Filiform corrosion of aluminium sheet. III. Microstructure of reactive surfaces

Metal surface properties, which induce susceptibility to filiform corrosion (FFC) of certain impure aluminium alloys by high temperature annealing, were investigated by analytical transmission electron microscopy (TEM). Analysis of cross-sectional foils revealed grain refined surface layers (GRSLs), approximately 1 mu m thick on the rolled sheet, consisting of subgrains in the size range 20 to 200 nm. This layer was electrochemically more active than the bulk of the metal, and it was preferentially attacked by FFC of the painted substrate. By cold-rolling, a GRSL was also formed; however, such a layer did not cause surface activation and FFC. Annealing the etched surface not covered by a GRSL also activated the surface, indicating, together with the behaviour of cold-rolled surfaces, that GRSLs alone were not responsible for the surface activity. The analytical techniques employed were not able to detect any chemical modification of the surface which could explain the high surface reactivity. Possible mechanisms for the formation of reactive surface layers and the GRSLs are discussed. An obvious measure for avoiding FFC on painted aluminium sheet is to remove the active layer by etching or other chemical treatments prior to application of coating. (C) 1998 Elsevier Science Ltd. All rights reserved.