Morphology Characterisation of Pitting Corrosion on Sensitized Austenitic Stainless Steel by Digital Image Analysis

Abstract

Pit morphology on sensitized 310S stainless steel has been studied using an image processing method based on reflected light microscopy. Salt Spray (fog) test has been used to induce the pitting corrosion. Morphological pits characters do not depend on sensitization heat treatments applied here. Nucleation rates and growth may be associated with quantity and distribution of chromium carbides. This being so, condition I (heating up to 1065°C during 1 h and air cooling) and condition II (heating up to 1065°C during 1 h and air cooling followed by reheating up to 670°C during 5 h and again air cooling) are the most susceptible to pitting, in particular the first one. In these two conditions, pits are nucleated in grains and in grain boundaries, while in condition III (heating up to 1065°C during 1 h and air cooling followed by reheating up to 620°C during 24 h and again air cooling), pits are preferentially nucleated in boundaries of small grains. Thence, pits usually grow more rapidly in depth than in width, being able to occur partial or total grains separation. Pits are mainly hemispherical, near-hemispherical, near-conical, and near-cylindrical without significant geometric transition associated with an increasing exposure period.