Investigating cracks and crazes on coated steel with simultaneous SVET and EIS

The corrosion of coated steel with microcracks in the coating was monitored as time progressed using EIS and SVET simultaneously, i.e. to the same sample during the same measurement. The microcracks were created by environmental stress cracking after a controlled deformation.Both methods revealed an initial stage with an exponentially increasing corrosion current, and a second stage where the corrosion current increases linearly. SVET measurements showed that only a few of the cracks present participate in the corrosion process and that the corrosion positions are constant. EIS measurements showed that the cracks are partially crazes, and that the corrosion mechanism changes from the first to the second stage. The mechanistic information could be combined to show the transition from uniform corrosion through pores to a local cell type mechanism, even though the cracking process is stochastic and the electrochemical data shows a large variation between samples.