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.
|Journal||Journal of the Electrochemical Society|
|State||Published - Jan 2012|
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Renewable Energy, Sustainability and the Environment
- Surfaces, Coatings and Films
- Materials Chemistry