Filiform and pitting corrosion of aluminium alloy

The aim of this work is to investigate the localised corrosion behaviour of aluminium alloy AA6111 and the relationship between artificial ageing and corrosion susceptibility for the alloy. Filiform corrosion (FFC) on AA6111 is investigated in the first chapter. Scanning Kelvin Probe (SKP) potentiometry is used to measure the effect of artificial ageing and the presence of a deformed surface layer on the open circuit potential of the bare surface of AA6111 sheet in humid air. The SKP is then used to follow the kinetics of chloride-induced FFC and to map the evolving potentials which are characteristic of the FFC localised corrosion cell. It was shown that abrasion of AA6111 introduces a deformed surface layer which exhibits a significantly more negative potential than the bulk alloy. When FFC is initiated on polymer coated AA6111 in the presence of the deformed surface layer the observed attack is rapid and superficial; a slower and more deeply penetrating attack occurs in its absence. AA6111 is relatively resistant to FFC in the T4 temper but susceptibility increases as a result of artificial ageing. In the second chapter the methods used to initiate and propagate FFC in the first chapter were adapted and then evaluated on their ability to predict the FFC performance of a set of coated architectural aluminium sheet products. The method is shown to perform well when compared to standard accelerated corrosion tests. In the final chapter the Scanning Vibrating Electrode Technique (SVET) is used to investigate the localised corrosion of AA6111 in the T4 condition and after artificial ageing for between 0.17 and 56 hours at 185° C. Pitting corrosion was the dominant corrosion type. AA6111 is shown to be relatively resistant to pitting corrosion in the T4 temper but susceptibility increases as a result of artificial ageing.