aluminum corrosion 1

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Trans. Indian Inst. Met.

Vol.57, No. 6, December 2004, pp. 593-610

REVIEW

1. INTRODUCTION

Heat treatable aluminum alloys are widely used in

aircraft structural applications and are susceptible to

localized corrosion in chloride environments, such as

pitting, crevice corrosion, intergranular corrosion,

exfoliation corrosion and stress corrosion cracking.This article reviews the some aspects of passivity and

pitting of Al alloys. Specifically, metastable and

stable pits, pitting mechanism, effect of intermetallics

and effect of welding parameters on pitting corrosion

of age hardenable Al-alloys.

2. PITTING CORROSION OF Al

ALLOYS

Pitting corrosion is defined as localized accelerated

dissolution of metals that occurs as a result of abreakdown of the protective passive film on the metal/

alloy surface1. In an aggressive environment,

typically containing halide ions, pits initiate and grow

in an autocatalytic manner, where the local

environment within the pits becomes more aggressive

because of decrease in pH and increase in chloride

concentration, which further accelerates the pit growth.

The pit growth usually takes a variety of shapes2

(Fig. 1). Pit shapes can be simply divided into

isotropic and anisotropic groups. Shapes in

Fig. 1 a-e are isotropic, while those in Fig. 1f are

anisotropic and are called microstructural orientated

pitting. The variation in pit shape could mainly depend

on the microstructure of metals or alloys such as

alloy composition and aspect ratio of grains. Even

though there are some differences in pitting corrosion

between stainless steels and Al alloys, e.g., hydrogen

bubbles form at the active pit surface in Al alloys,

both materials basically share a similar mechanism.

In general, pitting corrosion involves three stages:

pitting initiation, metastable pitting, and pitting

growth.

2.1 Pit Initiation

As mentioned above, aggressive anions such as

chloride are believed to cause passive film breakdown.

However, the exact mechanism of the passive film

breakdown is still unclear. A number of models have

been proposed to explain passive film breakdown orpit initiation3-9. Three main models are 1) adsorption

mechanism 2) penetration mechanism and 3) film

breaking mechanism (Fig. 2). These models have been

reviewed in depth in the literature10-11.

The adsorption theory emphasizes the importance of

adsorption of aggressive anions like chloride ions.

A competitive adsorption of chloride ions and oxygen

finally may lead to film thinning. The penetration

model emphasizes the importance of anion penetration

and ion migration through the passive film.

MacDonald and coworkers5-7 have developed a point

PITTING CORROSION OF HEAT-TREATABLE

ALUMINIUM ALLOYS AND WELDS: A REVEIW

K.Srinivasa Rao and K.Prasad RaoDepartment of Metallurgical and Materials Engineering, IIT-Madras,Chennai-600036

E-mail:[email protected]

(Received 12 April 2004 ; in revised form 18 October 2004)

ABSTRACT

This review attempts to present the current understanding of the mechanism of pitting corrosion of heat treatable

aluminium alloys and their welds. The role of alloying elements and intermetallic phases on the corrosion

behavior of these alloys has been discussed. Pitting mechanism of aluminium- copper alloys is specifically

discussed. Finally effect of welding on the pitting corrosion of these alloys is also presented.


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TRANS. INDIAN INST. MET., VOL. 57, NO. 6, DECEMBER 2004

Fig. 1 : Variations in shape of Pits

ion penetration and migration, and stress-induced

breakdown of passive film. Although these models

obtained some experimental support, no comprehensive

or universal model can account for pitting corrosion

in all metal/environment systems. This indicates that

pit initiation is rather complicated and a combination

of these models could explain pitting for a certain

metal/environment system.

2.2 Metastable Pitting

Metastable pits are pits that survive for a very short

lifetime in the order of seconds or less. They can

initiate and grow to the micron size at potentials far

below the pitting potential and also above the pitting

potential during the induction time prior to the onset

of stable pitting. Figure 3 shows typical metastable

pit current transients on stainless steels, in chloride

solution under an applied anodic potential. The current

increases corresponding to the growth of metastable

pit followed by a sharp current decrease due to

defect model as a modified or related penetration

model. The point defect model addresses the transport

of cationic vacancies to the metal/oxide interface

controlling pit initiation instead of anion penetration.

The point defect model has been fitted to experimental

data such as pitting potential and induction time for

pitting corrosion of Al and Al alloys in halides.

However this model cannot explain metastable pitting,

and some assumptions such as the electrode potentialand vacancy migration in extremely high electric field

(on the order of 106 to 107 V/cm) are suspicious.

The film-breaking model involves the breakdown and

repair of the passive film simultaneously. Mechanical

stresses due to electrostriction and surface tension

cause the passive film breakdown, which is repaired

rapidly. According to this film-breaking model, pits

initiate as a result of the passive film breakdown

only when stable pits grow afterward.

In summary, these models address important aspects

of pit initiation such as aggressive ion adsorption,


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SRINIVASA RAO AND PRASAD RAO : PITTING CORROSION OF HEAT-TREATABLE ALUMINIUM

ALLOYS AND WELDS

Fig. 2 : Schematic of pit initiation models

These detailed studies show that the early

development of stable pits appears to be identical to

that of metastable pits, and the probability of stable

pitting is directly correlated to the intensity of

metastable pitting events. Metastable pits repassivate

probably when the porous cover ruptures and the pit

electrolyte is diluted. In contrast to a huge amount of

studies on corrosion of stainless steels, literature on

corrosion of Al or Al alloys is limited. Pride14 et al.

studied metastable pitting on pure Al. They found

that the number of metastable pits and the current

repassivation process. Since metastable pits experience

initiation, growth, and repassivation, a better

understanding of these three stages for the stable pit

can be gained through study of metastable pitting.

