Localized corrosion of 6056 T6 aluminium alloy in chloride media

Localized corrosion of 6056T6aluminium alloy in chloride media

Vale

rie Guillaumin,Georges Mankowski*Laboratoire de Cristallochimie,Re ?activite ?et Protection des Mate ?riaux,UPRESA CNRS 5071,ENSCT,118route de Narbonne,31077,Toulouse Cedex 04,France

Received 30September 1998;accepted 19March 1999

Abstract

The corrosion behaviour of 6056T6aluminium alloy was investigated in 1M NaCl solution.Pitting and intergranular corrosion were found to be dependent on each other since intergranular corrosion nucleated on pit walls and spread from them.Coarse intermetallic Al±Si±Mg-containing particles were found to be nucleation sites for pits.Pits ?rst developed within the grains and grew up through microscopic crystallographic tunnels.The intergranular corrosion mechanism of 6056T6alloy appeared to consist of preferential dissolution of the anodic Si and Cu depleted zone along grain boundaries whereas Cu±Si±Mg rich intergranular precipitates functioned as local cathodes and stimulated the dissolution of the depleted zone.#1999Elsevier Science Ltd.All rights reserved.

1.Introduction

Aluminium 6013T6alloy is a high strength Al±Mg±Si±Cu alloy recently developed by Alcoa for replacement of the traditional 2024T3alloy in structural areas such as fuselage and door skins,formers and aft pressure bulkhead skin,dorsal ?n and trailing edge panels [1,2].Indeed,its mechanical properties are very attractive [1,3,4]:weldability is comparable to that of 6061alloy;good stretch forming characteristics in the T4temper,comparable to that in 2024W,helps in reducing the fabrication cost;three percent lower density than the 2024alloy;0010-938X/00/$-see front matter #1999Elsevier Science Ltd.All rights reserved.PII:S 0010-938X (99)00053-0

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105±125

*Corresponding author.

higher tensile and compressive yield strengths when compared with the alclad 2024;fracture toughness and crack resistance higher than that of the alclad 2024.Moreover,6013T6alloy seems to be virtually immune to exfoliation and saltwater stress corrosion cracking [1,3,4].However,its lower resistance to fatigue corrosion when compared to both the bare 2024and alclad 2024seems to be due to intergranular corrosion [5,6].Indeed,6013T6alloy was found to be susceptible to intergranular corrosion,but very few investigations have studied its corrosion properties [7±10].In this study,the corrosion behaviour of 6056T6alloy,the French equivalent of the 6013T6alloy,was investigated in 1M NaCl solution.The purpose of this work was to understand the susceptibility of the alloy towards intergranular corrosion and other localized corrosion.Speci?c attention was given to coarse intermetallic particles whose corrosion behaviour inˉuenced the corrosion properties of the alloy.

2.Experimental method

Corrosion investigations were performed on a 3.2mm thick sheet of 6056T6alloy.Its composition is given in Table 1.The T6thermal treatment consisted of solution heat treating at 5508C,air quenching and tempering the alloy at 1758C for 8h.Specimens were prepared by cutting coupons of 10?10mm from the alloy plate.The edges of the specimens were covered with varnish,leaving the rolling plane area uncovered.Before testing,the samples were mechanically polished to 4000grit SiC paper,and then with 1m m diamond paste.They were ?nally rinsed with distilled water.

2.1.Polarisation measurements

The tests consisted of potentiokinetic polarization of the samples from a cathodic potential of à900mV/SCE to the highest potential of à400mV/SCE in naturally aerated 1M NaCl solution (pH=6).For several tests,the scanning potential was stopped at a given value,and the specimen was maintained at this ?nal potential for various times.All measurements were made at room temperature.All potentials quoted are relative to the saturated calomel electrode,the counter electrode was platinum.The potential sweep rate was 500mV/h.Table 1Composition of 6056T6alloys (wt%)

Al

Si Fe Cu Mn Mg Cr Zn Ti Zr 6056T6Base 0.920.190.870.550.860.0040.150.020.1

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2.2.Microscopic observations

