Investigating the effects of pre-deformation and aging conditions on localised corrosion behaviour of aluminium alloy 6061 via electrochemical polarisation

Sample preparation and heat-treatment conditions

Commercial 6061 alloy plates were received under T6 temper state with thickness of 5 mm. The received plates were first solution heat treated at 530 °C for 1 h, with water quenching performed in both 25 and 75 °C water to achieve different quenching speed. The pre-deformation process was done on some AQ samples by stretching them on a tensile testing machine along the longitudinal direction to a 10% permanent set. Natural aging treatment was then done by leaving the samples in room temperature for at least 5 days to reach stable hardness, while artificial peak aging treatment was conducted by heat treating the samples at 160 °C for 16 h. For convenience, shortened sample names were given. AQ stands for the AQ 6061 alloy sample which was quenched in 25 °C water. T3 and T4 stand for the naturally aged sample with and without pre-deformation, respectively. T8 and T6, on the other hand, stand for the artificially peak aged sample with and without pre-deformation, respectively. The SC-T6 was the only sample quenched in 75 °C water, where ‘SC’ stands for slow cooling rate. The chemical composition of 6061 alloy and schematic drawing of sample preparation are shown in Table 1 and Figure 1.

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Figure 1

Schematic drawing of sample preparation of 6061 alloy plate.

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Table 1

Chemical composition of 6061 alloy applied in this study (wt-%).

Element	Mg	Si	Cu	Cr	Fe	Mn	Ti	Al
6061 alloy	0.8	0.5	0.2	0.2	0.3	0.1	0.1	Bal.

Electrochemical measurement

Electrochemical testing was performed in de-aerated 3.5 wt-% NaCl solution on a VersaSTAT4 electrochemical workstation. Testing solution was purged with Ar gas for at least 1 h with a minimum flow rate of 1 L min⁻¹ prior to testing. An Ag/AgCl reference electrode and a Pt counter electrode were used in a testing cell with a working electrode exposure area of 1 cm². Samples were polished up to #5000 grit SiC paper before being mounted to the cell as a working electrode. A 3 min cathodic cleaning at −1.2 V vs. RE followed by open circuit potential (OCP) measurement was done after sample exposure. The time required for each sample to reach steady OCP varies between 30 and 45 min. As soon as a steady OCP value is reached, potentiodynamic polarisation was performed first to determine the basic electrochemical characteristics of the samples. Potentiodynamic scan was carried out from 50 mV below OCP to −550 mV vs. RE with a scanning rate of 0.167 mV s⁻¹. Potentiodynamic polarisation of each specimen was duplicated at least twice to ensure experimental consistency.

Potentiostatic polarisation at −650, −625, −600 and −675 mV vs. RE was then performed as accelerated corrosion tests. Samples were first potentiodynamically scanned to the destined potential with the same parameter as shown above, and then potentiostatically held for 18 000 s. The output current during potentiostatic scan was recorded to determine the severity of the corrosion. Both exposed surface and cross-section of the potentiostatically scanned sample were examined under OM to observe for the existence of localised corrosion. Duplication of potentiostatic polarisation was done at least once to ensure the current transient is reproducible.

3D reconstruction of pitting distribution

3D modelling and image stacking techniques were carried out in order to reconstruct and investigate the distribution of pits in naturally aged 6061 alloys. T3 and T4 specimens were first potentiostatically scanned at −600 mV vs. RE for 18 000 s with identical parameters mentioned above. After the specimen was removed from the cell, OM image of the exposed surface was captured under 50× magnification. The specimen was then carefully polished with #5000 grit SiC paper to gently remove material from the exposed surface. Once approximately 5 μm of specimen thickness was removed, the specimen was cleaned with alcohol and compressed air, and the newly exposed surface was photographed again under the same magnification. This process was conducted repeatedly until no more corroded pits can be found from the remaining material. The captured images were first aligned and post-processed to eliminate unwanted scratches and minor pits caused by polishing. Then, the images were binarised and stacked using the MATLAB program. During binarisation, when the areas of empty pits in the images were set to 0 and the remaining areas were set to 1, the image of the remaining material after corrosion testing will be shown. On the opposite, when the binarisation setting is reversed, areas of the corroded pits can be revealed.

Microstructure characterisation

Thin-foil TEM samples of T3, T4, T6, T8 and SC-T6 were prepared with a diamond cutter and twin-jet-electro-polished at −30 °C in a 1:1 solution of nitric acid and methanol. Transmission electron microscopy was performed using JEM 2100 operating at 200 kV. Bright-field images of the GB areas of these 6061 alloy samples were taken up to 1 M× magnification.