Metastable pitting phenomenon was first observed in

stainless steel in the early 1970s12. Frankel and

coworkers used the term of metastable pitting for the

first time13. Over the past 30 years, metastable pitting

has been systematically investigated by analyzing pit

current density for individual metastable pits and

stochastic approaches to groups of metastable pits.


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Fig. 3 : Metastable pit transients observed on 302 stainless

steel polarized at 420mV SCE in 0.1M NaCl

solution.

spikes increase with increasing applied potential below

pitting potential and the chloride concentration. A

critical transition from metastable pitting to stable

pitting in Al has been found in their study.

2.3 Pit Growth

Above the pitting potential, stable pits grow at a rate

depending on alloy composition, local pit environment

and pit bottom potential. Due to the autocatalyticnature of pitting corrosion, the local pit environment

and bottom potential is severe enough to prevent

repassivation. Pit growth can be controlled by each

or combinations of three factors mainly charge-

transfer, ohmic and mass transport 15-18. For a

hemispherical pit, different rate controlling factors

would lead to specific relationships between current

I, current density i, pit radius or depth r, time t, and

potential E.

. Under charge transfer control, Tafels law

describes i exp E.

. Under ohmic control, it can be derived I r

and i I/r2 1/r. From Faradays law,

i dr/dt, leading to r t 1/2 and thus

I t 1/2 and i t -1/2. Ohms law determines

i E .

. Under mass transport control, according to Ficks

laws, i 1/r, thus i t-1/2. i is E independent.

The similar i-t relationship for ohmic control and

mass transport control makes it difficult to distinguish.

For a 3D bulk sample, the non-steady state nature of

pit deepening and the problem with accurate

measurements of pit current density complicate the

clear identification of the i-E relationship . In a

conventional measurement of i-E relationship, current

may come from several pits with unknown active

surface areas and presumably is evenly distributed on

the pits. However, the assumption of even distribution

is not possible since different pits initiated at different

potentials grow at different rates. Artificial pit

electrodes, formed by imbedding a wire in epoxy

have been extensively used to study iron and stainless

steel behavior 19. The artificial pit electrode geometry

forms a single pit in which the whole electrode area

is active, generates a natural pit environment, and

provides an ideal one-dimensional transport condition.For Al and Al alloys, similar to artificial pit

electrodes, artificial crevice electrodes have been used

since large crevice area facilitates the escape of H2

bubbles 20,21. The results indicate that pits can grow

either in the active state without salt film precipitation

or in a salt-film-covered state. The active state is

dominated by ohmic control while a salt-film-covered

state is dominated by mass transport control. Other

single pit techniques include the exposure of small

area, laser irradiation of a small spot, and implantation

of an activating species at a small spot 22-25.

These studies suggested different viewpoints of either

ohmic control or mass transport control.Besides the

electrochemical methods, non-electrochemical

techniques have been also used. Hunkeler and Bohni 26

measured the time for pit to penetrate Al foils of

varying thickness to determine the pit growth rate.

They found that at fixed applied potential, pit depth

d and current density i were time dependent:

d t1/2 and i t-1/2. Pit growth on Al was ohmic

controlled since the growth rate was correlated to the

conductivity of the electrolyte. Detailed studies of2D pit in Al and other types of thin films by Frankel

and coworkers 13 found that the high current density

increased linearly with potential and reached a limiting

value at higher potentials (Fig. 4). Therefore, the pit

growth at the beginning is controlled by ohmic control

and after some time controlled by the mass

transport27-29.

2.4 Pitting Stability

Local pit environment and chemistry are believed to

be very important for pit growth and repassivation.


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ALLOYS AND WELDS

Among the various species present within pits such

as metal cations, metal hydroxide, Cl- and H+,

acidification within pits as a result of hydrolysis is

generally recognized to be a critical factor.

Galvele 30, 31 calculated the acidification in 1D pits,

based on metal dissolution, hydrolysis, and mass

transport. He found that a critical value of the product

x.i (x is pit depth and i is current density), was the

critical acidification within pits to sustain pit growth(Fig. 5). This critical product can be used to explain

the pitting potential and repassivation potential, and

determine the current density required to initiate pitting

and to sustain pit growth at a defect of a given size

in passive film such as crack. Although, for some

metals, other factors like chloride concentration are

more important than acidification, they will roughly

scale with acidification. Thus the critical value x.i

(sometimes Ipit

/rpit

used) can be used as criteria for

pitting stability. Williams et al.32 correlated pit

stabilization with metastable pitting. They suggested

that Ipit

/rpit

for metastable pits formed on steels must

exceed 4 10-2 A/cm2 for stable growth.

