In situ pH measurement during the formation of conversion coatings on an aluminum alloy (AA2024).

The measurement of interfacial pH change is important for understanding the formation mechanism of conversion coatings that are used to protect metals from corrosion. In this work, we used a tungsten microelectrode to measure the interfacial pH change near the surface of an aluminium alloy (AA2024) during the formation of two conversion coatings: (i) a trivalent chromium pretreatment (TCP) and (ii) a Ti-based, non-chromium-containing coating. The tungsten microelectrode exhibited an open circuit potential (OCP) that changed linearly as a function of the solution pH with a slope of -64 mV per pH. The microelectrode was positioned near the AA2024-T3 surface and its potential was measured as a function of time along with the OCP of the alloy sample during formation of the two coatings. The microelectrode exhibited a negative shift in potential immediately upon initiation of the coating formation, consistent with a significant increase in the interfacial pH of 2-6 units depending on the coating system. The pH increase is attributed to proton-consuming cathodic reactions that occur on the alloy surface once the passivating oxide layer is dissolved: hydrogen evolution and oxygen reduction. The increased pH causes the hydrolysis of the soluble fluorometalate precursor species in the baths, which precipitate forming a hydrated metal oxide coating (e.g., ZrO2·nH2O).