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

研制了一种在腐蚀性溶液中工作的扫描隧道显微镜（STM）。该系统包括在空气中工作的STM和在溶液中工作的STM。电化学电池是四电极型（双恒电位型），因此与电化学反应相关的电流可以最小化：控制隧道尖端的电位，使电化学电流减少到隧道电流的百分之一数量级。通过使用这种双恒电位技术，可以在控制样品电位的情况下实现STM成像。另一种最小化电化学电流的技术是玻璃涂层Pt-Ir隧道尖端。此外，该系统具有一个小型的三点弯曲型试验机，因此可以在外加载荷下进行成像。利用该系统，可以对不锈钢试样在3.5% NaCl溶液中微点蚀的形成和生长进行现场观察。成像结果表明，利用STM系统进行的现场观察可以在研究应力腐蚀开裂、晶间溶解和点蚀等腐蚀损伤机制的条件下进行。

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.