At a higher current density during pit growth, a salt

film may form on the pit surface due to saturation ofionic species. For Al pits in chloride solution, this

salt film was considered to be aluminum chloride

(AlCl3) or aluminum oxy-chlorides such as

Al(OH)2Cl and Al(OH)Cl

2according to measured

pH and possible hydrolysis processes 33-36 . Upon salt

film precipitation, as described above, the pit growth

is under mass transport control. A salt film can

enhance pitting stability by acting as buffer of ionic

species that can dissolve into pit to sustain a severe

condition in the pit environment such as high acid

concentration.

The potential distribution in pits is considered to be

another important factor to stabilize pit growth. When

the IR drop is less than a critical value, pit growth

stops due to repassivation, if the alloy undergoes an

active/passive transition in the pit environment37-39.

In fact, all of the factors above might be generalized

to pit growth current density, since a pit must maintain

a minimum current density for stabilized growth.

However, the critical pit current density and effect

of environment factors need to be investigated further.

2.5 Criteria for Evaluation of Pitting Corrosion

in Al Alloys

Many electrochemical studies of pitting corrosion have

found that there exist characteristic potentials. Using

cyclic polarization techniques, two characteristic

potentials can be determined, which correspond to

pit initiation and repassivation (Fig. 6). One is pitting

potential (EP), sometimes called critical potential or

breakdown potential (EB), above which stable pits

initiate and grow rapidly. The other is repassivation

potential (ER), sometimes called protection potential,

Fig. 4 : Anodic and net current densities change as afunction of potential for 100 nm Al film in 0.1M

NaCl solution.

Fig. 5 : Concentration of Al3+, Al(OH)2+, and H+ as a

function of the product of the depth x and the

current density in a unidirectional pit.


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below which growing pits repassivate and stop

growing. It should be noted that the values of these

two characteristic potentials can depend somewhat

upon the methods used and potential scan rate.

Moreover, since pitting corrosion is considered to be

stochastic, stochastic approaches have been developed

to handle the scatter of pitting potential 40 . Both EP

and ER

have been extensively used to evaluate the

susceptibility to pitting corrosion of various materials

in a given environment. It is generally recognizedthat materials exhibiting higher E

Pand E

Rare more

resistant to pitting corrosion.

Electrochemical impedance spectroscopy (EIS) uses

a range of low magnitude polarizing voltages, like

linear polarization. However, EIS voltages cycle from

peak anodic to cathodic magnitudes using a spectrum

of alternating current (AC) voltage frequencies,

instead of a range of single magnitude and polarity

direct current (DC) voltages. Data recorded in the

form of Bode and Nyquist plots can provide electrode

capacitance and charge-transfer kinetics and as themethod does not involve a potential scan,

measurements can be made in low conductivity

solutions with high accuracy. Figure 7 shows a simple

electrochemical equivalent circuit and the

corresponding data plots41. The magnitude of the

high frequency impedance where the impedance

magnitude is independent of frequency corresponds

to Rs. The difference in magnitude between the low

frequency and the high frequency independent regions

corresponds to Rp.These resistances are identical to

those on the Nyquist format plot. Low frequency

Fig. 6 : Schematic cyclic polarization showing EP

and ER

Fig. 7 : (a) Equivalent electrical circuit model for simple

corroding electrode, (b), (c) its Bodes and

(d) Nyquist plots

(a)

(b)

(c)

(d)


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ALLOYS AND WELDS

impedance value (Ohm.cm2), where the phase angle

approaches zerois accurate polarization resistance of

the alloy in a given environment. Recently EIS

technique has been recognized as the accurate method

of determining the corrosion resistance of aluminium

alloys in solutions containing aggressive halide ions.

Determining polarization resistance of the alloys

immersed in sodium chloride solution as function of

time and its variation may be criteria for the

measuring pitting corrosion resistance of aluminium

alloys. Recently Q.Meng et..al 42 studied the

corrosion behaviour of 7xxx alloys with varying

Copper content using immersion tests in aerated

chloride solutions (Fig.8) and concluded that the

polarization resistance decreases as the Cu contentincreases, which has been attributed to the Cu

enrichment on the surface.

Coarse intermetallic particles play a crucial role in

the corrosion behavior of Al alloys. The micro

galvanic coupling between the matrix and the

intermetallic particles is generally believed to result

in pitting corrosion and further develop intergranular

cracking (IGC) into the deep structure of Al alloys.

In this section, the role of alloying elements in solid

solution and intermetallic particles in pitting corrosion

of Al alloys will be reviewed.

3.1 Alloying Elements

Muller and Galvele43 first studied the role of alloying

elements in pitting corrosion of Al-Zn, Al-Mg, and

Al-Cu binary alloys in dearated 1 M NaCl. Zn, Mg,and Cu as alloying elements have different effects on

the pitting potential of Al alloys (Fig. 9). Pitting

potential decreased greatly with increasing Zn content

up to 3wt% and remained the same with further

increase in Zn content. There was no influence of

Mg on pitting potential. Pitting potential increased

3. ROLE OF ALLOYING ADDITIONS

AND INTERMETALLICS INLOCALIZED CORROSION OF Al

ALLOYS

Addition of alloying elements, especially Cu, can

significantly increase the mechanical strength of Al

alloys such as Al-Cu-Mg alloys (2xxx series) and

Al-Zn-Mg-Cu alloys (7xxx series) by precipitation

hardening. Due to the limited solubility of many

elements in aluminum, alloying elements are often

distributed not only in the Al solid solution, but also

in fine precipitates and coarse intermetallic particles.

