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

Abstract

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; \\ding172dry nitrogen gas, \\ding173wet air with 90% relative humidity and \\ding174an acid sodium chloride (ISO) solution. Under env.\\ding173, alloy No. 5 with coarse grains shows a decrease in elongation comparing with that under the inert env.\\ding172, 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.\\ding174, 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.