GB area microstructure

The microhardness values of the 6061 alloy T3, T4, T6 and T8 samples produced in this study were tested to be 85.3, 75.8, 121.1 and 122.8 HV, respectively, which agrees with material standards and majority of researches. Bright-field TEM images were taken up to 1 M× in order to correlate the relationship between corrosion behaviour and microstructural characteristics of 6061 alloy under different aging conditions. Figure 2 shows the GB area TEM micrographs taken under 100 k×. The evolution and chemical compositions of the intermetallic precipitates of 6XXX series Al alloy have been widely investigated for many years, hence precipitates identification using selected area diffraction pattern is not included in this study. The discussion is focused mainly on the size and distribution of the precipitates and the microstructural changes observed around GB areas. A typical T6 artificially peak aged microstructure is demonstrated in Figure 2(a). Needle-shaped β″ phase is believed to be the dominant strengthening phase in the matrix under peak aged condition with composition close to Mg₂Si. Slight traces of Cu-containing lath-shaped Q′ phase is also present in the T6 sample. Similar matrix microstructure has been reported for 6111 Al alloy under T6 condition, with volume fraction of β″ and Q′ phase being 80 and 20%, respectively [20]. Since the Cu content of 6061 alloy is noticeable lower than that of the 6111 alloy, it is reasonable to expect lesser amount of Q′ phase existing in 6061 alloy under peak aged condition. The existence of equilibrium Q-phase GBPs, as well as PFZs, can be clearly seen around the GB area of the T6 sample, which held major responsibility for the poor IGC resistance of artificially peak aged 6061 alloy. The width of PFZs in Figure 2(a) of the T6 sample is measured to be around 20-30 nm. The microstructure of the SC-T6 sample is shown in Figure 2(b). With retarded quenching speed, the SC-T6 sample shows coarser matrix precipitates, larger GBPs and wider PFZs than the T6 sample, despite sharing the same artificial aging parameter. The width of PFZs of the SC-T6 sample is found to be around 50-60 nm, which is significantly wider than that of the T6 sample. Delayed quenching greatly reduces the number of supersaturated vacancies within the matrix, and vacancies closer to the GBs are allowed to diffuse into the GBs. This leads to the decrease in precipitate quantity and increase in precipitate size, GBP size along with the width of PFZs. The aforementioned microstructural changes caused by delayed quenching are almost undetectable through microhardness testing, but will later show significant influence on corrosion resistance in potentiostatic polarisation.

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Figure 2

Bright-field GB area TEM micrographs of 6061 alloy under several temper conditions. (a) T6, (b) SC-T6, (c) T4, (d) T3 and (e) T8.

Figure 2(c) shows the microstructure of naturally aged 6061 alloy. Since GP-zones were reported to be the major hardening phase in natural aging and are generally in high coherency with the matrix and too small to be detected [21], no visible GBPs or matrix precipitates can be found in the T4 sample under 100 k× magnification. On the other hand, with the introduction of pre-deformation, multiple dislocations were generated near the GB area and also within the matrix in the T3 sample as shown in Figure 2(d). The strain hardening effect caused by pre-deformation raises the material hardness of T3 sample for approximately 10 HV compared with that of the T4 sample, and the gap remained throughout the 5 days as the two samples were naturally aged under room temperature. Other than the dislocations, no visible GBPs or matrix precipitates can be found in the T3 sample under 100 k× magnification. Microstructure of the pre-deformed and artificially peak aged T8 sample is shown in Figure 2(e). Some dislocations near the GB caused by pre-deformation remain visible after the artificial aging. However, the formation of GBPs and PFZs still occurs around the GB of the T8 sample during artificial aging, but with some dissimilarities compare with the T6 sample. Reduction in both size and amount of GBPs can be seen in the T8 sample. As shown in the highlighted area in Figure 2(e), precipitates were also found to exist closer to the GBs around areas nearby the dislocations, therefore slightly decreasing the width of PFZs (less than 20 nm) in certain areas. The effects of these findings on the corrosion behaviour of the T8 sample will be illustrated in the following paragraphs. It has been reported that in 6061 alloy, cold rolling process decreases the time required for the T8 sample to reach maximum hardness during artificial aging [21,22]. Strain-induced dislocations may act as diffusion shortcuts that enhances precipitation speed [23]. The 10% pre-deformation, however, is performed by stretching instead of cold rolling in the present work. The peak hardness for T8 sample appears between 12 and 14 h of aging time under 160 °C, a touch earlier than the 14 h required for the T6 sample to reach maximum microhardness. But with the tested microhardness values being similar between 12 and 14 h, the 14 h aged sample was eventually chosen for further testing mainly to match the aging time as T6 for experimental convenience. Hence, slightly overaged matrix microstructure with the presence of Q′ phases can be found in the T8 sample due to the accelerated precipitation caused by pre-deformation.

