The Corrosion Resistance and Paint Adhesion Properties of Chromate Conversion Coatings on Aluminium and Its Alloys

A non-proprietary process for the chromate conversion coating (chromate filming) of aluminium and its alloys has been evaluated with respect to both corrosion resistance of, and paint adhesion to, the chromate films. The process involves immersion of the metal in an aqueous chromic acid/sodium dichromate/sodium fluoride solution for three minutes at 30C. Iridescent, yellow-coloured films result. Alternative times and temperatures of immersion, pretreatments of the metal, washing and drying of chromate-filmed test pieces, and modes of application were examined. The chromate film thicknesses were measured and their corrosion resistance compared. Except for thin films (Less than 50nm) corrosion resistance did not vary markedly with thickness. Comparisons were also made with two proprietary processes and no major differences were found in corrosion resistance or paint adhesion properties of the different chromate films. Of four methods used for assessing corrosion resistance, exposure to continuous 5% neutral salt fog was the best, and paint adhesion was evaluated by using two British Standards Institution test methods. The findings in this Report will be used as the basis for a Defence Standard for chromate conversion coatings for aluminium and aluminium alloys.