Formation of nanocrystalline surface layers by annealing and their role in filiform corrosion of aluminum sheet

Annealing of specific rolled aluminum alloys at temperatures above 300 deg C causes a significant increase in the filiform corrosion susceptibility of the painted substrate. As demonstrated for a selected substrate (AA8006), the increased susceptibility is related to the formation of a thin, (electro)chemically active nanocrystalline layer at the surface as a result of the annealing treatment. The corrosion resistance is significantly improved by removing the layer by etching during surface treatment. Possible causes of layer formation and its activity are discussed.