Material Factors of Grain Boundary Sensitization in Stainless Steels Detected by 10% Oxalic Acid Etching Test Method

Abstract

Material factors which cause intergranular corrosion (etched grain boundaries) of Type 304 stainless steel in 10% oxalic acid etching and nitric acid-fluoric acid etching test methods are studied. It has been clarified that the material factor of grain boundary etching in 10% oxalic acid etching test is never the Cr depletion at grain boundaries, but the Cr carbide precipitated at GB. Also, it has been found that the Cr depletion at grain boundaries is the only cause for GB etching in nitric acid-fluoric acid etching test and Cr depleted zone is completely healed by the diffusion of Cr from the matrix in the heat treatment at 800°C for more than 250h.