The Effect of Alkaline-Etch Pretreatment on the Pitting Corrosion of Wrought Aluminum

Pitting corrosion of aluminum alloys is sensitive to the type of surface treatment used prior to a corrosion test. In particular, significant differences in the pitting behavior are observed between the as-received and etched surfaces of several wrought alloys. It is shown that the as-received surface is generally more resistant to pitting, since it is coated by a thicker oxide and because the cathodic intermetallic phases are covered by a thin layer of matrix alloy. Alkaline etching dissolves this layer, thereby increasing the cathodic area. Moreover, the exposed, Fe-containing intermetallics become enriched with iron at their surfaces as a result of dealloying of the more active aluminum, and these factors lead to a significant increase in the cathodic reaction rate. Enrichment of certain other components, especially manganese, at the surface of intermetallics and matrix alloy improves the pitting resistance.