Influence of Sensitization Treatment of Duplex Stainless Steels in Systems Containing Nitric Acid

The corrosion resistance of S32304, S31803 and S32750 duplex stainless steels in nitric acid environment was investigated by coupon test and immersion test. The results show that the three kinds of duplex stainless steels in solid solution state had low corrosion rate and good corrosion resistance in nitric acid environment. After sensitization treatment, the corrosion rates of the three steels were increased in turn, and the sensitized S32750 sample showed obvious selective corrosion. With the increase of the content of Cr and Mo in the dual-phase steels, more σ phase would precipitate at the phase boundary during the sensitization process, and it would grow into the interior of the ferrite, accompanied by the generation of secondary austenite. In the nitric acid environment, the sensitized S32750 duplex stainless steel was more susceptible to selective corrosion, in which corrosion occurred in the secondary austenite first and then spread to the austenite phase, and finally the ferrite phase remained.