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

Abstract

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 CO 2 , i.e., carbonic acid (H 2 CO 3 ) and acetic acid (CH 3 COOH/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.

Effect of H2S on the CO2 corrosion of carbon steel in acidic solutions

“…Fig. 18 presents a comparison of pH values at 25 • C in CO 2 environments, between the bulk solution versus that in near surface region over corroding steel [24]. For the current pHs 3 and 4 corrosion tests in this work, similar surface pH values (≈6) were measured and was found to be higher than the bulk solution pH value.…”

“…This suggests that it is impossible to form mackinawite by precipitation out of bulk solution at pHs 3 and 4 conditions. However, recent research in our lab on CO 2 corrosion has shown that, the pH measured near the surface of corroding carbon steel is higher than the bulk solution pH, in-spite of the fact that CO 2 has a good buffering capacity [24]. Fig.…”

Effect of bicarbonate on corrosion of carbon steel in CO2 saturated brines

“…In localized CO 2 corrosion, a pit surface is usually bare and is surrounded by scaled/passivated cathode surface (Han et al, 2007(Han et al, , 2010a(Han et al, , 2011b. A high pH can develop in an active pit and allow concentrated bicarbonate to form, potentially playing an important role in localized corrosion (Han et al, 2010b). In industrial practice, several failures due to bicarbonateinduced corrosion have been reported and reviewed by Zhang and Martin (2011).…”

Localized Corrosion of Mild Steel in Marginally Sour Environments

“…It has been previously shown that the surface pH could vary from the bulk pH by 1 unit to 2 units depending on the level of mixing. 49 This suggests that both iron carbonate and mackinawite were likely supersaturated at the steel surface, which is a necessary precondition for formation of solid corrosion products layers. Figure 3 shows the SEM images of the surface of exposed specimens.…”