Investigation of filiform corrosion of epoxy-coated 1045 carbon steel by scanning Kelvin probe force microscopy

The mechanism for filiform corrosion (FFC) is thought to involve oxygen diffusion through the tail to the active head. The primary cathodic region is near the back of the head (at the head/tail boundary), where oxygen concentration is higher, and the primary anodic region is at the front edge of the head of the filament. Although there is experimental support for this mechanism, a high-resolution description of the FFC process has not been presented. The aim of this study was to provide detailed information about the mechanism of FFC on coated steel using the high spatial resolution of scanning Kelvin probe force microscopy. Segments of active filaments were successfully investigated through 150 and 300 nm thin epoxy coatings in air of 93% relative humidity. Volta potential and topographic maps showed separation of active anodes and cathodes in the head and revealed the presence of voids associated with delamination of the coating along the edge of the tail. The morphology of filaments and Volta potential distributions were strongly dependent on the film thickness. Differences in growth characteristics were explained by mass transport considerations. (C) 2004 The Electrochemical Society.