After polarization,observations using optical microscopy and scanning electron microscopy(SEM)with a LEO435VP apparatus were performed on the samples in order to examine and locate pits or intergranular corrosion.The specimens were also observed using transmission electron microscopy(TEM)using a JEOL JM2010,in order to observe intergranular precipitation and very small precipitates.To study the intergranular corrosion mechanism,foils prepared by electropolishing for TEM observations were subsequently immersed in two di erent dilute HCl solutions.The?rst one was made by adding two drops of concentrated HCl(36%)per liter of distilled water(pH=3.3),the second one was made by adding8drops of concentrated HCl(36%)per liter of distilled water (pH=2.7).The foils were immersed for2min in the less concentrated HCl solution,and after,2min in the other solution.The TEM foils were?nally rinsed with methanol and dried in air.Energy dispersed spectrometery(EDS)analysis was also performed during TEM observations to determine the composition of intergranular and other small precipitates.

2.3.Phase shifting interferometric microscopy(PSIM)[11]

This method,based on classical interferometry,was used to study the reactivity of coarse intermetallic particles and to quantify the dissolution of these particles through2D and3D pro?les.The advantage of PSIM lies in the acquisition of4 images of the same surface,but with a di erence of l/4in optical path length between two consecutive images.As accurate repositioning of the samples was possible,the pro?les plotted before and after the test were compared.The technique was chosen since the lateral resolution was well suited to the dimensions of the intermetallic particles.Moreover,this technique a ords a very high vertical resolution of1nm.Consequently,even slight modi?cations occurring in the shape of the intermetallic particles or in the matrix around them can be measured.

3.Results and discussion

Intergranular corrosion is usually caused by the precipitation of second-phase precipitates in the grain boundary.During precipitation,an adjacent denuded zone free of precipitates(PFZ)is formed,which has a di erent solution potential than the matrix.The degree of susceptibility of an alloy to intergranular corrosion depends on its microstructure,in particular the amount,size and distribution of intergranular precipitates,and the composition and the width of the PFZ. Obviously,these characteristics result from the metallurgical history and thermal treatment of the alloy.Likewise,many studies have proved that coarse intermetallic particles inˉuence the corrosion behaviour of aluminium alloys. Indeed,corrosion properties of2024T351alloy were found to depend on the

behaviour of the coarse Al±Cu±Mg-containing particles [12,13].Thus,this investigation began with a microstructural characterisation of the 6056T6alloy.

3.1.Microstructure

The overall thermomechanical treatment led to the 6056T6alloy having an average grain size of 30±40m m.Both optical and SEM observations show the presence of a very irregular distribution of coarse intermetallic particles (Fig.1).Indeed,particles can be very close to each other,but sometimes they can be at very wide intervals.The global fraction of particles with an analysed surface area equal to 0.35mm 2was found to represent 2.7%of the total area.Two types of coarse intermetallic particles were distinguished:Dmore or less globular Al±Si±Mg-containing particles with a size between 2±20m m for the longest side.They appeared blue coloured by optical observation and represent 0.7%of the total area.Dmore irregularly shaped Al±Si±Mn±Fe-containing particles which size is between 2±20m m (for the longest side).Optical observations made them appear pink.They were more numerous and represented 2%of the total area.Such coarse intermetallic particles have also been observed by Blanc et al.[12]in the 6056T6alloy.Lectard et al.[14]and Conte and Sanders [15]have also observed a -(AlFeMnSi)particles in 6013alloy.TEM observations were performed to reveal smaller particles.EDS analysis was also used to determine their chemical composition.In the matrix:Dneedle shaped hardening Al±Cu±Si-containing precipitates with a long side of around 100nm were found (Fig.2).They are unlikely to be b 'or b 0which

are

Fig.1.Coarse intermetallic Si±Mg-rich and Si±Mn±Fe-rich particles in 6056T6alloy.

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the precursors of the stable b -Mg 2Si phase which is generally encountered in 6013alloy.As the wt%copper was higher than 0.25%,the precursors y 'or y 0of the y -Al 2Cu phase could also have been observed [16].However,the analysed copper concentration was so small that it seemed improbable that the hardening precipitates belonged to this chemical type.They most likely belong to

another

Fig.2.Hardening Al±Cu±Si-containing precipitates in the

matrix.

