Galvanic corrosion of laser weldments of AA6061 aluminium alloy

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Corrosion Science 49 (2007) 4339–4351 www.elsevier.com/locate/corsci

Galvanic corrosion of laser weldments of AA6061 aluminium alloy A.B.M. Mujibur Rahman a

a,1

, S. Kumar a, A.R. Gerson

b,*

Ian Wark Research Institute, University of South Australia, Mawson Lakes, SA 5095, Australia b Applied Centre for Structural and Synchrotron Studies, University of South Australia, Mawson Lakes, SA 5095, Australia Received 5 December 2005; accepted 27 April 2007 Available online 24 July 2007

Abstract Galvanic corrosion of laser welded AA6061 aluminium alloy, arising from the varying rest potentials of the various weldment regions, was examined. The weld fusion zone is found to be the most cathodic region of the weldment while the base material is the most anodic region. The rate of galvanic corrosion, controlled by the cathodic process at the weld fusion zone, increases with time until a steady state maximum is reached. On galvanic corrosion the corrosion potential of the weld fusion zone shifts in the positive direction and the free corrosion current increases. It is proposed that the cathodic process at the weld fusion zone causes a local increase in pH that in turn causes dissolution of the surface film resulting in the loss of Al to solution and the increase of intermetallic phases. The increase in galvanic corrosion may result from either the build up of the intermetallic phases in the surface layer and/or significant increase in surface area of the weld fusion zone due to the porous nature of the surface layer.  2007 Elsevier Ltd. All rights reserved. Keywords: A. Aluminium; B. Potentiostatic; C. Galvanic couple; C. Welding

*

1

Corresponding author. Tel.: +61 8 8302 5545; fax: +61 8 8302 3044. E-mail address: [email protected] (A.R. Gerson). Current address: Origin Energy, P.O. Box 133, Hindmarsh, SA 5007, Australia.

0010-938X/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2007.04.010

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1. Introduction Welding as a fabrication method is one of the simplest ways to make a gas or liquid tight joint and laser welding competes favourably with alternative welding processes on the basis of all-round performance. However, welding introduces microstructural and compositional heterogeneities that can result in substantially enhanced corrosion and hence aluminium alloy weldment corrosion has been the subject of considerable research [1–3]. Al–Mg–Si alloys are widely employed in welded fabrications. The inherent corrosion resistance of these alloys and their filler metals (Al–Mg or Al–Si) is good and, individually, they are all suitable for marine service. However, in seawater the electrode potentials of Al–Mg alloys tend to be anodic as compared to Al–Mg–Si alloys, whereas Al–Si alloys are cathodic [4]. It has been recommended that in welded structures subjected to corrosive environments the weldment should be cathodic with respect to the parent alloy [5] so as to prevent metal loss to solution. However, alumina tends to become unstable at a pH greater than nine [6] and galvanic corrosion may occur even if the weldment is cathodic. AA6061 aluminium alloy, an Al–Mg–Si alloy, has numerous industrial applications [7] and is readily weldable. To avoid hot cracking in the welds, AA4043 filler wire (Al–Si) is generally used [8]. For these reasons AA6061 aluminium alloy with AA4043 filler wire laser welds were selected for examination of their galvanic corrosion behaviour. 2. Experimental method Aluminium alloy AA6061 sheets were cut into 2 mm · 120 mm · 120 mm pieces. These and the AA4043 filler wire were cleaned with acetone. Full penetration bead-on-plate welds were then produced on these sheets using a GSI Lumonics AMS 356 (3.5 kW continuous wave Nd:YAG laser). The parameters used to prepare the welds are shown in Table 1. A reduced wire feed rate, compared to general industry practice of 2.25 m min1, was used. The nominal compositions of the materials used to prepare the weld are shown in Table 2. The working electrodes, consisting of the weld fusion zone, the heat affected zone or the base metal, used for the galvanic potential measurements (Section 3.2), were prepared using epoxy resin and plastic tubing so that only these regions were exposed to the test solution (Fig. 1). Working electrodes consisting of the weld fusion zone were used for the galvanic corrosion measurements and potentiodynamic polarisation measurements. The base metal counter electrodes, used for the corrosion measurements (Section 3.2), were prepared using silicon sealant leaving approximately 14.4 cm2 exposed to electrolyte Table 1 The welding parameters used Spot size/diameter Incident power Focus position above material surface Welding speed Top shield (25% argon and 75% helium) Bottom shield (argon) Wire (1.6 mm diameter) feed rate

