Current transients during repeated micro-indentation test of passive iron surface in pH 8.4 borate buffer solution

Micro-indentation test was performed for passive iron surface in deaerated pH 8.4 borate solution to correlate the passive film breakdown with mechanical deformation of substrate. Loading and unloading of a conical micro-indenter to the passive surface were repeated 10 times at an identical position. Elastic–plastic deformation occurred during the first indentation, while elastic deformation is predominant during the second and following indentations. The anodic current appears at the commencement of the first loading, with the maximum current generated at the maximum load. A more obvious current peak appears when the indenter just lost the contact with the iron surface during the first unloading. The current peaks observed during the subsequent indentations decreased markedly compared with the first indentation. Thus, the passive film breakdown by elastic deformation of iron substrate is less significant compared with that by plastic deformation under the present conditions. There is a linear correlation between the electric charge required for repassivation of the ruptured passive film and the work for plastic deformation.