Use of video techniques to study the electrochemical activation and corrosion of aluminium alloys

Abstract

In situ video techniques have been used to examine the galvanostatic initiation of pits on two Al–Mg–Sn–Ga alloys and an Al-In alloy demonstrating the switch-on of the anodic-discharge reaction. Surface magnifications in the range ×25 to ×200 have been employed. Surface effects were studied during electrochemical activation and deactivation of the alloys and during the voltage-oscillation phenomenon observed with Al–In alloys when under discharge in alkaline solutions. Alloys undergoing non-superactive behaviour exhibited a much more crystallographic pitting behaviour than those displaying superactive behaviour, the latter showing much more rounded smooth-sided pits. The time required for battery alloys to activate fully, i.e. switch-on, was related to the number of pits per unit area generated. The formation of hydrogen bubbles during switch-on caused a very noisy voltage transient. The voltage oscillatory behaviour of Al–In under discharge was explained by use of the observation that during voltage rise and fall there was sympathetic growth and dissolution of a surface film. This cyclic film growth and dissolution was proposed to cause the voltage oscillation through its oscillatory effect on the specimen area undergoing corrosion.