600 lm 3200 W Surface focus 3 m min1 40 dm3 min1 10 dm3 min1 1.15 m min1

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Table 2 Nominal compositions (wt.%) of the materials used to prepare the welds

AA6061 AA4043

Si

Fe

Cu

Mg

Cr

Al

Others

0.4–0.8 4.5–6.0

0.7 0.8

0.15–0.4 0.3

0.8–1.2 0.1

0.04–0.35 –

95.85–97.21 93.80–92.30

0.7 0.5

Potentiostat Interface RE PC

CE(a)

CE(b) WE

Fig. 1. Schematic of the experimental setup. For the corrosion potential measurements no counter electrode (CE) was used, for the corrosion experiments, CE(a), consisting of base metal was employed and for the potentiodynamic polarisation measurements CE(b), a platinum counter electrode was employed. The reference electrode (RE) was a saturated calomel electrode. The right hand schematic within the inset showing the working electrode is a vertical view of a bead-on-plate weld showing the weld fusion zone centrally, surrounded by the heat affected zone and then the base metal. The working electrodes were developed by slicing along the region of interest and then embedding the sample in resin so that only that region was exposed to solution.

(CE(a) on Fig. 1). A platinum counter electrode (CE(b) on Fig. 1) was used for the potentiodynamic polarisation measurements. Steel wire was used to obtain an electrical connection to the Gamry Instruments potentiostat. All electrochemical tests were carried out using a research grade PC4-750 potentiostat made by Gamry Instruments. The output current was resolved to 2.5 fA with a maximum measurable value of 750 mA. The voltage rating is resolved to 1 lV with a maximum possible value of 10 V. The potentiostat was controlled and data were digitally recorded using Framework DC 105 software. Experiments were carried out in 1.75 dm3 aerated (except where stated otherwise) 3.5% NaCl solution at room temperature (20 ± 2 C). All test solutions were made from analytical grade chemicals and demineralised water. Oxygen concentration was monitored and was found to be 8.8 mg dm3 throughout the measurements. A saturated calomel reference electrode was used as the reference cell and all potentials quoted here refer to this. Backscattered and secondary scanning electron microscope images were obtained to highlight microstructural and compositional changes in the weldment. The scanning electron microscope (Cambridge S250 Mark 1) used is equipped with an energy dispersive X-ray detector with a MORAN Scientific pulse processor, capable of 30,000 counts per

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second at 165 eV resolution. This is coupled to MORAN Scientific analysis and quantitative X-ray mapping software. A Microspec 6 xstal wavelength dispersive X-ray detector was employed for light and trace element analysis. 3. Results and discussion 3.1. Characterisation of the weldment The backscattered scanning electron microscopy image, shown in Fig. 2, of the weld edge shows the based metal, heat affected zone and weld fusion zone. Interdendritic segregation of intermetallic phases can be seen in the weld fusion zone. Thin lines of intergranular segregation can be seen in the narrow heat affected zone. Large intermetallic particles are visible in the base metal, however, intergranular segregation is not prominent. The compositions of the base metal and the weld fusion zone were analysed by Spectrometer Services Pty. Ltd. of Melbourne, Australia (Table 3). In the base metal the alloying elements Mg, Si, Cu and Cr are present as is the impurity Fe. The addition of Si rich filler wire (AA4043) results in an increased Si concentration in the weld fusion zone. It has been reported that the microstructure of the base metal contains two types of precipitates, Mg2Si phases and iron compounds, typically of the AlFeSi form [9]. In AA6056 aluminium alloy the large Al–Si–Mg and Al–Si–Mn–Fe containing particles vary from 2 to 20 lm in diameter [10]. The laser weldment microstructure of the AA6056 aluminium alloy indicates that large phases within the base metal dissolve and resegregate as fine interdendritic particles (size 20 nm) in the weld fusion zone [11]. 3.2. Corrosion of the weldment Rest potentials, using the three regions of the weldment as the working electrode, were measured with reference to the saturated calomel electrode. The experimental setup is shown schematically in Fig. 1 and no counter electrode was used. The standard ASTM

WFZ

BM

HAZ Intergranular segregation in HAZ

10µm

Fig. 2. The backscattered scanning electron microscopy image of a weld edge. The bright regions represent heavier elements. Base metal – BM; heat affected zone – HAZ, weld fusion zone – WFZ.

