Anodising as pre-treatment before organic coating of extruded and cast aluminium alloys

Anodising has been investigated as a replacement for chromating as pre-treatment prior to organic coating of aluminium. Both AC and DC anodising gave filiform corrosion properties that were comparable to chromated samples. A correlation between filiform corrosion performance and cathodic properties of converted surface was found. Chromating and hot AC anodising gave significant reduction in cathodic current density. The reduction is due to chromium oxide covering intermetallic particles after chromating, while iron rich intermetallics normally acting as cathodic sites were removed by hot AC anodising.