Fig. 8 : Polarization resistance determined by EIS tests as

a function of immersion time for AA7xxx-T6 in

aerated 0.5 M NaCl.

Fig. 9 : Variations of pitting potential as a function of

alloying element content of a) Al-Cu b) Al-Zn and

c) Al-Mg binary alloys.


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dramatically with increasing Cu content up to 5wt%.

Furthermore, they studied the corrosion morphology

of these three binary alloys. It was found that tunnel-

like pits formed on Al-3Zn, and crystallographically

shaped pits on Al-3Mg and Al-3Cu. Sato4 studied

metastable pitting on Al-Zn alloys. They found that

the rate of pit nucleation was potential dependent

regardless of the alloying addition. It was suggested

that Zn addition influenced the pit growth instead of

the pit nucleation events.

Since the mid 1980s, many studies have been conducted

on surface chemistry and corrosion properties of

stainless Al alloys containing W, Ta, Mo, Nb,

and Cr

44-49

. These studies provide some clues toexplain the role of alloying elements on pitting

potential. Due to the low solubility of the above

alloying elements in aluminum, thin films of

supersatuarated Al binary alloys have been prepared

by non-equilibrium methods such as sputter

deposition. The electrochemical studies revealed that

the pitting potential of aluminum can be dramatically

increased by the addition of these elements. One of

explanations is that enrichment in the passive film

plays an important role in improving pitting resistance.

Moshier and coworkers44-45using X-ray Photoelectron

Spectroscopy (XPS) conducted surface analysis of thepassive films formed on Al-Mo, Al-Ta, Al-Cr, and

Al-W alloys. They found significant incorporation

of the alloying elements into the passive film. It was

suggested that a more protective passive film enriched

with the solute atoms was responsible for improved

pitting resistance by impeding the ingress of chloride

ion through the passive film. Smialowska 50 suggested

that the solute elements in the active pit surface play

the critical role instead of solute in the passive film.

She proposed that the low solubility of the solute

oxide in the acidic pit environment is responsible forimproved pitting resistance. Another explanation has

been proposed by Frankel and coworkers27-29 based

on their measurement of thin film pit growth kinetics

for Al-Nb, Al-Mo, and Al-Cr thin films by sputter

deposition. They found that stable pits initiated at

potentials only about 30 mV higher than they

repassivated (Fig. 10). It was suggested that the

addition of noble alloying elements increased the

pitting and repassivation potential by ennobling the

dissolution kinetics of pit growth rather than the

passive film effect. However, the exact mechanism

by which alloying elements alter the dissolution

kinetics is still unclear. Regardless, this dissolution

kinetics viewpoint provides a new insight to understand

the role of alloying elements such as Zn, Mg, and

Cu in Al alloys in pitting corrosion. In the light of

the dissolution kinetics viewpoint, Ramgopal and

Frankel21 recently studied the dissolution kinetics of

Al-Zn, Al-Mg and Al-Cu binary alloys using the

artificial crevice electrode technique. It was found

that Zn, Mg, and Cu addition had different effectson repassivation potential and the dissolution kinetics.

The addition of Cu increased the repassivation

potential and lowered the dissolution kinetics. The

addition of Zn decreased the repassivation potential

and enhanced the dissolution kinetics. The addition

of Mg had little or no effect on the repassivation

potential by changing the dissolution kinetics. They

suggested that the role of alloying elements was to

mainly change the surface overpotential and thus

shifted the repassivation potentials.

3.2 Intermetallic Particles

Intermetallic particles (IMCs) can be grouped into

coarse intermetallic particles and fine precipitates. In

Al alloys, coarse intermetallic particles form during

the solidification process, while fine precipitates

including hardening precipitates in the matrix and

grain boundary precipitates form during the aging

process. The type and composition of intermetallics

varies with the Al alloy composition and heat

treatment51. The primary coarse intermetallics found

in Al-Cu-Mg alloy such as AA2024-T3 are Al2Cu

Fig. 10 : Pitting potentials for freshly deposited samples, Epand aged samples Epa along with repassivation

potentials ER, for pure Al and AlNb alloys.


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ALLOYS AND WELDS

(S),Al2CuMg (S) and Al

20Cu

2(Fe Mn)3 52-53. The

coarse intermetallics Al3Fe, Al

7Cu

2Fe, Al

2CuMg, and

Mg2

Si are found in Al-Zn-Mg-Cu alloys such as

AA7075-T6. The fine precipitates for AA2024-T3

and AA7075-T6 are Al2CuMg and Mg (Zn Cu Al)

2,

respectively. As mentioned earlier, intermetallic

particles play a crucial role in localized corrosion of

Al alloys. The coarse intermetallic particles mentioned

above can be further divided into two groups: active

and noble particles relative to the Al matrix. Al2Cu,

Al3Fe, Al

7Cu

2Fe, and Al

20Cu

2(Fe Mn)

3are found

to be noble to the matrix, while Al2CuMg and Mg

2Si

are active to the matrix. Buchheit 54 compiled the

corrosion potentials of various intermetallic phases in

Al alloys, showing that the intermetallics exhibitdifferent electrochemical properties from the matrix.