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Table 3 Elemental composition (wt.%) of the base metal (AA6061) and the weld fusion zone of the weldment

Base metal Weld fusion zone

Si

Fe

Cu

Mg

Cr

AI

Others

0.6 1.2

0.4 0.4

0.2 0.2

1.0 0.8

0.2 0.2

97.4 97.1

0.2 0.1

procedure [9] was employed except that the final polish of the working electrode was carried out with 1200 grit silicon carbide paper instead of No. 00 steel wool. Data was recorded five times per second for extended periods of time. The rest potential was estimated from the experimental results by following the ASTM standard procedure [12]. The variation in corrosion potential over the weldment is shown in Fig. 3 and the corrosion rest potentials of the weldment regions, derived from this data, are given in Table 4. The most cathodic region, the weld fusion zone, and the most anodic region, the base metal, of the weldment, as determined by the rest potential measurement were subjected to galvanic corrosion analysis. Galvanic corrosion was simulated by electrically connecting, through the Gamry Instruments potentiostat in zero resistance mode, the weld fusion zone (working electrode) to the base material (counter electrode) of the weldment (Fig. 1). Before the experiment a final polish was given to the base metal counter electrode and the weld fusion zone working electrode using 1200 grit silicon carbide paper. Both electrodes were then rinsed with acetone. After connection the current flowing through the couple and the couple potential with respect to the saturated calomel electrode were each measured once every 10 min for approximately 200 h. A high level of noise is present in the results of the galvanic corrosion experiment shown in Fig. 4 nevertheless it is clear the cathodic current density becomes more negative with time until its short-term average value reaches a steady value. The area of the cathode at the start of the experiment was 0.48 cm2 (estimated using Maxima image analyses software). The maximum value of the short-term average galvanic corrosion current was approximately 19 lA at completion of the experiment. Thus, the Time (s) 0

1000

2000

3000

Corrosion potential (mV)

-700

-740

-780

-820

Weld fusion zone Base metal Heat affected zone

-860

-900 Fig. 3. Corrosion potential plots of the various regions of the different weldment regions.

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Table 4 Corrosion potentials of the different weldment regions Material

Corrosion potential (mV)

Weld fusion zone Heat affected zone Base metal

716 727 744

50

Time (h)

100

150

-670

0

-680

-10

-690

-20

-700

-30

-710

-40

-720

-50

-730

-60

-740

-70

-750

-80

-760

-90

Galvanic current density (µA/cm²)

Couple potential (mV)

0

Fig. 4. Galvanic current density (top trace) and couple potential (bottom trace) of the weld fusion zone (cathode) and the base metal (anode).

maximum value of the short-term average galvanic current density was estimated, by dividing the current by the cathode area, to be approximately 40 lA cm2 (assuming no change in surface area). Galvanic corrosion exposure results in the formation of a thick porous surface film on the weld fusion zone (Fig. 5 including a cross-sectional elemental map of O) estimated to

Fig. 5. Images of the post-galvanic weld fusion zone: (a) topographic scanning electron microscope image of the surface film and (b) cross-sectional view of the oxygen distribution. The intensity bar to the right represents a scale of 0–50 wt.% for black to white.

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vary from 5 to 25 lm in thickness. Elemental maps (Fe, Cu, Cr and Si) of the cross-section of the weld fusion zone and its surface film are shown in Fig. 6. The maps suggest an increased concentration of Fe, Cu, Cr, Si and O in the surface film as compared to the weld fusion zone. Intermetallic phases can be seen as bright spots in the outermost post-galvanic corrosion surface (Fig. 7). Energy dispersive X-ray analysis of these regions revealed that they contain Al, Fe, Cu, Cr and Mg. It is evident from Fig. 7 that the dimension of intermetallic particles in the surface film is no more than 5 lm. To quantify the fraction of the total cathodic process that occurs at the outermost oxide/electrolyte interface two further stages were introduced onto the galvanic corrosion experiment. For the first additional stage of the galvanic corrosion experiment a postgalvanic corrosion weld fusion zone sample was dried, after rinsing with distilled water, by blowing warm air onto the sample surface followed by air-drying at ambient conditions for several hours. The dried weld fusion zone sample was then immersed in the electrolyte. As the sample came in contact with the electrolyte, the cathodic current density increased in magnitude (from zero in air) to 15 lA cm2 (Fig. 8). As it takes time for the electrolyte

Fig. 6. Elemental distributions (obtained using energy dispersive X-ray analysis) of the cross-section of the weld fusion zone obtained at the end of a galvanic corrosion experiment. The intensity bar between (a), the distribution of Fe, and (b), the distribution of Cu, represents a scale of 0–3 wt.% for black to white in (a) and (b). The intensity bar to the right hand side of the image (c), the distribution of Cr, represents a scale of 0–2 wt.% from black to white. The intensity bar to the right hand side of image (d), the distribution of Si, the image represents a scale of 0–4 wt.% from black to white.

