A new microelectrochemical method to study pit initiation on stainless steels

Microelectrochemical techniques using microcapillaries as microcells are powerful methods to study local phenomena on passive metal surfaces. The enhanced current resolution to pA and fA allows us to study local processes in the nm-range. The results obtained from stainless steels indicate that MnS-inclusions, acting as potential nucleation sites for pitting corrosion, are dissolved even in chloride-free solutions, whereas stable pit growth only occurs in the presence of chlorides. The microelectrochemical technique easily allows us to distinguish between active and inactive inclusions. Noise analysis of potentiostatic measurements revealed that the RMS current noise, increasing linearly with the exposed area, is largely caused by the dissolution of small inclusions. Furthermore, the results allowed us to correlate pitting potentials with the inclusion size, indicating that the inclusion;size has to be kept well below 1 mu m to improve substantially the pitting resistance of stainless steels. (C) 1997 Elsevier Science Ltd.