Pits are readily found at the periphery of noble

particles in Al alloys during exposure to chloride

solution. It is generally accepted that noble Fe- or

Cu-containing intermetallic particles act as cathodes

and support oxygen reduction. As a result, a high pH

local environment is established at the noble particles,

which causes grooving of the surrounding Al matrix

by alkaline dissolution. The alkaline attack must then

somehow switch to acid attack to result in a stable

pit, which requires an acid environment.

Electrochemical studies have been conducted on Al3Fe

and Al2Cu 55-57. Nisancioglu found that near the

open circuit potential in NaOH solution, Al3Fe

underwent a preferential dissolution of Al, which

resulted in an Fe rich surface55. It was suggested that

Fe enrichment on the Al3Fe surface is detrimental to

cathodic behavior due to the formation of a protective

Fe oxide. The presence of Mn and Si in Al3Fe can

reduce the effect of Fe on both anodic and cathodic

rates. Mazurkiewicz and Piotrowski57 found that

Al2Cu underwent dissolution to form Al and Cu ionsat the open circuit potential and under anodic

polarization in sulfate solutions. Cu ion release was

also found in Rotating Ring-Disk Electrode (RRDE)

experiments on Al2Cu and Al

7Cu

2Fe at the OCP and

under anodic and cathodic polarization in chloride

solution .

The corrosion potentials for Mg2Si (D) and Al

2CuMg

(S) particles in chloride solution are -1.59 and -0.92

V SCE, respectively. Both Mg-containing phases are

active to the matrix and act as anode. They are

susceptible to active dissolution or Mg dealloying

when exposed in acidic solution or chloride solution.

Mg2Si phase in AA6000 dealloyed in 0.1 M

phosphoric acid and MgO was found on the Mg2

Si

particles. Buchheit and coworkers58,60-62 studied the

electrochemical behavior of S (Al2CuMg) phase in

the form of both synthesized bulk and real phases in

AA2024-T3. They found that S phase supported rapid

anodic and cathodic reaction kinetics and selective

dissolution of Mg and Al readily occurred under

anodic and cathodic polarization. Dealloying of active

S phase left Cu-rich remnants, which was cathodic to

the matrix and therefore caused grooving by alkaline

dissolution and then pitting at the dealloyed S phase.

They also proposed that decomposition of Cu-rich

remnants of S phase resulted in Cu release andredistribution, which further accelerated corrosion of

the Al alloys. This hypothesis has been supported by

RRDE experiments on S particles. The details about

Cu enrichment and redistribution will be reviewed

below. In summary, alloying addition and various

intermetallic particles play an important role in the

corrosion properties of Al alloys.

3.3 Cu Enrichment and Redistribution

The critical role of intermetallic particles in localized

corrosion of Al alloys was described previously. Manystudies revealed that Cu-containing intermetallic

particles govern the corrosion of high strength Al

alloys due to the noble nature of the Cu rich particles.

In chloride environment, Cu-rich particles,

particularly Al2CuMg (S) particles, often lead to Cu

enrichment and redistribution, which in turn is

detrimental to corrosion resistance. Buchheit and

coworkers58,60-62 attributed Cu enrichment in AA2024

to dealloying of S-phase, which accounts to about

60% of total intermetallic particles (Fig. 11). S phase

is susceptible to dealloying in acidic solution orchloride solution. Selective dissolution of Mg and Al

leaves behind Cu rich sponge remnants. Buchheit

et..al62 further pointed out that Cu redistribution was

attributed to the formation and re deposition of Cu

ions although the corrosion potential of AA2024 is

well below the reversible potential for Cu/Cu2+.

There are two possible explanations for this seemingly

thermodynamic contradiction. They proposed that the

Cu rich sponge remnants undergo physical coarsening,

which results in non faradaical liberation of

mechanically and electrically isolated metallic Cu

clusters.


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Metallic Cu clusters suspended in the solution or

isolated in the corrosion product can achieve a

corrosion potential that is not controlled by the alloy

potential. In an aerated solution, metallic Cu is

oxidized into Cu ions. Dissolved Cu2+ ions can drift

around by solution convection and redeposit on the

alloy surface, reducing back to metallic Cu. This

then leads to the localized corrosion in other places.

Sieradzki and coworkers 63-64 proposed a different

viewpoint that Cu ions are formed directly from Cu

rich sponge remnants on the alloy surface. In this

view point, the curvature effect is thought to be

responsible for the formation of Cu ions rather than

the liberation of metallic Cu clusters. The curvature

of Cu rich clusters on the surface shifts the reduction

potential for Cu in the anodic direction (equation 1),

dramatically when the radius r is very small.

nFrEE

CuCu

CuCu

=

2(1)

whereCuE is the potential of the Cu rich remnant,

ECu is the reversible potential for Cu, n is

Fig.11 : (a) Optical picture and (b) Schematic representing Cu enrichment redistribution during localized corrosion in

AA2024-T3


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2 equiv/mol, F is faradays constant (96487 C/equiv),

9Cu

is the molar volume of Cu, r is the radius of

the surface curvature and ICu

is the surface energy

of Cu.

The formation of Cu ion is possible at the alloy

corrosion potential when the radius is about 40 nm.

