Scanning Micropipette Contract Method Measurement of Aluminum Alloy: Effect of Approach Parameters on Corrosion Potential

Abstract

Scanning electrochemical cell microscopy (SECCM) enables direct electrochemical measurements at microscopic sits by scanning a droplet cell over a substrate surface. Scanning micropipette contact method (SMCM) is a type of single-channel SECCM. It has been used to record the spatially resolved electrochemical activities across metal surfaces to investigate corrosion at the (sub)microscale. In SMCM, the applied potential during the approach of micropipette to the substrate (Eappr), generates a transient current upon droplet contact with the substrate. Once the transient current exceeds a set threshold, the micropipette is automatically halted. In the investigation of aluminum alloy, we found that Eappr affected the subsequent measurements of corrosion potential (Ecorr) in the open circuit potential (OCP) and potentiodynamic polarization (PDP), which was considered to be inconsequential previously. For aluminum alloys, the dense oxide film restricts the surface conductivity, increasing the difficulty of droplet landing. This leads to pipette-substrate contact and droplet-substrate contact landings using different Eappr. Additional oxygen flux from the droplet-oil interface in the droplet-substrate contact resulted in more positive Ecorr(OCP). In the anodic PDP at a high scan rate of 100 mV/s, Ecorr(PDP) moved away from Ecorr(OCP) to larger extent as Eappr increased to more cathodic values. The systematic interpretation of the effect of Eappr will promote the understanding of SMCM measurement especially in the field of metal corrosion.