Mechanism of intergranular attack in Zn–Al–Cd sacrificial anodes at elevated temperatures

Abstract

The premature failure of a Zn–0·3 Al–0·03Cd anode at 70°C in sea bed mud has been simulated in laboratory sea water using galvanostatic polarisation. The exposed surface of the anode suffered intense intergranular attack and some dissolution. Within the bulk of the material intergranular attack was observed, but no dissolution. Unpolarised alloys in a variety of environments exhibited the same type of attack; however, it could not be produced on pure zinc. No evidence could be found of segregation or precipitation of aluminium at grain boundaries. Moreover, specimens that had been solution treated to ensure a single phase microstructure suffered from intergranular attack in short term simulation tests. It is concluded that a previously proposed mechanism consisting of grain boundary precipitation of aluminium followed by its preferential dissolution is incorrect. It is proposed from analysis of the fracture morphology and the effect of test conditions that the failure is caused by hydrogen penetration.