Potentiodynamic polarisation and breakdown potential

Figure 3 shows the potentiodynamic polarisation curves of 6061 alloy under several temper conditions. The de-aerated testing solution separates the corrosion potential E _corr and the breakdown potential, revealing a passive region. It has been reported that the E _corr value of 6061-T6 increase toward negative in de-aerated chloride solution compared with aerated solution, as the breakdown potential in de-aerated solution roughly coincide with the E _corr value in aerated solution [24]. Generally, E _corr values in aerated solution are found to be stable within 100 mV range [24] and are considered as an indicator of the corrosion resistance. The E _corr values of all 6061 alloy samples tested in de-aerated solution sits between −800 and −900 mV, as shown in Figure 3. During experiment duplications, the E _corr values of the samples under the same temper condition were found to vary slightly within the range of 100 mV. This means that it is inappropriate to use the E _corr values of 6061 alloy samples as an indicator of corrosion resistance in de-aerated solution. The breakdown potential (E _br) on the other hand, where the corrosion current starts to increase rapidly with increasing scanning potential, remains steady as a reproducible feature [4] for samples under the same temper condition. Unlike some Al alloys such as AA7075 [25,26] and AA2024 [4] which demonstrate two breakdown potentials during potentiodynamic polarisation, only one breakdown potential was found in the polarisation curve of 6061 alloy. As shown in Figure 3, the AQ 6061 alloy possesses the highest E _br value (−640 mV approx.) of all specimens. The naturally aged T3 and T4 samples demonstrate higher E _br values (−660 mV approx.) than the artificially aged T6, T8 and SC-T6 samples (−680 mV approx.). E _br generally corresponds to the dissolution of a certain metallurgical phase or the rupture of passive film [27]. This result indicates that the corrosion resistance of the passive film of 6061 alloy decreases with increasing aging. However, a rapid potentiodynamic polarisation scan is inadequate to reveal the corrosion mechanism as well as differentiating localised and general corrosion. Also, the corrosion current output at certain potentiodynamic scanning potential cannot entirely represent a long-term corrosion rate. Hence, the results from potentiostatic polarisation is discussed in the following sections.

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Figure 3

Potentiodynamic polarisation curves of 6061 alloy under different temper conditions in de-aerated 3.5% NaCl solution at a scan rate of 0.167 mV s⁻¹.

Potentiostatic polarisation and the effect of aging conditions

Potentiostatic polarisation scans at −650, −625, −600 and −675 mV vs. RE were conducted for 18 000 s in de-aerated 3.5% NaCl solution. Scanning potentials were chosen based on the E _br values observed through potentiodynamic polarisation. Observations from potentiostatic polarisation scans and metallographic examinations are summarised in Table 2.

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Table 2

Summary of observations found in potentiostatic polarisation scans and metallography examination of the tested samples.

Scanning potential (vs. RE)	Sample	Average current density (µA cm−2)	Corrosion type	Note
−650 mV	AQ T3 T4 T6 SC-T6 T8	31.56 48.96 70.34 457.38 486.38 438.20	Pitting Pitting Pitting IGC and Pitting IGC and Pitting IGC	Slight IGC Large pits
−625 mV	T3 T4 T6 T8	239.96 229.87 602.10 582.58	Pitting Pitting IGC and Pitting IGC and Pitting	Generally distributed Restricted area (deeper)
−600 mV	AQ T3 T4	665.90 572.64 665.55	Pitting Pitting Pitting	Restricted area (deeper) Generally distributed Restricted area (deeper)
−675 mV	T3 T4 T6 T8	−0.21 −0.05 7.16 0.02	NA NA IGC only NA	Re-passivated Re-passivated No visible corrosion

