Investigation of sigma phase generated by simulated welding heat treatment on corrosion behavior of 2205 hydraulic control pipeline steel

Duplex stainless steel (DSS) hydraulic control pipelines readily form sigma phase during the heat treatment process, reducing corrosion resistance and leading to pipeline leakage and failure. This study employs various analysis methods to investigate the effect of sigma phase on the corrosion behavior of 2205 hydraulic control pipeline steel, focusing on the electronic characterization of passive film and pitting corrosion behavior. Results revealed that the sigma phase content reaches a maximum value of 13% at 850 °C within the heat treatment temperature range of 750 - 950 °C. The voltaic potential of the sigma phase is approximately 47 mV higher than the matrix, but the Cr-depleted zone around the sigma phase is about 85 mV lower than the matrix. The passive film of specimens containing sigma phase exhibits higher carrier densities and lower content of Cr(OH)3 and Cr2O3 compared to as-received specimens. The morphologies of the film show obvious groove defects, which provide a corrosion channel to the matrix. Moreover, specimens containing sigma phase exhibit higher intergranular corrosion susceptibility and lower pitting corrosion resistance. Pits preferentially nucleate and develop in the Cr-depleted zone around the sigma phase, with their quantity and size positively correlated with the sigma phase content.