A Study on Pitting Corrosion of Stainless Steels in Halide Solutions

Abstract

The role of Mo in the pitting behaviours of stainless steels in bromide solutions is a matter of current debate. While Mo has been widely acknowledged to increase the pitting resistance in chloride solutions, some authors have proposed that the beneficial effects of Mo are compromised in bromide solutions. In this thesis, the results of an investigation into the pitting resistances of three austenitic grades 304L, 316L and SMO as well as the duplex grade 329 in both chloride and bromide solutions at different temperatures are presented. Potentiodynamic polarization measurements showed that the temperature dependent pitting potentials of the stainless steels followed a linear relationship in sodium bromide but an exponential relationship in sodium chloride, evident of different rate-determining steps. X-ray photoelectron spectroscopy further revealed the formation of molybdates MoO₄^2- in the passive films of SMO and 329 in chloride solutions, but the formation was compromised in bromide solutions.