Figure 4 shows the polarisation curves, exposed surfaces and cross-sectional OM images of 6061 alloy samples potentiostatically scanned at −650 mV. Corresponding to Figure 3, the scanning potential of −650 mV vs. RE is below the E _br value of AQ specimen, and is slightly above and around 25-30 mV above the E _br values of naturally aged 6061 alloys and artificially aged 6061 alloys, respectively. Since the samples were first potentiodynamically scanned to the destined potential prior to potentiostatic scan, the initial current of potentiostatic scan is approximately the same as the one recorded at the end of potentiodynamic scan. An obvious gap in corrosion severity (current density) between naturally aged and artificially aged 6061 alloys exists at the start of potentiostatic scan in Figure 4(a). During the first half hour of scanning, corrosion current of the artificially aged 6061 alloys (T6, T8 and SC-T6) increased sharply, while those of the naturally aged 6061 alloys (T3 and T4) increased rather slightly. After the initial increase, corrosion current then remains relatively steady until the end of the test. From a data analytical point of view, the area under the potentiostatic polarisation curve represents the amount of material dissolved into the corrosive solution which can be considered as the average corrosion rate if divided by the time of testing when the corrosion mechanism and depth are not taken into account. In potentiostatic polarisation at −650 mV, the SC-T6 sample showed the highest average corrosion rate of the five 6061 alloy samples followed by T6, T8, T4, T3 and AQ, as summarised in Table 2. For the SC-T6 sample, the supersaturated vacancies adjacent to the GB area diffuse into the GB due to delayed quenching, resulting un-uniform precipitation across the GB area which increases the width of PFZs, thus, increasing the average corrosion rate. Examining from the exposed surface and cross-sectional images as shown in Figure 4(b,c), the prolonged potentiostatic polarisation also reveals the existence of pitting and IGC, which is an un-attainable feature by a rapid potentiodynamic polarisation scan. For the artificially aged 6061 alloys, the mixture of IGC and pitting can be found in the T6 and SC-T6 sample. Corrosion crept in mostly along with the GB, but in some areas, the matrix is also attacked after the matrix is exposed to the corrosive solution. The pre-deformed T8 sample sustained purer IGC with less corrosion within the matrix as shown in the cross-sectional metallography image (Figure 4(b)). The average corrosion rate of the T8 sample is also the lowest of the three artificially aged 6061 alloys.

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Figure 4

(a) Current transients for 6061 alloy samples potentiostatically held at −650 mV in de-aerated 3.5% NaCl solution, (b) cross-sectional view and (c) top view of the exposed surfaces.

As for the naturally aged and AQ 6061 alloys, minor existence of IGC was found on the exposed surface of the T4 sample but none on the pre-deformed T3 sample, even though the two samples are similar in corrosion rate and showed little difference from cross-sectional images. The dissimilarities between the T3 and T4 sample in corrosion rate and corrosion behaviour become more pronounced at higher scanning potentials which will be explained later. The AQ specimen, tested as reference on the other hand, exhibits the highest corrosion resistance since no precipitation has taken place yet in the solid solution matrix.

Increasing the potentiostatic scanning potential to −625 mV vs. RE, corrosion becomes more severe after the 18 000 s test, as shown in Figure 5(a). Compare with the potentiostatic polarisation at −650 mV, the naturally aged 6061 alloys showed more increase in average corrosion rate than the artificially aged 6061 alloys. As shown in Figure 5(b,c), a combination of IGC and matrix pitting dominates the corrosion of artificially aged 6061 alloys at this potential. As for the naturally aged 6061 alloys, the slight IGC found in the T4 sample in Figure 4(c) no longer exist at higher scanning potentials, and the two naturally aged samples are now only susceptible to pitting. However, the way of pitting between the T4 and the pre-deformed T3 sample starts to differentiate in both distribution and depth, despite little difference can be told by the current transient. The pits in the T3 sample seems more wide spread and remains closer to the exposed surface, while the pits in the T4 sample seems more localised and penetrating into a higher depth. It is worth noticing that beside the initial current increase, sudden increases in corrosion current during the scanning process of the artificially aged 6061 alloys can be observed in Figure 5(a). As shown in Figures 5 and 6, this sudden increase of corrosion current repeatedly appears in the experimental results, whenever the average corrosion rate of the tested sample reaches beyond a certain level (500 μA cm⁻² approx.). Luckily the sudden increase is limited within the range of 100 μA cm⁻² and does not affect the total trend of the current transient. Nevertheless, the true cause of this phenomena, whether it is due to diffusion factors or the sudden breakdown of certain passive films, will require future attention. For now, solution circulation has been attempted to exclude concentration polarisation during the experiments and does not alter such experimental results.

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Figure 5

(a) Current transients for 6061 alloy samples potentiostatically held at −625 mV in de-aerated 3.5% NaCl solution, (b) cross-sectional view and (c) top view of the exposed surfaces.

