Surface reactions during pickling of an aluminium-magnesium-silicon alloy in phosphoric acid

The surface products formed during pickling of a modified AA 6000 alloy (0.70Mg1.52Si) in 0.1 M phosphoric acid were analysed using electron spectroscopy for chemical analysis, Auger electron spectroscopy, scanning electron microscopy and energy dispersive X-ray spectroscopy. Cold-roiled and heat-treated samples were compared. Pure Si, Mg2Si and AlFeSi particles are present in the material after the heat treatment. Pickling gives a chemical dissolution of the surface oxide and an anodic dissolution of the Mg2Si particles. The degree of dissolution increases with the pickling time. On the cathodic AlFeSi particles, aluminium phosphate is deposited. After prolonged pickling, micropits are visible on the surface. It is suggested that this is due to the dissolution of small Mg2Si particles precipitated during the annealing process. (C) 2001 Elsevier Science Ltd. All rights reserved.