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In this study, pH measurement was performed in a thin electrolyte droplet with a thickness <1000 µm by the measurement of the equilibrium electrode potential of an Sb/Sb x O y electrode used as a pH sensor. The equilibrium potential of the Sb/Sb x O y electrode was evaluated by using the Kelvin probe (KP) technique. To investigate the potential response of the Sb electrode in a thin electrolyte droplet, the dependency of the Volta potential difference between the Sb and a gold wire as a KP on electrolyte droplet thickness was measured. The Volta potential difference had a linear response with respect to the buffer solution pH, independent of the droplet thickness. This result indicates that the KP technique, combined with an Sb electrode, is sensitive to the pH of a thin electrolyte droplet of thickness ²50 µm. This pH measurement technique was also applied to measure pH in a corrosion model of steel. The corrosion model consisted of two steel plates in the same plane as the anode and cathode, with a constant current between them. During the corrosion process, the pH value decreased from 6 to 5 near the anode and increased from 6 to 12 at the cathode. The changes in pH measured in the thin electrolyte droplet were in good agreement with the color changes of the solution containing pH indicators.

To investigate hydrogen absorption behavior into carbon steel during corrosion in an aqueous sodium chloride (NaCl) droplet, a simultaneous measurement system of the corrosion potential, E corr and hydrogen permeation current, i per was developed using the Kelvin probe (KP) technique and the Devanathan-Stachurski (DS) method, respectively. This system outputs the interrelation between corrosion and hydrogen absorption into steel throughout the drying process of an NaCl droplet. Our results showed that hydrogen absorption into the steel occurred when the E corr shifted in less noble direction under wet conditions, and ceased at a higher potential of E corr when the steel surface dried up. Based on the results of the transients of the i per , the amount of hydrogen absorbed during the drying of the NaCl droplet increased with NaCl concentration, which was attributed to the negative shift of the E corr. Furthermore, the amount of hydrogen absorbed within one wet-dry cycle changed with the number of cycles, due to the expansion of the corroded area and the formation of iron rust.