Observations related to the phosphating of aluminium alloy 7075-T6 using a spraying technique

Abstract

An initial review is made of the effects of changing parameters involved in forming a zinc phosphate coating layer by spraying an aluminium alloy 7075-T6. Various coatings were evaluated by X-ray photoelectron spectroscopy, scanning electron microscopy and adhesion tests. The coating solution reported represents a modification of that previously used for coating the same system by the dipping method (J. Mater. Sci. 31 (1996) 565), but for spraying it was necessary to add the accelerators KClO₃ and NaNO₂ in order to achieve an adequate phosphate layer. For this solution, the favoured coating conditions by spraying corresponded to 1 min at 85 °C. A significant effect on the phosphating process is also indicated for the polishing pre-treatment of the substrate; the coating formed on a surface polished by 1200 grid aluminium oxide sandpaper showed good adhesion to paint, whereas that polished with 1200 grid silicon carbide failed to form an adhesive phosphate coating. In general, conditions for spraying are less easy to control than for dipping, but directions are indicated for obtaining promising coatings by the former approach.