Corrosion behaviour of aluminium alloy machining defects and quality of subsequently formed anodised oxide coatings

Several types of defect have been reproduced on machined surfaces pf aluminium alloy 7010, and their corrosion response in saline solutions has been studied. The manner in which this corrosion influenced the quality of subsequently formed anodised coatings was also examined. It was found that chip buildup on the tool face could impart an extreme form of surface disturbance that was not necessarily revealed even by microscopic examination of the as machined surface. Such damaged al eas, when subjected to a corrodent, promoted what appeared to be a rapid cathodic reaction, with the immediate formation of a film that effectively thickened in discrete steps owing to the bull owing action of other advancing films. As the film thickened active metal attack occurred beneath it, first by general tunnelling through the machining flow zone, then by advancing in an intergranular manner into the undisturbed microstructure. The characteristic insoluble particles present appealed to play a completely separate role from the intergranular attack, although they were frequently engulfed by it. Some particles seemed to activate the formation of deep, smooth sided pits, but these were insignificant compared with the section loss presented by the intergranular attack, which was also very much deeper. Intergranular corrosion paths in the parent metal resulted in deep crevices in subsequently formed anodised oxide coatings that frequently penetrated the oxide/metal interface. These were associated with a residue of metal grains that had been electrically isolated from the oxidation process.