Dissolution and passivation of nickel-free austenitic stainless steel in concentrated acids and their mixtures

Abstract

Electrochemical polarization behavior of nickel-free austenitic stainless steel in concentrated phosphoric (H3PO4), sulfuric (H2SO4), acetic (CH3COOH), and formic (HCOOH) acids and binary and ternary solution mixtures was investigated by a potentiostatic polarization technique at 30 ± 1°C. The cathodic polarization curves were almost linear and similar in all binary and ternary solution mixtures and resembled that of the hydrogen evolution reaction. However, in ternary solution mixtures of H3PO4, H2SO4, and HCOOH, the shape of the curves indicated composite reactions of hydrogen and oxygen reduction. Anodic polarization curves showed active, passive, and transpassive behaviors in all the solution mixtures. When H2SO4, CH3COOH, or HCOOH was added to H3PO4, the aggressive nature of the binary solution mixtures toward the alloy was intensified in the same order. Thus, maximum corrosion was observed in the case of binary solutions of H3PO4 with HCOOH. However, in the ternary solution mixtures, the corrosion of the alloy decreased. In the transpassive region, a secondary current peak and current plateau were observed in most cases. However, electropolishing was not achieved in these solution mixtures. The mass-transfer-controlled condition is probably not achieved in such cases even when a current plateau is observed. Resulting surface morphologies were characterized with scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smoothing action was observed in the transpassive region. © 2001, NACE International.