Influence of Altered Surface Layer on Corrosion Inhibition of AA5083

The 5XXX series aluminum alloys are well known lightweight materials that provide excellent properties for marine applications. However, alloys with Mg concentration greater than 3.5% can sensitize, even at elevated service temperatures, by precipitation of β phase (Mg₂Al₃) along grain boundaries. Preferential dissolution of β phase on sensitized AA5XXX makes the alloy susceptible to intergranular corrosion (IGC) and stress corrosion cracking. Recent studies of sensitized AA5083 found that sustained β phase dissolution and IGC are not observed even at potentials well above the E_b of bulk β phase. High resolution TEM characterization of polished surfaces on sensitized AA5083 indicated the presence of an altered surface layer (ASL) with smaller grains and no grain boundary β phase. The corrosion behavior in chloride solution was different for samples that were etched to remove the ASL and the behavior of solutionized and sensitized AA5083 were clearly different after etching. Furthermore, corrosion inhibition with K₂CrO₄ was significantly influenced by removal of the ASL. Chromate acted as strong cathodic inhibitor on solutionized AA5083 samples both before and after etching to remove the ASL. Chromate was also a cathodic inhibitor for sensitized AA5083 with the ASL. However, mixed inhibition was observed for the AA5083 after etching, as a strong anodic inhibition was also observed on this sample on which β phase was apparently exposed at the surface.