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Figure 6

(a) Current transients for 6061 alloy samples potentiostatically held at −600 mV in de-aerated 3.5% NaCl solution, (b) cross-sectional view and (c) top view of the exposed surfaces.

Effect of pre-deformation on IGC and pitting resistance

At higher scanning potential above breakdown potential, pre-deformation begins to show noticeable influence on both corrosion rate and mechanism of naturally aged 6061 alloy, as shown in Figure 5. Potentiostatic polarisation at −600 mV vs. RE and 3D reconstruction of the tested samples were done to double check such findings, with results illustrated in Figures 6 and 7. The AQ 6061 alloy is again tested as a reference at this scanning potential. As shown in Figure 6, the difference between the T3 and T4 samples becomes even more obvious. The pre-deformed T3 sample not only demonstrates a lower average corrosion rate in Figure 6(a), but the pitting is also restricted closer to the surface, acting similar to general corrosion. Conversely, the non-pre-deformed T4 sample demonstrates a higher corrosion rate during the beginning of the potentiostatic scan. The pits also continue to remain localised and penetrating much deeper into the material, as shown in Figure 6(b). However, it is inaccurate to determine the pitting depth alone by metallographically inspecting a single cross-section of each sample. Therefore, results of 3D reconstruction of the tested samples are illustrated in Figure 7. Such an approach investigates the distribution of pitting corrosion over a wider area which provides a more comprehensive understanding to the effect of pre-deformation on pitting distribution. Results shown in Figures 6 and 7 are in high correspondence. The corrosion in the T3 sample covers a larger area on the exposed surface but digging only 85 μm into the material, while the corrosion in the T4 sample seems less severe from the top view but penetrating up to 300 μm into the material, which is more than three times deeper than the T3 sample. Consider the average corrosion rate of the T3 sample (572.64 µA cm⁻²), which is only 93 µA cm⁻² lower than that of the T4 sample (665.55 µA cm⁻²), it is clear that the pitting behaviour of the T3 sample is less harmful to the material in terms of corrosion depth. Surprisingly, at −600 mV, corrosion behaviour of the AQ sample behaves rather similar to the T4 sample, which demonstrates localised and deep pits. Such a finding is considered reasonable because microstructures of the AQ and T4 sample are nearly the same except for the presence of GP-zones in the T4 sample. Besides, −600 mV scanning potential is already much higher than the E _br values of both of these 6061 alloy samples, so the difference in E _br values between the two samples can no longer affect the result in long-term potentiostatic polarisation. One point worth noticing is that pitting in naturally aged 6061 alloys tend to spread horizontally after the pits develop into sub-surface areas. This is why some pits seem isolated from the surface in the metallographically prepared cross-sectional images as shown in Figures 4–6.

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Figure 7

3D reconstruction of T3 (a,c,e,g) and T4 (b,d,f,h) sample after potentiostatic polarisation at −600 mV in de-aerated 3.5% NaCl solution. (a,b) Solid view of the materials left after potentiostatic scan, (c,d) distribution of empty corroded areas (distribution of pits) after potentiostatic scans, (e,f) cross-sectional views from the highlighted direction in (c and d) and (g,h) binarised images before stacking and 3D reconstruction.