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Fig. 7. Back scattered scanning electron microscopy image of the surface film of the post-galvanic corrosion weld fusion zone. The bright regions represent heavier elements present as intermetallic phases in the surface film.

Time (s) 0

1000

2000

3000

Cathodic current density (µA/cm²)

0 -10 -20

At outermost oxide/electrolyte interface

-30 -40 -50

At steady state

-60 Fig. 8. The variation of cathodic current density of the weld fusion zone as the electrolyte advances through the dry surface film to wet the entire film and achieve steady state.

to diffuse into the inner oxide layers, wet the entire surface film and achieve steady state, this initial value is indicative of electrolyte contact with the outer surface of the weld fusion zone only. When the steady state condition was reached the average value of the cathodic current density was 37 lA cm2 (assuming no change in surface area). This value is consistent with the value observed (40 lA cm2) during the initial galvanic corrosion experiment (Fig. 4). A steady state O2 concentration gradient between the bulk electrolyte and the outermost oxide/electrolyte interface requires some time to establish. At the end of the experiment depicted in Fig. 8, i.e. after steady state was reached, the second additional stage of the galvanic corrosion experiment was carried out. The electrolyte was stirred manually to

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Time (s) 0

1000

2000

3000

4000

5000

Couple potential (mV)

-500

-600

In aerated solution

-60 -100

-650 Stopped additional aeration

-700

-20 Started deaeration using N2 Stopped solution aeration

-140 -180

-750 -220

-800 -850

Galvanic current density (µA/cm²)

Started additional aeration

-450

-550

6000 20

-400

-260

Fig. 9. The top trace shows the change of cathodic galvanic current in the weld fusion zone with variation in O2 (air) supply. The bottom trace shows the consequent variation in galvanic couple potential. The sudden increase in galvanic current following N2 supply was likely to be due to the turbulence created by the N2 flow.

disturb any O2 concentration gradient established during previous phase. This resulted in the average cathodic current density increasing in magnitude from 37 to 48 lA cm2. As 15 lA cm2 is approximately 30% of 48 lA cm2, it is estimated that approximately 30% of the total cathodic process occurs at the outermost oxide/electrolyte interface. To understand what controls the galvanic corrosion, another experiment was carried out in which the O2 supply to the weld fusion zone was varied (Fig. 9). Initially the solution was aerated using one aerator. After 1150 s an additional aerator was used to supply air directly to the weld fusion zone, resulting in an increase in magnitude of the galvanic current density by a factor of approximately 2.5. The two aerators were continued for a short period of time, after which the additional aerator was switched off. The galvanic current density returned to its original value. After approximately 3200 s, aeration was completely stopped resulting in a reduction in magnitude of the galvanic current density by a factor of 10. It is evident that the galvanic corrosion depends predominantly on O2 supply to the weld fusion zone. The cathodic process in the weld fusion zone is therefore the rate controlling process. Eq. (1) represents the anodic reaction in the weldment and Eqs. (2) and (3) represent the cathodic reactions that may occur in the weld fusion zone. The results described above suggest that Eq. (2) dominates: Al ! 3e þ Al3þ 

O2 þ 2H2 O þ 4e ! 4OH 2Hþ þ 2e ! H2

ð1Þ 

ð2Þ ð3Þ

From Eq. (2), it appears that the rate of the cathodic process in the surface film may be limited by either the diffusion of O2 and/or the conduction of e to the cathodic reaction site.

