Development of Scanning Tunneling Microscope Operating in Corrosiove Solution and In Situ Observation of Corrosion Damage of Metals

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A scanning tunneling microscope (STM) operating in a corrosive solution has been developed. The system consists of the STM operating in air and the one in a solution. The electrochemical cell is of a four-electrode type (bi-potentiostatic type), thereby the current associated with electrochemical reaction can be minimized : the potential of the tunneling tip is controlled so that the electrochemical current is reduced to the order of one-hundredth of the tunneling current. By using this bi-potentiostatic technique, the STM imaging can be achieved with the controlled potential of a sample. The other technique for minimizing the electrochemical current is a glass-coating Pt-Ir tunneling tip. Further, the system has a small, three-point bending type testing machine, thereby the imaging under an applied load can be done. By using this system, in situ observation of initiation and growth of micro-pitting on a stainless steel sample immersed in a 3.5% NaCl solution could be done. The imaging demonstrated that the in situ observation with the developed STM system can be operated under the conditions useful for investigating the mechanisms of corrosion damage including stress corrosion cracking, intergranular dissolution and pitting corrosion.