Growth of hydrous oxide films on aluminium alloy AA6351 in glycol–water solution

Abstract

Growth of hydrated alumina films on low copper grade aluminium alloy AA6351 during immersion in deionised water and 50 vol.-% propylene glycol–water solution at their respective boiling points and at 80°C has been investigated using gravimetric and electron optical techniques. Generally, pseudoboehmite evolves under all conditions to cover the alloy surface; however, at 80°C in deionised water, bayerite crystals also develop and eventually cover the original filmed surface. In the presence of propylene glycol, reduced film hydration levels are evident which, with reduced film solubility, explain the absence of bayerite at 80°C. After prolonged immersion in both solutions at their boiling points, fine pitting is evident, localised at second phase material sites; the onset of localised attack is detected by changes in the potential and current noise responses as well as the currents flowing between coupled electrodes.