Fig.3.Round and rod shaped dispersoids in the matrix.

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chemical type but analyses on such small particles were not precise enough to make conclusions.Dtwo types of dispersoids were found (Fig.3).Some were rather round with a size range of 0.08±0.4m m,and contained Al (74.3wt%),Mn (14.4wt%),Si (7.2wt%),Cu (2.2wt%),and Fe (0.6wt%).Others were rod shaped with the longest size of 0.35±1.2m m and contained Al (81.3wt%),Mn (10.2wt%),Si (4.6wt%),Cu (2.2wt%),and Fe (0.3w%).These two dispersoid compositions seemed very close to each other so,it was assumed that these two types of dispersoid belonged to the same chemical type and that they were oriented in perpendicular two directions.At the grain boundaries,a large number of needle shaped precipitates with average dimensions 20?40nm were found (Fig.4)and they contained Al (89.7wt%),Si (5.8wt%),and Cu (3.3wt%).Some were longer:24?130nm and contained Al (62.9wt%),Si (15.3wt%),Mg (10.4wt%),and Cu (10wt%).The PFZ along grain boundaries was not very clear (Fig.5).Indeed,sometimes hardening precipitates were found up to twenty nanometers from the grain boundary (Fig.5a)and at other times,they seemed to be very close to it (Fig.5b).EDS analysis of a grain boundary showed that it contained 2.24wt%Cu and 0.09wt%Si,whereas the matrix contained 2.56wt%Cu and 0.51wt%Si (such local analysis was possible with the JEOL 2010apparatus which allows the utilization of a 7nm analysis spot).The same copper and silicon contents were found over a zone 40nm wide on both sides of the grain boundary.Consequently,the grain boundaries presented a slight depletion in copper and a strong depletion in silicon,in comparison with the rest of the matrix.As TEM polar pieces are made of copper,results obtained always showed a copper content higher than the real

one.

Fig.4.Needle shaped Al±Si±Cu-containing precipitates at grain boundary.

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Indeed,the matrix copper content was a little higher than that indicated in Table 1.The hardening precipitates of Al±Mg±Si alloys have been widely studied by TEM analysis [16±24].However,very little work has been devoted to the 6013alloy microstructure.Blanc et al.[12],who worked on the susceptibility of pitting corrosion of 6056T6alloy have found approximately the same

precipitate

Fig.5.(a)Hardening precipitates up to twenty nanometers from a grain boundary.(b)Hardening precipitates very close to a grain

boundary.

Fig.6.Potentiokinetic polarization curve of 6056T6alloy in 1M NaCl solution.

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distribution.Jeniski et al.[25]have found 0.1±0.3m m dispersoids of a Al 12Mn 3Si and a -(AlFeMnSi)in the 6013alloy,depending on the local iron content.

3.2.Polarizaion curve in sodium chloride solution

Fig.6shows a typical potentiokinetic polarization curve obtained in 1M NaCl for 6056T6alloy.The curve shows only one breakdown potential around à730mV,close to the corrosion potential (E corr =à720mV).On and after the breakdown potential,both pitting and intergranular corrosion developed.In the ?rst stage of attack,large pits were observed as well as intergranular corrosion which took place on pit walls.Indeed,Fig.7shows a pit of 20±30m m in diameter which developed on the surface of a 6056T6sample after potentiokinetic polarization to à700mV (i.e.,30mV higher than the breakdown potential).On both sides of the pit,two grain boundaries were attacked,and intergranular corrosion spread from the pit following the grain boundaries.Fig.8presents observations of a sample after potentiokinetic polarization to à400mV.This potential was very high,so the attack became more severe compared to polarization to à700mV.Indeed,intergranular corrosion was stronger and more extensive,but stayed concentrated around the pits.Intergranular corrosion still took place on pit walls and spread from them (Fig.8a).Zones where pits did not develop did not allow intergranular corrosion to develop,and remained unattacked.Thus,these two forms of corrosion seemed to be dependent on each other.Various authors [3±6,9,10]who studied the

corrosion Fig.7.Pit on 6056T6surface after potentiokinetic polarization to à700mV,the potential was maintained for 5min at à700mV.

