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

Abstract

The detection of H 2 O 2 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 H 2 O 2 produced in 0.5 M Na 2 SO 4 . The activation of DCFH-DA to form the redox probe, DCFH, and its oxidation by H 2 O 2 to the fluorescent dichlorofluorescein was used for detection. Immersion tests were performed in naturally-aerated 0.5 M Na 2 SO 4 + 0.1 mM DCFH-DA for periods of time up to 96 h under open circuit conditions. The H 2 O 2 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 H 2 O 2 production, the following mechanism for transient formation of Cr(VI) in TCP coatings is confirmed: (i) dissolved oxygen is reduced to H 2 O 2 on the alloy surface presumably at cathodic intermetallic sites, (ii) the H 2 O 2 then diffuses to nearby coating sites to oxidize insoluble Cr(OH) 3 to soluble Cr(VI) species (e.g., CrO 4 2− ) 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 .

Investigation of the Trivalent Chromium Process Conversion Coating as a Sealant for Anodized AA2024-T3

“…Importantly, both are over 1000× higher than the value for a unanodized (degreased and deoxidized only) alloy. [25][26][27][28][29][30][41][42][43][44] The remainder of the curves in the mid (capacitive) and high frequency (solution and electrode ohmic resistance) regions are overlapping with no distinct differences.…”

“…Based on past studies of TCP conversion coating formation on aluminum alloys, it is presumed the sealant layer consists of relatively insoluble ZrO 2 and Cr(OH) 3 . [25][26][27][28][29][30]41,43,44 These chemical species are formed by hydrolysis of the coating bath precursors, H 2 ZrF 6 and Cr 2 (SO 4 ) 3 , under alkaline conditions according to the following two chemical reactions:…”

Electrochemical Quantification of H2O2 Released by Airway Cells Growing in Different Culture Media

“…The first peak at −0.5 V is attributed to the reduction in dissolved oxygen to H 2 O 2 , following Equation (2) [ 80 , 81 ] …”

Editors’ Choice—Dealloying-Driven Cerium Precipitation on Intermetallic Particles in Aerospace Aluminium Alloys

“…23,37,38 Hydrogen peroxide (H 2 O 2 ) as an oxidising agent can be responsible for the Ce (III)-to-Ce(IV) transformation as its presence has recently been shown as a consequence of oxygen reduction. 39 Besides, the oxidation of Ce(III) can take place directly over the Cu-rich particle where the solution pH and potential can be high enough; mind the E-pH diagram for cerium. 25 Figure 8 shows the FIB/SEM 3D reconstruction of an S-phase particle on an AA2024-T3 exposed for 24 h. Figure 8a reveals a buried S-phase remnant and its adjacent alloy matrix covered by a thick layer of cerium (hydr)oxide layer.…”