Besides the Cu enrichment and redistribution from

the S phase, other arguments have been made to

explain Cu surface enrichment, which do not require

any Cu oxidation or long-range redistribution of Cu

from the S phase. An argument is that Cu on the

surface around the intermetallics comes from the

surrounding matrix 65.According to this viewpoint,

the active intermetallic particles such as the S phaserapidly dealloy leaving behind the porous Cu rich

remnants, which act as local cathodic sites. The

oxygen reduction reaction occurring at these cathodic

sites increases local pH to alkaline. In local alkaline

solution, adjacent Al matrix around intermetallics

dissolves also leaving behind Cu, which is originally

in the Al matrix. Cu enrichment from both S phase

dealloying and matrix dealloying is possible. Cu

enrichment and redistribution from the S phase is

dominant when AA2024-T3 is immersed in chloride

solution for a short time, whereas, matrix dealloying

contributes to Cu enrichment and redistribution morethan S phase delloying after long time immersion.

4. PITTING MECHANISM IN Al-Cu

ALLOYS

Although the addition of copper increases the strength

of aluminium it dramatically decreases the corrosion

resistance of the metal to seawater. Copper has a

limited solubility in aluminium (2wt%) and unless

the liquid metal is rapidly cooled, copper will not be

uniformly distributed throughout the grains of thealuminium phase 66. If precipitation hardening

(increase in hardness of the metal due to the

precipitation of the CuAl2

inter-metallic phase) occurs,

the areas around the grain boundaries become depleted

in copper and as such become more anodic (more

reactive) than the rest of the grain. Under these

conditions the metal is subject to inter-granular

corrosion. In the absence of complicating factors the

more reactive metal or Corrosion potentials for a

solution containing 53g 1 1 NaCl, 3g 1-1 H20

2

from65 metal phase will have a more negative

corrosion potential (see Table 1). The difference of100mV in the E

corrvalues for pure aluminium and

aluminium with 2% copper in solid solution is quite

large and can lead to markedly different corrosion

rates across the different phases in a sheet of metal.

The copper corrosion products were formed by the

oxidation of the CuAl2

units in the metal structure.

A very com-mon mineral formed during the corrosion

of copper in seawater (pH 82) is cuprous oxide

(Cu2O). Under the same conditions an aluminium

oxide (Al20

3) will form as aluminium corrodes.

Because of the more reactive nature of aluminium

compared with copper, Cu20 or any other copper

mineral can be converted back to the metal by reaction

with aluminium metal,

2A1 + 3Cu20 A1

20

3+ 6Cu (2)

The formal cell potential for the above reaction is

+198 volts and so the process is spontaneous.

Examination of the Pourbaix diagram (Fig. 12)

(Eh

vs pH) for copper in seawater 67 and for

aluminium 68 shows that none of the copper corrosion

products can co-exist in contact with aluminium metalunder equilib-rium conditions. In the light of this it

is not surprising to find that copper deposits on parts

of the aluminium surface.

In the pitting of aluminium (Fig. 13) the deposited

copper acts as a cathodic site for the facile reduction

of oxygen 69, viz.

02

+ 2H20 + 4e 40H (3)

Noble impurities such as Al3Fe act in a similar

fashion. Chloride ions are known to be absorbed

Table 1

Solid solution or Ecorr

, the corrosion

constituent potentialVolts rel. N.H.E.,

25C

Cu +007 noble

Al + 4% Cu in solid solution -036

CuAl2

-040

Al + 2% Cu in solid solution -042

Al -052 active


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604


Fig. 12 : Potential pH diagram for the system Cu NH3 Cl

H2O Al. The diagram is based on data from [68]

and [74]. Region 1 is the stability domain for

Cu(NH3)12+ region 2 for Cu(NH

3)22+ and region

3 for Cu(NH3)32+. Solution conditions are NH

3=

NH4 = 025M, Cl = 2 X 10-3M, Al = 10-6M,

CU = 1 X 10 4M.

onto aluminium 70 and as little as 15ppm chloride

can initiate pit growth due to breakdown of the

protective oxide film 71.

The anodic reaction occurs at the bottom of the pit

Al Al3+ + 3e (4)

and the aluminium ions migrate towards the

inter-facial region where hydrolysis occurs,

Al3+

+ 3H20 Al(OH)3 + 3H+

(5)

which makes the pit acidic. Chloride ions migrate

into the pit to form aluminium chloride (A1Cl3) which

dissolves in the solution. Because of the low pH the

aluminium may also corrode with the evolution of

hydrogen .

2A1 + 6H+ 3H2

+ 2A13+ (6)

There is a critical bulk chloride concentration needed

to keep the pit propagating (16M) which is higher

than normal seawater (057M)

There is an equilibrium between the formation of

aluminium oxide and AlCl3, at the interfacial region

(the area between the metal and the corrosive medium)

viz.

A12O

3+ 6H+ + 6Cl- 2AlCl3 aq + 3H2O (7)

When aluminium chloride is formed a pit develops

and when alumina (Al2O

3) forms the pit will passivate.

The chloride ions directly affect the corrosion

potential of aluminium in fresh water. The higher

the chloride ion concentration the more negative is

the corrosion potential and the faster the metal will

corrode (in the absence of complicating factors).

Chloride ions accelerate the corrosion process but

whether this is due to oxide film breakdown orassisting the anodic reaction, is not known. To be

effective in arresting further corrosion the treatment

process must remove the re-deposited copper from

the remaining metal surface and also remove chloride

ions.

