Mesh-capped probe design for direct pH measurements at an actively corroding metal surface

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Quantitative characterization of surface reaction species improves understanding of reaction mechanisms, especially for heterogeneous reactions such as in corrosion and electrolysis. A universal mesh-capped surface pH probe was developed in order to investigate surface proton concentrations for aqueous metal dissolution/deposition reactions. This technique can be generally applied to characterize the surface pH either for bare metal reactions or metals corroding beneath a corrosion product film. Measurements with the new probe design have shown a good degree of reproducibility. The surface pH measurements during corrosion of mild steel have shown that a higher surface pH is achieved at the steel surface as compared with that of the bulk solution, just as predicted from theory. Chemical buffering effects on surface pH were observed during corrosion by contrasting the measurements obtained in HCl deaerated solutions with those containing CO2, i.e., carbonic acid (H2CO3) and acetic acid (CH3COOH/HAc) solutions. It was found that the surface pH was mainly controlled by the water chemistry and chemical buffer capacity. The surface pH is affected by the deposited corrosion scale porosity. A higher deviation of surface pH compared to the bulk value was observed for more dense layers with lower porosity.