OBSERVATIONS RELATED TO THE PHOSPHATING OF ALUMINUM-ALLOY 7075-T6 USING A SPRAYING TECHNIQUE

An initial review is made of the effects of changing parameters involv ed 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 coa ting solution reported represents a modification of that previously us ed for coating the same system by the dipping method (J. Mater. Sci. 3 1 (1996) 565), but for spraying it was necessary to add the accelerato rs KClO3 and NaNO2 in order to achieve an adequate phosphate layer. Fo r this solution, the favoured coating conditions by spraying correspon ded to 1 min at 85 degrees C. A significant effect on the phosphating process is also indicated for the polishing pre-treatment of the subst rate; the coating formed on a surface polished by 1200 grid aluminium oxide sandpaper showed good adhesion to paint, whereas that polished w ith 1200 grid silicon carbide failed to form an adhesive phosphate coa ting. In general, conditions for spraying are less easy to control tha n for dipping, but directions are indicated for obtaining promising co atings by the former approach. (C) 1998 Chapman & Hall.