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properties of 6013T6alloy have found that pitting and intergranular corrosion were often encountered together.Pits can reach a very large size (200±300m m)and can show crystallographic facets.A tendency to grow following certain crystallographic planes was observed in Fig.8b.Other authors [26,27]have previously observed crystallographic shaped pits in the matrix of di erent aluminium alloys.In Fig.8a,pits are signi?cantly rami?ed presenting a rough pro?le.They seemed to grow following the model

of Fig.8.6056T6surface after potentiokinetic polarization to à400mV/SCE.(a)Microscopic observation of the whole surface.(b)SEM observation of a pit.

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Reigada et al.[28].This model is based on the idea that microscopic crystallographic tunnels are randomly nucleated on the pit wall,and quickly penetrate the metal.The occurrence and size of the tunnels depends on the local chloride concentration and potential.Under mild conditions,tunnelling is a rare process,the size of the tunnels is small,so the pit pro?le becomes rough.With increasing aggressiveness,both the occurrence and the size of the tunnels increases,leading to the formation of smooth,hemispherical pits.Frankel [29]who studied pit morphology on pure aluminium in chloride solutions has shown that the pit shape depended on the pit growth potential.At low potentials,the pit current density increases linearly with potential and pit pro?le is extremely rough.At higher potentials,the pit current density is independent of potential and the pit pro?le becomes round and smooth.Fig.9a shows the microscopic observation of a sample surface after potentiokinetic polarization to à400mV.Pitting as well as intergranular corrosion were observed.Fig.9b presents the same zone of the sample surface,after a short polishing with 1m m diamond paste.The attack appears much more severe after polishing.Pits are more extensive and microscopic tunnels are conspicuous,much more than in Fig.9a.The pit propagation by microscopic tunnels is there very clear.The propagation of the attack spread under the passive ?lm but spread also in depth.Moreover,as tunnelling was also observed on both sides of the grain boundaries,it is assumed that once the grain boundaries were attacked,microscopic tunnels nucleated on the grain boundary walls,and penetrated the metal.Finally,tunnelling seems to be the way of propagation of pitting but also takes place during intergranular corrosion propagation.The question remains as to how pitting and intergranular corrosion are

related.Fig.9.Microscopic observation of 6056T6surface after potentiokinetic polarization to à400mV/SCE.(a)Just after the experiment.(b)After a short polishing with 1m m diamond paste.

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Fig.10shows a specimen surface after a potentiokinetic polarization to à400mV.This SEM observation was taken with a 608tilt.Across the corroded surface many voluminous corrosion products were observed.Many of them were circular shaped with a hole in their centre.It can easily be assumed that each hole beneath each circular shaped corrosion product is a pit.Thus,the pit environment was isolated from the bulk solution by the voluminous aluminium hydroxide corrosion products and it became acidic and high in chloride concentration.Indeed,local acidi?cation is one of the possible mechanism of pit propagation [30±33].It can then be assumed that once the pit formed and the pit environment acidi?ed because of the formation of the voluminous corrosion product,the microscopic tunnels nucleated on pit walls.They penetrated the metal and spread within the grain.They could also penetrate along grain boundaries which were in contact with the growing pit.Then,intergranular corrosion started on pit walls and spread from them.Burleigh et al.[9,10]who studied intergranular corrosion of 6013T6alloy in NaCl/H 2O 2corrosive medium,have found that tall chimneys of corrosion products could maintain the acidic environment of pits and subsequently caused intergranular corrosion.This argument strongly supports ?ndings of the present work.Finally,it was inferred from these results that the breakdown potential corresponds to the matrix breakdown potential.On and after this potential,pits developed in the matrix and led to the formation of voluminous corrosion products.Then,the pit environment becomes acidic and high in chloride concentration.In this aggressive medium,the pits grow up through microscopic tunnels.When some of them penetrate grain boundaries,intergranular corrosion starts,and tunnelling also takes part in intergranular

corrosion.