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3.3. Metal loss from the weld fusion zone The film that forms on the surface of aluminium is known to be stable in the pH range 4–9 [6]. Cathodic activity can result in increased pH on the surface of aluminium, and metal loss [13]. This process is referred to as the cathodic corrosion of aluminium and involves simultaneous formation and dissolution of the surface film [14]. The film dissolves at the oxide/electrolyte interface and grows at the oxide/alloy interface. The possible reactions involved in the growth and dissolution processes for Al2O3(s) are: 4AlðsÞ þ 3O2 ! 2Al2 O3ðsÞ Al2 O3ðsÞ þ 3H2 O ! 2AlðOHÞ3ðsÞ

ð4Þ ð5Þ

AlðOHÞ3ðsÞ þ OH ! AlðOHÞ 4

ð6Þ

It is probable that a significant portion of the cathodic current is involved in Al dissolution (Eqs. (2) and (4–6) combined): 

4O2 þ 8H2 O þ 4e þ 4Al ! 4AlðOHÞ4

ð7Þ

The cathodic current density in the weld fusion zone (Fig. 4) was fitted to an empirical equation:   6t I ¼ 40  106 1  e5:4410 ð8Þ Using Eq. (8), the total charge delivered to the weld fusion zone during this experiment (i.e. the experiment giving rise to Fig. 4 run for 160 h) was found to be approximately 16 C cm2. If one electron is assumed to be involved in the dissolution of one aluminium atom of the oxide (Eq. (7)) the thickness loss (x) may be calculated: x¼

Qm eAd

ð9Þ

where Q is the charge (C cm2) involved in the cathodic process, e is the charge of an electron, A is Avogadro’s number, m is the molecular mass of aluminium and d is the density of aluminium. The thickness loss calculated was 16 lm. The post-galvanic corrosion surface film was partially removed with a water soaked cotton swab, acetone spray jet and dried. The resulting average depth value (measured using a Surtronic 3+ surface roughness analyser, Taylor Hobson), relative to the unaffected expoxy film which was initially co-planar, was approximately 7 lm with a maximum of 15 lm (Fig. 10). The difference between the calculated and the measured value of the thickness loss is reasonable. 3.4. Galvanic corrosion increase with time Potentiodynamic polarisation measurements were carried out to determine the reason for the increase in magnitude of galvanic current with time. For these measurements a platinum counter electrode (CE(b) in Fig. 1) was used in conjunction with a luggin capillary tube to minimise the influence of solution voltage drop. The working electrode potential sweep was started from a cathodic potential of 1100 to 400 mV at a scan rate of 1 mV s1. The current flow between the working electrode and a platinum counter electrode was measured as a function of the working electrode potential.

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Measurement number 0

2000

4000

6000

8000

2 0

The surface of epoxy resin of the working electrode of the weld fusion zone

Depth (µm)

-2 -4 -6 -8 -10 -12 -14 -16

The surface of the weld fusion zone

Fig. 10. The depth profile of the surface of the weld fusion zone obtained after the partial removal of its postgalvanic corrosion surface film.

Potentiodynamic polarisation measurements were carried out with base metal and weld fusion zone working electrodes each in both the pre- and post-galvanic corrosion condition. For the pre-galvanic corrosion measurements the electrode surfaces were freshly polished with 1200 grit silicon carbide paper and rinsed with acetone. An additional experiment was also carried out with the post-galvanic corrosion weld fusion zone sample after removing its weakly adherent surface film. The film was removed with a cotton swab that had been soaked in distilled water. The sample was then thoroughly rinsed with acetone. Fig. 11 shows the results of the potentiodynamic polarisation experiments. There occurs a shift of corrosion potential of the base metal (Fig. 11a) to the negative, however, there is no significant change of the free corrosion current between the pre- and post-galvanic corrosion conditions. Fig. 11b shows the polarisation plots from the weld fusion zone for the three different surface conditions. The corrosion potential and free corrosion current of the weld fusion zone shift towards the positive on galvanic corrosion. Thus on galvanic corrosion the difference between the corrosion potentials of the base metal and the weld fusion zone increases. This increase in the difference of corrosion potentials may account for the increase of the weldment galvanic current (Fig. 3) and may be a result of either the accumulation of intermetallic phases or the increased effective cathodic area due to the formation of undulating porous film on the surface of the weld fusion (Fig. 5a). However, it is apparent from the dimension of intermetallics (less than 5 lm, Fig. 7) as opposed to the thickness of the surface film (5–25 lm) that the accumulated intermetallics in the surface film may provide limited direct electrical connection throughout the surface film. It is difficult to quantify the increase in surface area of the weld fusion zone of the working electrode on galvanic corrosion however we estimate an increase in surface area of at least three times with a factor of greater than ten not being unreasonable (Fig. 7). This may also play a role in the change of relative potential of the base metal and weld fusion zone and the increase in galvanic current density on galvanic corrosion of the weld fusion zone.

