Formation of zinc phosphate coatings on AA6061 aluminum alloy

Conditions for forming zinc phosphate conversion coatings on AA6061 aluminu m alloy have been investigated by characterizing coatings formed for differ ent parameters of the coating bath. Morphological and compositional informa tion on the coatings was assessed by SEM, EDX and XPS, and simple adhesion tests were undertaken to indicate the strengths of coating attachment. The emphasis was to identify conditions that give high coverage, uniform coatin gs of small, strongly adhered, zinc phosphate crystals. The use of low-zinc solutions (e.g. an atomic Zn/P ratio of 0.07) and normal-zinc solutions (Z n/P ratio 0.25) were compared; coatings formed by the two solution types ap pear comparable at pH 2, although at pH 4 the low-zinc solution is more eff ective. Fluoride in the concentration range 200-400 ppm is indicated to be a useful additive for the normal-zinc coating bath and in the 600-1000 ppm range for the low-zinc process. The use of acid etching in the pre-treatmen t appears to yield better coatings than when mechanical polishing alone is used. (C) 2001 Kluwer Academic Publishers.