Fig.10.SEM observations of 6056T6surface taken with a 608tilt,after potentiokinetic polarization to à400mV/SCE.

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3.3.Intergranular corrosion mechanism

After immersion in dilute HCl solutions,foils were ?rstly examined by optical microscopy.On one of its sides,the foil hole was quite rami?ed,presenting a rough pro?le (Fig.11)such as the pit pro?le in Fig.8a.So,it could be assumed that a pit developed and spread from the hole to the grain interior by microscopic crystallographic tunnels.TEM observation of the foil showed that this zone was quite oxidized.It appeared rather black by TEM observation,and the attack was so strong it that it was not possible to observe matrix or dispersoids any longer.The rest of the foil remained completely undamaged,and no grain boundaries were found to be attacked.However,grain boundaries encountered in the oxidized zone were found to be seriously attacked (Fig.12).A 0.5m m wide band on both sides of the grain boundaries was dissolved whereas grain boundary precipitates remained undamaged and were still observed.Thus,it was assumed that the Si and Cu depleted zone along the grain boundaries was anodic in comparison to the grain boundary precipitates and dissolved preferentially leaving the grain boundary precipitates unattacked.It is important to highlight the fact that attacked grain boundaries were only observed in the oxidized zone.This result con?rms the previous hypothesis:intergranular corrosion nucleates on pit walls and spreads from them to the metal interior following the depleted zone along the grain boundaries.Indeed,outside the corroded zone (i.e.,the pit),grain boundaries were completely undamaged.From the literature [34],two intergranular corrosion mechanisms have been proposed:DThe ?rst one consists of preferential dissolution of grain

boundary

Fig.11.TEM foil of 6056T6sample after immersion in HCl dilute solutions.

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precipitates.If the initial grain boundary precipitate is b 'or b ,these particles could serve as anodic sites and dissolve preferentially.As these particles are quite anodic compared to aluminium,their dissolution shifts the alloy potential in the cathodic direction,thereby diminishing the intergranular corrosion driving force.DThe second one consists of preferential dissolution of the PFZ along grain boundaries.If silicon is the initial grain boundary precipitate,the particles could function as minute cathodes stimulating electrochemical attack of the PFZ

zone Fig.12.Grain boundary attack containing Cu±Si rich intergranular

precipitates.

Fig.13.3D pro?les of an Al±Cu±Mn±Fe containing particle.(a)After polishing.(b)After 5min at free potential in 1M NaCl solution.

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along grain boundaries.The Si depleted PFZ zone would tend to be slightly anodic to the grain centres.The intergranular corrosion mechanism of 6056T6alloy seems to correspond to the second proposed mechanism.Indeed,the Si and Cu depleted zone along grain boundaries seems to be anodic in comparison to both the Si±Mg±Cu rich precipitates and grain centres and consequently dissolves preferentially,whereas intergranular precipitates function as local cathodes.Moreover,pitting corrosion helps intergranular corrosion to develop since intergranular corrosion nucleates on pit walls.

3.4.Reactivity of coarse intermetallic particles in chloride solution

The role of coarse intermetallic particles in aluminium alloys has been previously stated in this study.Moreover,Blanc et al.[12]showed that coarse intermetallic particles in 6056T6alloy are reactive in sulphate media.Therefore,to study the behaviour of coarse intermetallic particles in detail,phase shifting interferometric microscopy was performed on 6056T6alloy.Firstly,the particle behaviour was studied after 5min immersion in 1M NaCl solution at free potential.Fig.13b shows the 3D pro?le plotted for an Al±Si±Mn±Fe-containing particle after 5min at free potential in 1M NaCl solution.Fig.14b and 15b show 3D pro?les plotted for two di erent Al±Si±Mg-containing particles after 5min immersion in 1M NaCl.The 3D pro?les plotted before the polarization are shown in Figs.13a,14a and 15a.In Fig.13a,the Al±Si±Mn±Fe-containing particle shows a more strongly marked relief than does the Al±Si±Mg-containing particle in Figs.14a and 15a.This is due to di erences of hardness between the matrix and the coarse intermetallic particles which leads to such relief after

polishing.