5. CORROSION OF WELDS

Welding is an important method of fabrication and

leads to physical, chemical and metallurgical changes

in aluminium alloys. One of the reasons for thechemical changes in the welds is due to the different

chemical compositions of the filler materials used.

Weld thermal cycle also causes microstructural changes

in the weld metal and heat affected zone (HAZ).

These alloys after welding will be subjected to either

postweld natural aging (T-4) or post weld artificial

aging (T-6). AA6061 and AA2014 alloys are

subjected to single step aging while 7020 alloys is

subjected to two step aging. Though artificial aging

results in higher strength values compared to natural

aging, some times natural aging is preferred from

the viewpoint of toughness.

Though literature on the corrosion behaviour of

aluminium alloys is available, the same cannot be

said of their welded counterparts presumably in view

of the chemical and microstructural changes caused

by welding. The literature survey also indicated that

there were no detailed and comparative studies

available on corrosion behavior of welds of heat

treatable aluminium alloys with respect to changes in

welding and heat treatment conditions.


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605


ALLOYS AND WELDS

When localized corrosion does occur in aluminum

welds, it may take the form of preferential attack of

the weld bead, pitting, intergranular attack or

exfoliation may occur in a HAZ a short distance

from weld bead. Welds in Al-Mg-Si alloys (AA6061)

generally have a good resistance to atmospheric

corrosion, but in specifically corrosive environments

like seawater localized corrosion may occur. Welds

in Al-Cu alloys (AA2014, AA2219, AA2026), Al-

Zn-Mg-Cu alloys (AA7075) and Al-Zn-Mg alloys

(7039) have less corrosion resistance, due to

metallurgical changes in the HAZ. Reheat treatment

of the welded part might restore the original corrosion

resistance, but this is rarely possible.

Some of the heat treatable alloys particularly those

containing substantial amounts of copper and zinc,

may have their resistance to corrosion lowered by the

heat of welding. These alloys exhibit grain boundary

precipitation in the HAZ and of this zone is normally

anodic to the remainder of the weldment. In a

corrosive environment selective corrosion on the grain

boundaries may take place and in the presence of

stress this corrosion can proceed more rapidly. Post

weld heat treatment provides a more homogeneous

microstructure and improves the corrosion resistance

of these alloys. Welds in Al-Zn-Mg alloy were seento be attacked preferentially in an area adjacent to

the weld bead when exposed to a corrosive environment

in the as welded condition. Post weld aging for a

sufficient at a high enough temperature eliminated

this preferential attack. Insufficient aging resulted in

a knife-edge attack parallel to at some distance from

weld.

Gas tungsten arc welding (GTAW) and Gas MetalArc Welding (GMAW) processes are widely used for

joining aluminium alloys for various applications like

aerospace, defence and automotive industries. The

resistance to corrosion of aluminium alloy welds is

affected by the alloy being welded and by the filler

alloy and the technique used. Galvanic cells that cause

corrosion can be created because of the corrosion

potential differences among the base metal, the filler

metal and the heat-affected regions where

microstructural changes have been produced.

In the Aluminium-copper alloys, the heat-affectedzone (HAZ) becomes cathodic, where as in aluminum-

zinc alloys, it becomes anodic to the remainder of

the weldment The corrosion potentials across the weld

zone for a 5xxx, 2xxx and 7xxx series weldment are

shown in Fig. 14.These differences in potential can

lead to localized corrosion as demonstrated by

corrosion of the alloy 7005 shown in Fig. 15. The

HAZ in the 5xxx alloy is mildly cathodic, where as

the 2xxx alloy exihibits a greater cathodic potential.

The 7xxx series HAZ is anodic to the unaffected

material and would be of great concern. Selection ofproper filler wire is important to avoid cracking during

welding and to optimize corrosion resistance. When

the solution potential of the filler is same as that of

the base metal (4043 for 6061-T6 alloy), optimum

corrosion resistance is obtained. In some cases,

Fig. 13 : Schematic diagram showing pitting corrosion in aluminium


14/18

606


Fig. 14 : Effect of the heat of welding on microstructure, hardness and corrosion potential of welds of three aluminium

alloys. (a) Alloy 5456-H3 with 5356 filler (b) Alloy 2219-T87 base metal with 2319 filler (c) Alloy 7039-T6 base

metal with 5183 filler

intermetallic phase formed by the base metal and

filler wire determines the final corrosion resistance

of the weld, for example magnesium silicide formed

during welding 5xxx alloy with 4043 filler can be

highly anodic to all other parts of the weldment72.

In general, the welding procedure that has the least

influence on microstructure has the least chance of

reducing the corrosion resistance of aluminium

weldments. The alloy with the more negative potential

in the weldment will attempt to protect the other

part. Thus if the weld metal is anodic to the base

metal (as 5356 weld in 6061-T6), the small weld can

be attacked preferentially to protect the larger surface

area of the base metal.


