Chromium-Free Coating for Al Alloy Corrosion Protection Based on a Novel Ti/Mg Oxyfluoride

Abstract

Chromium-free corrosion protection of aluminum alloys comparable to that of chromates (Cr(VI)) is demonstrated using a TiF6 2− fluoro-anion-based coating recipe containing boric acid and a Mg salt. Boric acid drove TiF6 2− hydrolysis enabling up to micrometer-scale thick coatings. The Mg salt led to deposition of a novel crystalline phase Ti/Mg oxyfluoride coating with an approximate composition of Ti5MgO7F8. Corrosion protection performance was characterized by polarization resistance measurements during immersion in 5% aqueous NaCl solution and neutral salt fog exposure following ASTM B.117. The performance is discussed in terms of the presence of fluoride in the coating enabling active behavior during its gradual depletion (in corrosion environments) toward a final composition approaching TiO2. This active corrosion protection mechanism may be more generally applicable for transition metal-based coatings that do not have high oxidation state oxo-anions, but do have stable fluoro-anions.