Fig.14.3D pro?les of an Al±Si±Mg-containing particle.(a)After polishing.(b)After 5min at free potential in 1M NaCl solution.

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Comparison of Figs.13a and b shows no signi?cant modi?cation of the Al±Si±Mn±Fe-containing particle.All the other 3D pro?les plotted for this type of particle led to the same result.Consequently,the Al±Si±Mn±Fe-containing particles were not reactive,and remained undamaged after 5min immersion in 1M NaCl at free https://www.360docs.net/doc/b512210029.html,parison of Figs.14a and b shows that the Al±Si±Mg-containing particle was largely dissolved.After immersion in the electrolyte,it appeared as a hole on the sample surface.Nevertheless,comparison of Figs.15a and b shows that

this Fig.15.3D pro?les of an Al±Si±Mg-containing particle.(a)After polishing.(b)After 5min at free potential in 1M NaCl

solution.

Fig.16.2D pro?les of the Al±Si±Mg-containing particle in the Fig.14before and after immersion for 5min in 1M NaCl solution at free potential.

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Al±Si±Mg-containing particle partly dissolved and still appeared in relief on the surface.Moreover,the attack was not homogeneous.Thus,this particle seemed to be less reactive than Fig.14particle.The other 3D pro?les plotted on this type of particle led to the same result.Some of them were slightly dissolved,others were completely dissolved.It can be inferred from these results that the Al±Si±Mn±Fe-containing particles are not reactive during immersion in 1M NaCl solution at free potential whereas,the Al±Si±Mg-containing particles are reactive.To compare the reactivity of Al±Si±Mg-containing particles in di erent electrolytes and at di erent potentials,the following experiments were conducted:5min in distilled water at free potential,5min in 1M NaCl solution at à900mV,and 5min in 1M NaCl solution at à750mV.However,3D graphic representation did not allow comparison of the dissolution of the Al±Si±Mg-containing particle according to the experimental conditions.Indeed,the hole masked by the surrounding surface could not be seen correctly,and it was not possible to measure its height,2D pro?les seemed to be more adapted to quantitative measurement of hole depth.Moreover,it was easier to make 2D pro?les coincide using the non-attacked surrounding matrix level as reference.Fig.16represents the 2D pro?les corresponding to the Al±Si±Mg-containing particle seen in Fig.14,before and after immersion in the electrolyte.As was done previously [12],to estimate the amount of material lost,the surface areas between the particle pro?les and the matrix level (S R before immersion and S A after immersion)were measured.This allowed the surface area lost,S L ,to

be Fig.17.Mean attack depth of Al±Mg±Si-containing particles after immersion for 5min in (a)Distilled water at free potential.(b)1M NaCl solution at free potential.(c)1M NaCl solution at à900mV.(d)1M NaCl solution at à750mV.

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Fig.18.Al±Si±Mg-containing particles after potentiokinetic polarization to10mV higher than the corrosion potential,the potential was maintained at this?nal value for5min.

calculated.Since,the particle sizes varied over a wide range,a parameter R was de?ned as follows:

