles. The presence of fly

ash particles acts as sites to initiate pits. In particular, the interfaces between

reinforcement and matrix material, where the matrix surface is broken, will act

as pit initiators.

7. In the case of the AA2024-fly ash composites, the anodic current density was

reduced, decreasing the amount and size of the pits.

8. The ∆E is a measure of the width of the passive region on the polarization

curve and provides an indication of the susceptibility to pitting. Lower ∆E

values were observed for composites than base alloy.