Detection of H2O2 from the Reduction of Dissolved Oxygen on TCP-Coated AA2024-T3 : Impact on the Transient Formation of Cr(VI)

The detection of H2O2 formed in naturally-aerated electrolyte solution contacting uncoated and TCP-coated AA20243-T3 alloy surfaces is reported on. This molecule is produced from the reduction of dissolved oxygen and it has been implicated in the mechanism of transient formation of Cr(VI) in trivalent chromium process (TCP) conversion coatings. Linear sweep voltammetry and a spectrophotometric assay involving 2',7'-dichloro-dihydrofluorescein diacetate (DCFH-DA) were used to detect H2O2 produced in 0.5 M Na2SO4. The activation of DCFH-DA to form the redox probe, DCFH, and its oxidation by H2O2 to the fluorescent dichlorofluorescein was used for detection. Immersion tests were performed in naturally-aerated 0.5 M Na2SO4 + 0.1 mM DCFH-DA for periods of time up to 96 h under open circuit conditions. The H2O2 concentration was greater in solution exposed to uncoated than to TCP-coated AA2024-T3 for equivalent times due to the inhibition of the oxygen reduction reaction kinetics by the conversion coating. With this direct evidence of H2O2 production, the following mechanism for transient formation of Cr(VI) in TCP coatings is confirmed: (i) dissolved oxygen is reduced to H2O2 on the alloy surface presumably at cathodic intermetallic sites, (ii) the H2O2 then diffuses to nearby coating sites to oxidize insoluble Cr(OH)3 to soluble Cr(VI) species (e.g., CrO42−) and (iii) the transiently formed Cr(VI) species diffuses to nearby corroding sites on the alloy where the inhibitor gets reduced back to passivating Cr(OH)3.