Nanoscopic Analysis of Aqueous Corrosion by Scanning Tunneling Microscopy

Scanning probe microscopy (SPM), such as scanneling tunneling microscopy (STM) and atomic force microscopy (AFM), can be used in a vacuum, in air, and in aqueous solution. No other device can detect submicron-range changes in surface morphology caused by electrochemical reactions. The possibility of in-situ quantitative evaluation of microcorrosion in the micron range was studied using STM. The corrosion behavior of Cu particles was studied in 0.1 M copper sulfate (CuSO₄) + 0.1 M sulfuric acid (H₂SO₄) aqueous solution. The local corrosion rate was estimated by the subtraction of STM images. Accuracy of the estimated corrosion rate was examined by measuring the drift of vertical direction with time. The drift of vertical direction in 10 min was < 10 nm. The average corrosion rate obtained by STM image subtraction correlated well with the corrosion rate calculated from Faraday’s conversion of the anodic current density. The local corrosion rate was found to be up to three times greater than the average corrosion rate. From these results, it was concluded that local corrosion behavior can be studied very accurately using STM.