R S L

d R

R took into account the size of the particles in the attack plane with d R ,being the dimension of the particle measured in the 2D pro?le plotted before immersion.The parameter R allowed better comparisons to be made between the lost material for particles for di erent sizes.R corresponded to a mean depth and was expressed in m m.When the attack was not homogeneous,like as in Fig.15b,the 2D pro?le was plotted at the maximum attack of the particle,R was calculated from this pro?le.Fig.17shows the di erent mean values of R obtained from 4to 6particles after the experiments.After 5min immersion at free potential in distilled water,the particles reacted as they reacted after 5min immersion at free potential in 1M NaCl solution:some of them were slightly dissolved,others were completely dissolved.For both immersions at free potential,the dissolution was very low,R was around 0.02m m.This may have been due to the fact that it took more than 5min for the potential to become stable,and this,whatever the electrolyte.At the beginning of immersion in 1M NaCl solution,the free potential was around à1300mV and it reached à730mV more than 5min later.Consequently,the dissolution of the particles at free potential was slight.After immersion for 5min in 1M NaCl at à900mV or at à750mV,all of the particles were much more dissolved and appeared like a hole on the sample surface.Also,R ,of around 0.05m m,was two times higher compared to R at free potential.Thus,cathodic polarization caused a greater dissolution of the particles.Whatever the applied potential,à900mV or à750mV,the values of R were rather equal.It can be inferred from these results that the Al±Si±Mg-containing particles are strongly reactive in 1M NaCl solution at a cathodic potential.After the breakdown potential,the dissolution of the particles was so strong that it was not possible to use PSIM any longer.Consequently,SEM observations were conducted to study the behaviour of the Al±Si±Mg-containing particles after the breakdown potential.Before the test,a large zone of a sample was examined by optical microscopy and all the Al±Si±Mg-containing particles were located.After polarization to 10mV higher than the corrosion potential,the same zone was again examined by SEM.All over this zone,all the pits encountered nucleated on,or just around,the Al±Si±Mg-containing particles (Fig.18).Thus,these particles can be considered as nucleation sites for pits,and consequently for intergranular corrosion.Besides,in the initial stages of the attack,as seen in Fig.7,the 20±30m m pit showed exactly the same size as the average size of the particles.Several hypotheses can be considered:Dif particle dissolution was strong,the particle could become the nucleation site for the pit (Fig.18a).

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V.Guillaumin,G.Mankowski/Corrosion Science42(2000)105±125123Dif the particle was only partly dissolved,it grew hollow.It can then be assumed that in this hole,a local acidi?cation of the environment occurred, weaker than in the?rst hypothesis.If the particles were in contact with grain boundaries,it can be assumed,that intergranular corrosion started since the PFZ along the grain boundaries was anodic in comparison with the rest of the matrix. Following the PFZ in the core of the metal,if at a given moment,the attack reached the sample surface,a pit could develop.Then,the pit was a few micrometers from the particle as in Figs.18b±d.Figs.18e and f,where intergranular corrosion propagates under the surface of the metal from one particle to another,supports this hypothesis.

In both cases,pitting corrosion seems to be a consequence of preferential dissolution of the coarse intermetallic Al±Si±Mg-containing particles.The dissolution of these particles makes these zones fragile,and helps pitting corrosion to occur.In Fig.8a,the large zones where pitting and intergranular corrosion did not occur were free of Al±Si±Mg-containing particles.That is why they remained completely undamaged.

4.Conclusion

The potentiokinetic polarization curve of6056T6alloy in1M NaCl solution presents one breakdown potential which corresponds to the matrix breakdown potential.After it,both pitting and intergranular corrosion develop.These two forms of corrosion seem to be dependent on each other since intergranular corrosion starts on pit walls,spreads from them,but stays concentrated around them.

Pits?rstly develop within the grains leading to the formation of voluminous corrosion products.Then,the pit environment becomes acidic and high in chloride concentration.In this very aggressive medium,pits grow up through microscopic crystallographic tunnels.When some of them penetrate a grain boundary which is contact with the growing pit,intergranular corrosion starts.

Coarse intermetallic Al±Si±Mg-containing particles are strongly reactive in1M NaCl solution and seem to be nucleation sites for pits and consequently for intergranular corrosion.

Finally,the intergranular corrosion mechanism of6056T6alloy consists of preferential dissolution of the anodic Cu and Si depleted zone along grain boundaries whereas Si±Mg±Cu rich intergranular precipitates function as local cathodes and stimulate the electrochemical attack of this depleted zone.

Acknowledgements

This work was carried out in the framework of the``Laboratoire Re gional pour l'Ame lioration des Mate riaux Stucturaux pour l'Ae ronautique''with the?nancial support of the Re gion Midi-Pyre ne es.Special recognition goes to Michelle

Reversat and Djar Oquab for the SEM work and to Josette Poujardieu for TEM work.

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