2016 Volume 65 Issue 7 Pages 307-312
SUS304 stainless steels with the metal/glass crevice were tested potentiostatically in artificial seawater. The propagation behavior of crevice corrosion during the test was observed by the original apparatus. Geometric analysis of the crevice corrosion propagation behavior with time was carried out by using crevice corrosion shape parameters that were newly defined in this paper. The main results were as follows:
1） The corrosion depth of initiation site was about 20μm, and the place of maximum corrosion depth was located near crevice edge in any applied potential. The maximum corrosion depth was deeper under the higher applied potential. The corrosion depth of the dark gray area was below about 5μm. From these results, crevice corrosion mainly occurred at the interface between passive area and corroded area until the tip of corrosion reached to the edge of crevice. This means the dark gray part was only the trace of corrosion with corrosion products.
2）The shape of corrosion area changed to a quasi-elliptical shape from a apparently round one with crevice corrosion propagation. This tendency became stronger at higher potential and/or as the corrosion area got close to the crevice edge.
3）The major axis length (a) was divided into two parts at the initiation site, aedge for the edge of crevice and amp for the center of crevice. aedge became large exponentially as higher applied potential and as approaching the edge of crevice. However, aedge became slightly small with time, and the potential dependence was small.
4）It was considered that the expanding speed of the crevice corrosion was determined by the potential of the corrosion tip. The current density at the tip was larger as the tip close to the crevice edge by increasing of the potential of the tip. As the result of this phenomenon, the expanding speed was faster as the tip position close to the crevice edge.
