Austenitic/ferritic duplex stainless steels, which contain 22-25%Cr-3%Mo-5-7%Ni-0.15%N, show excellent corrosion resistance in many corrosive environments. When duplex stainless steels were heat-treated, an intermetalic precipitate such as sigma phase was formed. In this paper, effect of sigma phase on corrosion resistance of duplex stainless steels was studied. Precipitates of sigma phase did not affect intergranular corrosion resistance of duplex stainless steels in the Huey and the Streicher tests. In the Strauss test, cracking was observed in test specimens in which sigma phase precipitates, when bending even before corrosion test was carried out. On the other hand, sigma phase greatly affected pitting corrosion resistance. Corrosion rate in Cl- ion including solution increases with area fraction of sigma phase. General corrosion resistance must be determined by Cr content in the fresh austenite phase which precipitated with sigma phase spontaneously. As a result, pitting corrosion test and bending test were the most sensitive methods in detecting sigma phase precipitate. High ferrite phase content of duplex stainless steels leads to prevention of sigma phase formation and improves pitting corrosion resistance of heat-treated materials.