Results from Figures 5–7 have shown that pre-deformation can significantly influence the corrosion behaviour of naturally aged 6061 alloys during potentiostatic scans above their breakdown potentials. The corrosion of the pre-deformed and naturally aged T3 sample is restricted closer to the surface which causes less harm to the material. Possible explanations of this phenomenon are illustrated in Figure 8. Figure 8(a) shows the dislocations found in the T3 sample near the GB under 500 k×. In Figure 3(d), some dislocations in the T3 sample originate from the GB and extend into the grain, while some dislocations originate within the grain matrix. Even though these dislocations also exist in the artificially aged T8 sample, the T8 sample demonstrates totally different corrosion behaviour than the T3 sample, indicating that these dislocations may not be the only factor affecting the IGC sensitivity of 6061 alloy. Looking even closer at Figure 8(b,c), considerable precipitates can be found very close to the dislocation and the GB. These small precipitates can be identified as β″ by the strained contrast of the adjacent matrix [20]. Precipitates this close to GB were not found in the T4 sample, not to mention those artificially aged samples with clear PFZs and large GBPs. It can be suggested that 6061-T3 demonstrates the most homogeneous precipitation across GB areas, thus exhibits the highest localised corrosion resistance among all samples tested in this study. Presumably, the pre-deformation process may have produced considerable misorientation sites other than the visible dislocations. For areas neighbouring the GB, these potential sites for precipitate nucleation cover the lack in AQ vacancies, and with the dislocations acting as diffusion shortcuts [23], the precipitation of small β″ is encouraged. For artificial aging conditions on the other hand, as soon as the material temperature was raised after quenching to conduct subsequent artificial aging, the AQ vacancies adjacent to the GB will be given the energy to rearrange or diffuse into the GB, forming vacancy free zones before the nucleation of precipitates. However, comparing the microstructure of the T8 sample to the other artificially aged 6061 alloys in Figure 2, the T8 sample possesses GBs alike naturally aged 6061 alloys, with only little and few GBPs present, as shown in Figure 2(e). Nevertheless, despite PFZs were still found in the T8 sample, a more distinctive IGC as well as a lower average corrosion rate can be found in the T8 sample compare to other artificially aged samples, as shown in Figure 4. This suggests that the pre-deformation process can still affect the formation of PFZ during artificial aging. The strain-induced dislocations and misorientation sites may still act as additional precipitation sites which might help narrowing the vacancy free zone for the pre-deformed 6061 alloy during artificial aging, and in the end lead to narrower PFZs (highlighted area in Figure 2(e)) and less GBPs. This also means that the pre-deformation parameter and the amount of deformation generated by such process could be crucial factors influencing the precipitation behaviour around the GB area under artificial aging condition. The pre-deformation in this study was done by tensile stretching, which is harder to achieve higher thickness reduction than cold rolling. Hence, investigating the effects of different pre-deformation methods and parameters on the GB-related microstructure and corrosion behaviour of artificially aged 6061 alloy will be the primary goal in future studies.

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Figure 8

Bright-field GB area TEM micrographs of 6061 alloy T3 sample. (a) Dislocation taken at 500 k×, (b) dislocation taken at 1 M× and (c) precipitates taken at 1 M×.

Additionally, pre-deformation was found to enhance the ability of re-passivation for artificially aged T8 sample at lower potentiostatic scanning potential, as shown in Figure 9. The scanning potential of −675 mV is under the E _br values of the naturally aged samples, hence in Figure 9(a), the T3 and T4 sample re-passivate without a doubt. The initial anodic corrosion current of both naturally aged samples drop rapidly and turn into cathodic current as soon as the scanning potential of −675 mV is reached and held. The pre-deformed and artificially aged T8 sample nearly succeed in re-passivating during the −675 mV scan, demonstrating an average corrosion current density of just 0.02 µA cm⁻², even though the scanning potential is just around its E _br value. The non-pre-deformed T6 sample however, failed to re-passivate, despite sharing the same E _br value as the T8 sample. The current transient of the T6 sample remains anodic during the 5 h scanning at −675 mV, demonstrating an average corrosion current density of 7.16 µA cm⁻². No visible corrosion can be observed from the T3, T4 and T8 samples either from cross-sectional view or top view of the exposed surface. Sadly, despite the low scanning potential and corrosion rate, clear IGC still makes an appearance on the T6 sample, as shown in Figure 9(b). For 6061-T8, the slightly improved corrosion resistance compared with 6061-T6 could be caused by the decrease in the amount and size of GBPs and the marginally narrower PFZs. The slightly overaged microstructure with the Q′ phase extracting Cu from the matrix and reducing the micro-coupling between the matrix and the PFZ [18] is also considered as an additional factor.

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Figure 9

(a) Current transients for 6061 alloy samples potentiostatically held at −675 mV in de-aerated 3.5% NaCl solution and (b) IGC observed from the exposed surface of T6 sample.

Comparing with AA2024 [4], which has been tested with similar methods in this study, one point is worth noticing. Two breakdown potentials were observed in potentiodynamic polarised AA2024-T3 in de-aerated chloride solution, and the more noble one represented the transition between pitting and IGC [4]. In this study, the 6061 alloy demonstrates only one breakdown potential during the potentiodynamic polarisation, and the transition between pitting and IGC does not occur. The breakdown potential of 6061 alloy observed in potentiodynamic polarisation only indicates the initial of severe corrosion, either pitting or IGC. More importantly, the corrosion mechanism of 6061 alloys of various temper conditions remains unchanged under different potentiostatic scanning potential. Consequently, it is suggested that potentiostatic polarisation could be a suitable and accurate method for identifying the IGC sensitivity of 6061 alloys.