Morphology perspective on chromic acid anodizing replacement by thin film sulfuric acid anodizing

Conventional chromic acid anodizing (CAA) with dichromate seal provides both superior corrosion protection and strong adhesion without significant fatigue debit, yet hexavalent chrome is toxic and needs to be replaced. Nickel acetate seal removes most of Cr (VI) from the CAA coating. Thin Film Sulfuric Acid Anodizing (TFSAA) replacing CAA eliminates Cr (VI) usage in the anodizing process. TFSAA coating with non-Chromated seal is the most desirable for Cr (VI) elimination. An Electrochemically stressed (EC-stressed) cyclic test, designed for fast screening by us, was used to compare CAA with dichromate seal, CAA with nickel acetate seal, and TFSAA with an environmentally friendly two-step seal. In EC-stressed test, CAA with dichromate seal performed worst, CAA with nickel acetate seal provided much better barrier properties and TFSAA with two-step seal performed the best for both barrier properties and corrosion resistance. Coating morphology and barrier properties are important factors for different coating applications.