Contribution of Hydrogen Embrittlement to SCC Process in Excess Si Type Al-Mg-Si Alloys

For underaged Al-Mg-Si alloys with excess Si, No.~5(Al-0.7 mass%Mg-1.1 mass%Si) and No.~C5 with 0.2 mass%Cr addition, SSRT tests have been carried out to reveal the contribution of hydrogen embrittlement (HE) to SCC processes at strain rate 6.9× 10-7 s-1 under three environments; (1)dry nitrogen gas, (2)wet air with 90% relative humidity and (3)an acid sodium chloride (ISO) solution. Under env.(2), alloy No.~5 with coarse grains shows a decrease in elongation comparing with that under the inert env.(1), while alloy No.~C5 with finer gains exhibits rather an increase. The small areas of intergranular(IG)- and transgranular(TG)-facets both with a feature of wavy slips are observed in contact with the free-surface of the specimens, respectively, which are regarded as an evidence of hydrogen-enhanced localized plasticity. Under env.(3), alloy No.~5 shows a high susceptibility to SCC, while alloy No.~C5 exhibits a low one improved through Cr addition. On SCC fracture surface of alloy No.~5, three modes of IG one with crystallographic pits, IG another with fine ledges and TG one with an appearance of quasi-cleavage are presented, which indicates that the mechanism of plastic deformation localization induced by anodic dissolution plays a dominant role in the SCC. Even though HE is involved in the SCC process, the effect is estimated to remain small.