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a

-1100

-1000

Potential (mV) -900 -800

-700 -3

-5

Corrosion potential

-6

Free corrosion current

-7

Log (current density (A/cm²))

-4

-8

b

-1100

Potential (mV) -900 -700

-500 -1 -2 -3 -4 -5

Free corrosion current

Log (current density (A/cm²))

Corrosion potential

-6 Fig. 11. (a) Polarisation plots of the base metal AA6061. The grey data set represents the polarisation plot pregalvanic corrosion and the black data set post-galvanic corrosion. The intersections of the anodic and cathodic Tafel slopes of the polarisation curves indicate the free corrosion potentials and currents. (b) Polarisation plots of the different surface conditions of the weld fusion zone, pre-galvanic corrosion (bottom, light grey), post-galvanic corrosion (middle, dark grey) and post-galvanic corrosion with the surface film removed (top, black).

4. Summary The weld fusion zone and base metal are, respectively, the cathode and anode within a bead-on-plate laser weldment of AA6061 base metal and AA4043 filler wire. Due to the difference in the rest potentials between the weld fusion zone and the base metal galvanic corrosion occurs in the weldment. The galvanic current increases substantially in magnitude with time until it reaches a maximum value. During galvanic corrosion, the corrosion potential of the base metal shifts towards the negative direction while that of the weld fusion zone shifts towards the positive direction. There is no significant change in the free corrosion current of the base metal, but a substantial change occurs in the weld fusion

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zone. The cathodic process is the rate controlling process and the O2 reduction reaction was found to play the dominant role in the cathodic process of the weld fusion zone. A thick porous surface film forms on the weld fusion zone. A substantial cathodic process occurs near the outermost oxide/electrolyte interface of the film. This is expected to result in an increase in the local pH (Eq. (2)) at this interface and cause dissolution of the film (Eq. (6)). The dissolution is balanced by oxide growth at the alloy/oxide interface. The dissolution and growth of the film result in metal loss from the weld fusion zone and the accumulation of intermetallic phases. The increase in galvanic current with time may be due to the thickening of the porous surface film, resulting in increased surface area and/or accumulation of intermetallic phases in the surface film. References [1] C.M. Liao, W.R. Horng, W.C. Kuo, Short-transverse SCC investigation on the HAZ of a welded Al–Zn–Mg alloy with a specially designed DCB specimen, Scripta Metallurgica et Materialia 26 (1992) 109–112. [2] A. Calatayud, M. Rodenas, C. Ferrer, V. Amigo, M.D. Salvador, Effect of welding on the microstructure and stress corrosion cracking susceptibility of AA7028 alloy, Welding International 11 (12) (1997) 973–977. [3] J. Onoro, C. Ranninger, Exfoliation corrosion behaviour of welded high strength aluminium alloys, British Corrosion Journal 28 (2) (1993) 137–141. [4] K.G. Compton, J.A. Turley, Electrochemical examination of fused joints between metals. Galvanic and pitting corrosion field and laboratory studies, ASTM STP 576 (1976) 56–68. [5] M.F. Gittos, Welding Al–Mg–Si Alloys, vol. 27, The Welding Institute Research Bulletin, 1986. [6] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solution, first english ed., Pergamon Press, 1966, p. 170. [7] B. Irving, Welding the four most popular aluminium alloys, Welding Journal (1994) 51–55. [8] S.R. Yeomans (Ed.), Welding in Structural Engineering: Steels and Aluminium Alloys, Department of Civil Engineering, Australian Defence Force Academy and University College, The University of New South Wales, 1994. [9] F. Paray, B. Kulunk, J. Gruzleski, Effect of strontium on microstructure and properties of aluminium based extrusion alloy-6061, Materials Science and Technology 12 (4) (1996) 315–322. [10] V. Guillaumin, G. Mankowski, Localised corrosion of 6056 T6 aluminium alloy in chloride media, Corrosion Science 42 (2000) 105–125. [11] D. Fabregue, A. Deschamps, Microstructural study of laser welds A16056–AS12 in relation with hot tearing, Materials Science Forum 396–402 (2002) 1567–1572. [12] Standard practice for measurement of corrosion potential of aluminium alloys, Annual Book of ASTM Standards, Section 3, vol. 03.02, Designation G69-81, 1997. [13] H. Kaesche, Corrosion of Metals: Physicochemical Principles and Current Problems, Springer-Verlag, Berlin/Heidelberg, 2003. [14] S.-M. Moon, S.-I. Pyun, The corrosion of pure aluminium during cathodic polarisation in aqueous solutions, Corrosion Science 39 (2) (1997) 399–408.