Influence of heat treatment and surface conditioning on filiform corrosionof aluminium alloys AA3005 and AA5754

The effect of high-temperature heat treatment combined with different clean ing and pretreatment practices on filiform corrosion resistance has been in vestigated for aluminium alloys AA3005 and AA5754. High-temperature heat tr eatment severely reduces the filiform corrosion resistance of alloy AA3005, while the corrosion properties of alloy 5754 are only moderately affected. The drastic loss of filiform corrosion resistance of alloy AA3005 after hi gh-temperature heat treatment is attributed to preferential microstructural changes in a heavily deformed, micrograined surface layer caused by large surface shear strain during rolling. The enhanced deformation of the near-s urface region promotes precipitation of a fine dispersion of intermetallic particles during subsequent heat treatment. The higher density of intermeta llic particles combined with lower supersaturated solid solution levels of manganese in the surface layer as compared to the bulk structure makes the heat-treated AA3005 material susceptible to superficial corrosion attacks a nd results in poor filiform corrosion resistance, Application of a commerci al acid cleaning/chromating pre-treatment did not restore the filiform corr osion resistance of heat-treated alloy AA3005. Alloy AA5754, containing low er levels of manganese and iron than AA3005, did not undergo similar prefer ential microstructural changes during heat treatment. A moderate increase i n the amount of filiform corrosion of heat-treated AA5754 samples is attrib uted to poor protective properties of the thick, magnesium enriched, therma lly formed surface oxide on this alloy. Any cleaning/pre-treatment practice that removes the thermally formed oxide on this alloy results in a very hi gh filiform corrosion resistance. (C) 2001 Elsevier Science Ltd. All rights reserved.