Molybdate-based Conversion Coatings for Corrosion Protection of Aluminum Alloys
Materials Science and Engineering
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In order to prevent the corrosion of aluminum alloys, a chromate conversion coating (CCC) is applied to the surface. However, chromate is a carcinogen. Thus, the development of chromate-free and environmentally- friendly replacement coatings is currently being pursued. This thesis describes the development, formulation, surface characterization and corrosion resistance properties of an environmentally-benign, molybdate-based conversion coating (MoCC) for the protection of aluminum substrates. In this study, a molybdate conversion coating (MoCC) was developed on aluminum alloy AA2024-T6. The coating ennobled the corrosion potential of AA2024-T6 from ~ -670mV to ~ -600mV versus Ag/AgCl reference electrode. The coating was determined to protect the underlying aluminum alloy via anodic inhibition. Further, the number of pits was reduced on the coated sample when compared to as-received AA2024-T6. The repassivation ability of the so formed MoCC on AA2024-T6 was tested by scratching the sample with a glass tip and measuring the OCP. Immediately after scratching, the OCP dropped significantly due to the exposed underlying alloy. However, within 5 seconds the OCP traced back to its pre-scratch potential indicating that MoCC possess the ability to `self-heal'. Scanning electron microscopy revealed the surface morphology to consist of a mud cracked pattern that was similar to what would be seen on a sample coated with a chromium conversion coating. Multiple techniques were used to characterize the coating chemistry. Ultraviolet-Visible, Raman, Fourier Transform-Infrared and energy dispersive spectroscopies showed the formation of a molybdate based conversion coating on the aluminum alloy surface. X-ray photoelectron spectroscopy (XPS) provided a more thorough analysis of the coating. XPS showed the coating to be composed of multiple molybdenum based species: MoO2, Mo2O5, MoO42- 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. Based on these analytical techniques it was shown that a protective layer was successfully formed on an aluminum alloy AA2024-T6 sample. The thin layer on top shows the species responsible for the repassivation behavior, while the underlying layer is the densest part of the coating and forms a protective barrier. A model for the structure of the molybdate conversion coating on the AA2024-T6 has been proposed.