15/18

607


ALLOYS AND WELDS

Conventional Continuous current welding (CCW)

technique of gas tungsten arc welding (CCTIG) limits

the use of base metal property like strength and

corrosion resistance73. Due to steep thermal gradients,

characteristic of CCW, the segregation of elements

and liquid film formation at the grain boundaries

leads to hot cracking in the fusion zone and poor

corrosion resistance. Combination of pulsed current

and magnetic arc oscillation technique using AC TIG

process will improve the hot cracking resistance,

reduce the grain size and micro segregation, and hence

improve corrosion resistance of fusion zone. The

intermetallic phases like CuAl2

in AA2219 and Mg2Si

in AA 6061 Al-alloys are harmful with respect to

weldability and corrosion resistance. Theseintermetallic particles induce liquation in the partial

melted zone(PMZ) of the weldment leading to

cracking and galvanic coupling effect of these phases

with the surrounding matrix results in poor corrosion

resistance. Recently, Huang and Kou75 studied the

liquation mechanisms in AA2219, 6061, 2014 and

7075 alloys and suggested that proper care has to be

taken in selecting parameters and filler during GMA

and GTA welding of Al-alloys to avoid liquation

cracking. It is difficult to avoid the PMZ in Al-

alloys completely, but can be minimized by

Unpublished work of the authors on age-hardneble

Al-alloys (AA2014,AA6061,AA7020) deposited with

Al-5% Si (AA4043) and Al-5%Mg (AA5356) fillers

showed a strong dependence of corrosion behaviour

on the chemical and metallurgical changes of Al-

alloy welds due to welding and post weld heat

treatments.

Fig. 15 : Welded Jiont of alloy 7005 with 5356 filler aftera one year exposure to sea water (a) As welded

joint showing severe localized corrosion in HAZ

(b) Post weld aged effect. Corrosion potentials

measured in 53g/l NaCl plus 3g/l H2O2 versus

SCE.

Fig. 16 : PMZ areas of GTA Welds of 2219-T6 after Corrosion Testing.


16/18

608


controlling the welding parameters and selecting proper

filler wire suitable for the base metal composition

and thermal temper. Recent unpublished work of the

authors proved that PMZ is a strong function of

prior thermal temper of the Al-alloy like T-4, T-6

and T-87.Similarly it was also found that width of

the PMZ is less in pulsed current (PC) GTA welding

compared to that of continuous current (CC) GTA

welding of Al-alloys. Improvement in corrosion

resistance in Pulsed GTA welds of 2219 and 6061

alloys has been attributed to the decrease in segregation

and refinement of eutectic network of the weld metal.

Corrosion studies on Heat Affected Zones with PMZs

in CC and PC GTA welds of AA2219 (T6 and T87)

and AA6061 (T4 and T6) indicated the significantinfluence of prior welding technique and prior thermal

temper. HAZs of PC GTA welds of 2219-T87 and

6061-T4 are found be having higher corrosion

resistance when compared to that of T-6 temper.

Corrosion damage is extensive in the PMZ area of

the HAZ due to grain boundary eutectic enrichment

and segregation of alloying elements during welding

and this is evident from the optical microscopy and

scanning electron microscopy studies on GTA welds

of 2219-T6 alloy (Fig. 16). Authors76 recently made

an attempt to study the effect of prior copper removal

treatment on the corrosion resistance of the CC GTA

Welds of 2219-T6 and uniform pitting potential has

been achieved in all three zones of the weldment.

To summarize welding will have a strong influence

on the pitting corrosion of heat-treatable Al-alloys,

mainly microstructural changes in heat affected zone

and partially melted zone might lead to non-uniform

pitting potential across the weldment. Proper care

has to be taken in the selection of welding technique

and the filler wire, depending on the base metal

history of the Al-alloy.

6. CONCLUSIONS

1. In general, pitting corrosion involves three stages:

pitting initiation, metastable pitting, and pitting

growth. Three main models of pit initiation are

i) adsorption mechanism ii) penetration

mechanism and iii) film breaking mechanism

and these models address important aspects of

pit initiation such as aggressive ion adsorption,

ion penetration and migration, and stress-induced

breakdown of passive film respectively. The

number of metastable pits and the current spikes

increase with increasing applied potential below

pitting potential and the chloride concentration.

The potential distribution in pits is considered to

be another important factor to stabilize pit growth

and a pit must maintain a minimum current

density for stabilized growth.

2. Pitting potential ( EP) and Repassivation potential

( ER) can be used to evaluate the susceptibility

to pitting corrosion of aluminium alloys and

recently Electrochemical Impedance Spectroscopy

(EIS) has been recognized as the accurate method

of determining the corrosion resistance of

aluminium alloys in solutions containingaggressive halide ions.

3. The role of alloying elements was to mainly

change the surface over potential and shifting

the repassivation potential. Intermetallic particles

weaken the passive film and are sites for pit

nucleation. Cathodic intermetallics produce a

galvanic cell with aluminium matrix and act as

cathode for the oxygen reduction.These particles

selectively dissolve and remnants from the

particles dissolution- metallic Cu, Fe are still

more cathodic than the intermetallics. High pHlocal environment is established at these particles,

which causes grooving of the surrounding al-

matrix by alkaline dissolution.

4. Copper enrichment and re-distribution is the root

cause of pitting in aluminium-copper alloys.

Chloride ions which accelerate the corrosion

process may be attributed to oxide film break

down or assisting the anodic reaction.

5. Pitting corrosion resistance of heat-treatable

aluminium alloy welds depend strongly onwelding technique and the prior thermal temper.

Partially melted zone of the Al-alloy welds is

severely damaged by corrosion and is attributed

to the grain boundary eutectic enrichment and

segregation.

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