Influence of heat treatment on properties of chromate phosphate conversion coatings on aluminum.

Influence of heat treatment on organic coating adhesion and corrosion resistance of chromate phosphate conversion coatings was studied. Organic coating adhesion was evaluated on the basis of peeling strength of epoxy phenol paints at peel angle 180°. Corrosion resistance was investigated by means of salt spray test and retort test. Heat treatment at 200°C lowers adhesion property and corrosion resistance of the coatings. Studies by XPS, IR and DTA before and after heat treatment show that the main composition of the coatings is CrPO42-5H2O and that bound water of CrPO42-5H2O is dehydrated by heat treatment at 200°C×10min. TEM observation of the cross section of the coatings shows microcrack formation by heat treatment. Deterioration of adhesion property and corrosion resistance of the coatings by heat treatment is estimated to result from dehydration and microcrack formation of the film of conversion coating.