ELECTRON METALLOGRAPHY OF THE ALUMINUM-WATER VAPOR REACTION AND ITS RELEVANCE TO STRESS-CORROSION SUSCEPTIBILITY

The reaction of filmed surfaces of aluminium and certain aluminium alloys with water vapour saturated air at 70° C has been morphologically studied at high resolution using a JEOL 100C Temscan. Electron-transparent specimens, reacted for up to 24 h, have been examined using a combination of SE (secondary electron) and STE (scanning transmission electron) imaging modes to examine both the surface attack detail and the underlying microstructure of precisely located areas. Reaction on filmed surfaces is initiated by a hydrogen-induced blistering of the amorphous oxide film, the fracture of which results in the development of pseudoboehmite and bayerite on the bared metal surface. Alloying additions of magnesium localize the breakdown reaction at grain boundary-surface intersections, although this effect can be controlled by raising the solution heat treatment temperature to 550° C. The localization of the reaction results in hydrogen penetration of grain boundaries in magnesium-containing alloys, and this promotes a loss of grain-boundary strength and may lead to alloy failure by stress-corrosion cracking. © 1979 Chapman and Hall Ltd.