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5.4.3.1. Immersion testing

Figure 71 (d) and Figure 72 (d) demonstrate the low degree of corrosion which the AA5083-H321 material undergoes when immersed in 1 M NaCl for 6 weeks. These observations are consistent with other studies into the corrosion behaviour of AA5083 [55]. After sensitisation, the same pattern of localised corrosion around intermetallic particles still exists, however there are several large sites of intergranular corrosion, highlighted in Figure 72 (b), Figure 72 (e), a finding consistent with other studies of Al-Mg alloys [71, 81, 188]. Confocal Laser Scanning Microscopy of the AA5083-H321 sample after immersion, Figure 74, shows there to be no intergranular attack. This behaviour is similar to that seen in AA5083-H116 exposed to 1 M NaCl for 1 week, which has also shown localised pitting

corrosion [39], while AA5456-H116 showed pits up to 6 µm in diameter after immersion in 3.5 % NaCl for 2 weeks [89].

Measurements obtained in this study show the sensitised sample to have suffered from a large area of IGC and grain fallout after 6 weeks of corrosion, Figure 75. This is far greater than the average volume of the corroded regions, 23.1 µm3, reported in AA5456-H116 which had been sensitised for 100 hours at 150 °C and corroded for 2 weeks in a 3.5 % NaCl solution [89]. This is likely due to different sensitisation and corrosion times as well as the slightly different composition of the alloy.

In the HAZ of the sensitised then processed sample, the morphology of the corrosive attack, the etch response and TEM observations are all comparable to the AA5083-H321 material, indicating the removal of β-phase from grain boundaries returns the material to a state that is not susceptible to IGC.

5.4.3.2. Atmospheric corrosion

Atmospheric corrosion of both AA5083-H321 and sensitised material shows evidence of IGC. However that which occurs in the sensitised material covers a larger area, indicating a greater susceptibility, Figure 78 (d) and (e) and Figure 80 (a). No literature exists about the atmospheric corrosion of Al-Mg alloys; however grain boundary attack by this method of corrosion has been seen in the 2024 aluminium alloys [105, 189].

The occurrence of IGC in the AA5083-H321 sample under atmospheric corrosion but not under fully immersed corrosion may show the different types of corrosion that the testing conditions will promote. The droplet that was placed on the surface of the sample was of 1 M NaCl concentration, and would have then reached an equilibrium with regards to droplet

concentration and volume. As only single droplets were placed on the surface as opposed to the sample being fully immersed it is likely that the ability of oxygen to reach the surface of the sample at the thinner, outer edge of the droplet would be increased, thus increasing the rate of reduction reactions.

After 6 weeks of atmospheric corrosion, very little corrosion has taken place in the HAZ of the sensitised then processed sample Figure 78 (f). None of the 12 droplets on the HAZ region showed as much corrosion as any droplets that had been on the parent material under the same conditions, lending to the idea that the heat imparted to this region has removed the susceptible grain boundary phase to the point where corrosion behaviour has become better than the H321 temper.

5.4.3.3. Electrochemistry

It is evident from the results of potentiodynamic sweeps of the AA5083-H321, sensitised and processed samples that very little difference exists in the corrosion potential and breakdown potential of the various conditions, Figure 84.

Chang tested the intergranular susceptibility of AA5083 before and after applying a heat treatment of 500 °C for 1 hour [188]. The heat treatment was shown to significantly increase the intergranular corrosion susceptibility of the sample (tested by NAMLT), but most crucially lowered the corrosion potential from -842 to -992 mV(SCE) and the breakdown potential at 0.1 mA/cm2 from -653 to -740 mV(SCE). This was attributed to the greater reactivity of the β-phase which is highly anodic to the alloy matrix. While the amount of β- phase on the grain boundaries was not quantified or observed, the significant change in

breakdown potential which occurs compared to the current study implies that it may be relatively large compared to the small, discontinuous precipitates observed here.

Chang also followed the sensitisation with further annealing at 345°C for 1 hour, which had the effect of returning the NAMLT values to those of the AA5083-H321 sample and returning the breakdown potential to within 6 mV(SCE) of the base sample [188]. These results concluded that the IGC susceptibility of the sample could be “eliminated by dissolution of the continuous precipitation layer along the grain boundaries”, a finding consistent with both Summerson and Sprowls [190], and Dix [40].

Potentiostatic polarisation for 1 and 24 hours shows a dramatic increase in current density in the sensitised sample over the AA5083-H321 sample and sensitised then processed region, which show a passive current density, Figure 85 and Figure 87. A similar difference in polarisation behaviour has been seen in a 5083 alloy in the H116 temper and a sensitised condition [39].

The effect of the polarisation on the surface of the sample after 1 and 24 hours is shown in Figure 86 and Figure 88 where it is evident that the sensitised sample shows a much greater degree of attack than the AA5083-H321 sample.

5.5.Conclusions

1. Sensitisation of AA5083 for 336 hours at 100 °C leads to the formation of β-phase on grain boundaries, causing susceptibility to intergranular corrosion.

2. Friction surface processing of sensitised AA5083 leads to the formation of a heat-affected zone under the tool piece in which β-phase has been removed from the grain boundaries, leading to improved resistance to intergranular corrosion.

Chapter 6 – The effect of re-sensitisation of processed material on microstructure and