A NEW MICROELECTROCHEMICAL METHOD TO STUDY PIT INITIATION ON STAINLESS-STEELS

Microelectrochemical techniques using microcapillaries as microcells a re powerful methods to study local phenomena on passive metal surfaces . The enhanced current resolution to pA and fA allows us to study loca l processes in the nm-range. The results obtained from stainless steel s indicate that MnS-inclusions, acting as potential nucleation sites f or pitting corrosion, are dissolved even in chloride-free solutions, w hereas stable pit growth only occurs in the presence of chlorides. The microelectrochemical technique easily allows us to distinguish betwee n active and inactive inclusions. Noise analysis of potentiostatic mea surements revealed that the RMS current noise, increasing linearly wit h the exposed area, is largely caused by the dissolution of small incl usions. Furthermore, the results allowed us to correlate pitting poten tials with the inclusion size, indicating that the inclusion;size has to be kept well below 1 mu m to improve substantially the pitting resi stance of stainless steels. (C) 1997 Elsevier Science Ltd.