Transient Formation of Chromate in Trivalent Chromium Process (TCP) Coatings on AA2024 as Probed by Raman Spectroscopy

We report evidence for the transient formation of Cr(VI) oxide species in the Trivalent Chrome Process (TCP) coating on AA2024-T3 alloys. TCP is the leading replacement coating for the environmentally-unfriendly chromate conversion coating (CCC) that is commonly used to pretreat aerospace aluminum alloys. Transient formation of hexavalent chrome in these coatings is important to learn about as the presence of this species will affect the active level of corrosion protection as well as the environmental impact of the coating. Specifically, we tested the hypothesis that dissolved O₂ is reduced to H₂O₂ at the Cu-rich intermetallic sites and that H₂O₂ diffuses to nearby sites in the coating to oxidize native Cr(III) oxide (Cr(OH)₃ or Cr₂O₃) to Cr(VI) oxide (CrO₄ ^2-). Raman microprobe spectroscopy was used to spatially distinguish between the two species based on the different vibrational modes of Cr(III)-O (520-580 cm^-1) and Cr(VI)-O (840-904 cm^-1). Results are presented that support a mechanism whereby the formation of Cr(VI) oxide species is linked to the production of H₂O₂ locally within the coating. The H₂O₂ is presumed to arise from the 2e^- reduction of dissolved O₂ at Cu-rich intermetallic sites. A variety of Cr(VI) oxides and mixed oxides are formed in the coating during full immersion testing in air-saturated electrolyte solutions or aging in air.