Constant-distance mode AC-SECM for the visualisation of corrosion pits

The local visualisation of corrosion pits has so far failed to convincingly prove the advantage of scanning microscopy in overcoming Abbey’s Limit. Thus, for imaging pit initiation and instant pit growth the resolution of the methods used for detecting the related changes in local electrochemical activity has to be significantly improved. Progress into this direction has been achieved by combining shear force based constant-distance mode scanning electrochemical microscopy (SECM) with the alternating current mode of this technique (AC-SECM). A very small corrosion pit was generated by inducing active corrosion underneath an accurately positioned 5 μm diameter Pt disk SECM tip that served as counter electrode in the direct mode of SECM. Before and after pit initiation, constant-distance mode AC-SECM images were recorded and subsequently used to identify the area of broken passivity. The results demonstrate the feasibility of the suggested approach to image actively corroding sites with overall dimensions of only a few micrometers. Combination of constant-distance scanning with AC-SECM is an important prerequisite for further improving spatial resolution by employing smaller-diameter micro- or even nano-electrodes as SECM tips.