Molybdate-Based Conversion Coatings for Aluminum Alloys Part II: Coating Chemistry

Abstract

In order to prevent the corrosion of aluminum alloys, a chromate conversion coating (CCC) is applied to the surface. However, chromate is a carcinogen and substitutes are being studied for its replacement. In part I of this study, the authors described the development of an environmentally friendly and corrosion protective molybdate conversion coating (MoCC) for aluminum alloy AA2024-T6. In this part, MoCC is characterized and a structure is proposed. Multiple techniques were used to characterize the coating chemistry. Raman and Fourier Transform-Infrared spectroscopies showed the formation of a molybdate based conversion coating on the aluminum alloy surface. X-ray photoelectron spectroscopy (XPS) showed the coating to be composed of multiple molybdenum based species: MoO2, Mo2O5, MoO4 2- and MoO3. Further, MoCC was determined to consist of two layers; the surface layer was composed predominantly of oxidized Mo(VI) whereas the inner layer mostly consisted of reduced Mo(IV) and Mo(V) species.