Molybdate/Al(III) composite films on steel and zinc-plated steel by chemical conversion

Abstract A molybdate(VI)–Al(III) chemical conversion process was developed as an alternative to the chromate processes. Steel and zinc-plated steel specimens were treated in the solutions of 0.16 mol l −1 ammonium alum (AlNH 4 (SO 4 ) 2 · 12H 2 O) with small amounts of ammonium molybdate(VI) (0.002–0.016 mol l −1 (NH 4 ) 6 Mo 7 O 24 · 4H 2 O) at 60 °C for 10–30 min in an ultrasonic rinsing apparatus. The formed films were composed of oxyhydroxides containing Mo(V,VI), Al(III), Fe(II,III), and sulfate ions (and Zn(II) ions in the case of zinc-plated steel), and showed good corrosion resistance in an aerated 0.5 mol l −1 NaCl–0.15 mol l −1 H 3 BO 3 solution (pH=7). The films macerated during the corrosion test, but they did not detach and functioned as a protective layer. This process may be useful in forming undercoats for paints and polymer coatings on steel and zinc-plated steel.