Hydrogen embrittlement aluminium alloys

Later work on aluminium alloys has also focused more closely upon the role of hydrogen which had not previously been widely considered as an embrittling species in the stress-corrosion cracking process for these alloys. The idea was not new, however. Reports of intergranular failure under cathodic charging conditions had been made at a much earlier time . A reduction in stress-corrosion life and alloy ductility in a high purity Al-5Zn-3Mg alloy had been found in specimens pre-exposed to a 2% NaCI solution" , an effect that was accentuated if specimens were stressed". ... 

In more recent work embrittlement in water vapour-saturated air and in various aqueous solutions has been systematically examined together with the influence of strain rate, alloy composition and loading mode, all in conjunction with various metallographic techniques. The general conclusion is that stress-corrosion crack propagation in aluminium alloys under open circuit conditions is mainly caused by hydrogen embrittlement, but that there is a component of the fracture process that is caused by dissolution. The relative importance of these two processes may well vary between alloys of different composition or even between specimens of an alloy that have been heat treated differently. 

The embrittlement caused to aluminium alloys by pre-exposure to moist atmospheres or stress-corrosion environments is thought to be due to hydrogen in the atomic form. Intergranular bubbles of hydrogen, formed in association with certain precipitates, have been observed by HV and... 

Table 10.9 lists some common zinc anode alloys. In three cases aluminium is added to improve the uniformity of dissolution and thereby reduce the risk of mechanical detachment of undissolved anode material . Cadmium is added to encourage the formation of a soft corrosion product that readily crumbles and falls away so that it cannot accumulate to hinder dissolution. The Military Specification material was developed to avoid the alloy passivating as a result of the presence of iron . It later became apparent that this material suffered intergranular decohesion at elevated temperatures (>50°C) with the result that the material failed by fragmentation". The material specified by Det Norske Veritas was developed to overcome the problem the aluminium level was reduced under the mistaken impression that it produced the problem. It has since been shown that decohesion is due to a hydrogen embrittlement mechanism and that it can be overcome by the addition of small concentrations of titanium". It is not clear whether... 

Heat treatment after plating Heat treatment may be necessary after plating to improve the adhesion of coatings on aluminium and its alloys when certain processes, e.g. the Vogt process, are used, or to minimise hydrogen embrittlement of steel parts. Care is needed since heating may distort the part and impair the mechanical properties of the substrate. 

Until the end of the 1960s, the theory of hydrogen embrittlement was considered inapplicable to aluminium alloys, for two reasons ... 

Embrittlement of aluminium by hydrogen does not occur, even at very high pressure. Unlike steel, aluminium alloys are not sensitive to embrittlement in a dry hydrogen atmosphere. 

---