CONCLUSIONES Y RECOMENDACIONES

Etching with 60 °C phosphoric acid has been shown to preferentially attack any β-phase present in the alloy, and can give a visual indication of how susceptible to IGC an alloy is. When the AA5083-H321sample is polished, etched and observed under an SEM, very little attack is seen, Figure 70 (a) and (d). Very small, discreet sites have been removed but are not indicative of any amount of grain boundary precipitates being present.

In contrast, when the sensitised sample is etched, Figure 70 (b) and (e), an extensive network of susceptible grain boundaries is revealed. The attack or removal of β-phase is continuous and along every grain boundary where precipitates were shown to exist under TEM. This indicates while the sensitisation heat treatment was not sufficient to affect the large intermetallics, is has been able to promote segregation and precipitation on the grain boundaries.

When the HAZ of the sensitised then processed sample was etched under the same conditions, the resulting surface showed no continuous grain boundary etching, and only small, discreet attack sites, Figure 70 (c) and (f). This result is comparable with that of the AA5083-H321 sample, Figure 70 (d), and indicates that the heat from processing that creates the HAZ is sufficient to remove the susceptibility to IGC that the sensitisation brought about. There is, however, enough attack to able to make out the outlines of grains.

Figure 70 (a) (b) and (C) SEM images of the as AA5083-H321, AA5083-sensitised, and processed samples respectively plate after etching in phosphoric acid to reveal the degree of β-phase precipitation (10% solution at 60°C for 30 seconds). (d) (e) and (f) are magnified views of the same samples

5.2.Corrosion testing

5.2.1. Immersion testing

In order to observe the corrosion behaviour of the various conditions in this study, the sample was immersed in a corrosive solution and the morphology of the corrosion which takes place was observed. To do this, the top surface of the plate was mechanically removed to create a level surface, which included removing the nugget region which was made up of the FSP track seen in Figure 63 (c). Samples were polished in accordance with the methods outlined earlier, then immersed for 6 weeks after which optical and SEM analysis were carried out. Figure 71(a) (b) and (c) show micrographs of the corroded surfaces and the corrosion product left on the surface. The AA5083-H321 sample shows little corrosion product over the majority of the surface but does show several small sites where the corrosion product has built up. The sensitised sample shows a large degree of corrosion product over the whole of the surface implying that corrosion has taken place across the whole surface to a greater extent, and several sites where a large degree of corrosion product has built up. The sensitised then processed sample shows a good degree of corrosion product spread across the whole surface, however differs from the previous conditions in that no large sites of corrosion product can be seen.

Figure 71 (d) (e) and (f) are micrographs of the immersed samples after the corrosion product has been removed. The AA5083-H321 sample shows no signs of large scale corrosive attack, while in the sensitised sample a large site of attack is clearly visible. The sensitised then processed sample shows no large scale corrosive attack, much like the AA5083-H321 sample.

Figure 71 (a) (b) and (c) Optical images from the AA5083-H321, AA5083-sensitised and sensitised then processed samples respectively after immersion in 1M NaCl for 6 weeks. (d) (e) and (f) Optical images of same AA5083-H321, AA5083-sensitised and sensitised then processed surfaces respectively after corrosion product has been removed.

Figure 72 (a) (b) and (c) show detailed micrographs of the immersed samples where the corrosion product has been removed. The size of the grain fall-out from the sensitised sample can clearly be seen, when compared to the AA5083-H321 and AA5083-sensitised then processed samples which show no severe corrosion. Figure 72 (d) (e) and (f) show the corrosion in greater detail. In contrast, the sensitised then processed samples show only cathodic grooving around the Fe-based intermetallic particles, supporting the hypothesis that the processing can remove the susceptibility to IGC that sensitisation can bring about.

Figure 73 shows micrographs of the immersed samples after the corrosion product has been removed and the surface etched with phosphoric acid to reveal the underlying grain structure and highlight those which are susceptible. The corrosion of the AA5083-H321 sample does not follow any of the grain boundaries which the etch has exposed, showing the corrosion not to be intergranular in nature. The edge of the large site of grain fall out in the sensitised sample is detailed in Figure 73 (b), where the IGC propagation outwards from the centre of the site follows the susceptible grain boundaries which have been exposed. The processed sample shows neither susceptible grain boundaries, nor any form of intergranular corrosion, confirming that this material is not sensitised.

Figure 72 (a) (b) and (c) Optical images from the AA5083-H321, AA5083-sensitised and sensitised then processed samples, respectively, showing a close up of the corrosion on the surface. (d) (e) and (f) SEM images of same AA5083-H321, AA5083-sensitised and sensitised then processed surfaces respectively showing closer detail of the corrosion that has taken place.

Figure 73 (a), (b) and (c) Optical micrographs of the AA5083-H321, AA5083-sensitised and sensitised then processed samples after immersion in 1 M NaCl for 6 weeks, with the corrosion product removed, then etched in phosphoric acid to reveal the susceptible grain boundaries.