Development of Multi-Probe Scanning Environmental Microscope and In Situ Observation of Initiation of Corrosion Damage

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In order to perform in situ observation of an initiation stage of corrosion damage including pitting corrosion and stress corrosion cracking, we have developed a multi-probe scanning environmental microscope, which enables us observe local distribution of pH and chloride ion concentration. To measure precise distribution of pH and chloride ion concentration, the measuring system must be electrically isolated from the sample. For this purpose, we have developed a specially designed potentiostat and an electrometer. By using the developed measuring system, we have utilized a micron size Ag/AgCl microelectrode. The microelectrode is insensitive to local pH value, and therefore, a precise measurement of local chloride ion concentration becomes possible. We also developed a so called chronoamperometric method, where a micron size Pt microelectrode is cyclically polarized, and an associated electric current is measured : local concentration of chloride ion and pH values can be measured by using a current density by anodically, and cathodically polarized. Advantage of this method is that the reaction time of the microelectrode is faster than an microelectrode such as a Ag/AgCl electrode, which enables us measure faster ; this method is a kind of a multielectrode that can measure both pH and chloride ion concentration at a time. We discuss the ability and usefulness of these methods.