Imaging metastable pits on austenitic stainless steel in situ at the open-circuit corrosion potential

Abstract

Images describing metastable pits on 304 stainless steel obtained in situ in chloride solution at the open-circuit corrosion potential are presented. These images were obtained by scanning electrochemical microscopy and represent the oxidation of Fe 2+ emanating from the metastable pits as the probe tip passes over them. Each anodic transient on the tip is succeeded by a transient in the cathodic direction. We ascribe this to an indirect observation of the cathodic reaction. Reduction of oxygen as the cathodic reaction causes peroxide to be produced adjacent to the propagating pits. This reacts with Fe 2+ produced by the background passive current locally around the pit site. The microscope tip, set to detect Fe 2+, detects the reduced background current as a transient negative wing immediately after the anodic wing associated with the pit itself. The signal represents a reduced anodic background current. This negative wing on the probe current is not detected when the metastable pits are generated under potentiostatic polarisation. It is shown that the lifetime of the detected pits is a maximum of a few seconds.