A Mechanical–Electrochemical Approach for the Determination of Precursor Sites for Pitting Corrosion at the Microscale

The influence of metallurgical defects and residual surface stresses generated by polishing on the pitting susceptibility of duplex stainless steels was studied by combining macro- and microelectrochemical measurements with thermal-mechanical simulation and metallography tests. It has been shown that pits initiate in both phases at metallurgical point defects (such as oxide inclusions in the ferrite and dislocation lines in the austenite). By contrast, the surface stress state was the driving force for pit initiation along the austenite/ferrite interface. Experiments at the macroscale revealed that this process represents about 40% of the total number of pits observed. It has been demonstrated that the local stress gradient was the key-parameter in pit initiation and that the local average stress was the parameter governing the transition from metastable to stable pitting.