Effect of σ phase on pitting corrosion of 22-5 duplex stainless steel

Abstract

The effect of σ phase content on the pitting potential of 22-5 duplex stainless steel has been investigated in aqueous solutions of O·6M NaCl, with and without nitrogen purging, at both room temperature (22°C) and 80°C. A scratching technique was used to simulate the type of mechanical damage that may occur during service, resulting in localised attack due to disruption of the passive film, particularly in chloride containing environments. The amount of σ phase in the test specimens was varied up to a maximum of 18% by thermal treatment at 850°C for periods of time of up to 3 h to simulate microstructural changes due to welding. The test results showed that σ phase markedly decreases the pitting potential of 22-5 duplex stainless steel at both 22 and 80°C. Furthermore, the extent to which σ phase reduced the pitting potential was found to be strongly temperature dependent, being more pronounced at the higher temperature. Indeed, the increase in test temperature had a greater effect on the pitting potential than did the σ phase content. A significant observation was that nitrogen purging of the test solution rendered the specimens more susceptible to pitting corrosion than when tests were carried out without nitrogen purging. Microscopy revealed that most of the pits occurred where scratches passed through the ferrite phase of the duplex stainless steel specimens.