Effect of Si and Ce Addition on the Microstructure and Pitting Corrosion Resistance of Hyper-Duplex Stainless Steels

This paper presents the effect of cerium (Ce), silicon (Si), and Ce-Si addition, respectively, on the microstructural evolution and corresponding pitting corrosion resistance of four hyper-duplex stainless steels: base, Ce-containing, Si-containing, and Ce-Si-containing steels. In the solid solution-treated base steel, the pitting corrosion is initiated at the manganese (Mn) oxysulfide inclusions. The addition of Ce or Si suppresses the formation of these inclusions, thereby improving the pitting corrosion resistance of the steel. However, in the Ce and Siadded steel, the formation of Ce-rich oxysulfide decreases the pitting corrosion resistance. The pitting corrosion resistance of steels is significantly deteriorated by the subsequent sensitizing heat treatment at 850 degrees C due to the generation of regions in which the alloying elements are depleted. The addition of Ce is effective in retarding the precipitation of the s and. phases, and therefore, in preventing the degradation of the pitting corrosion resistance. However, in both Si-containing and Ce-Si-containing steels, the formation of the s phase is accelerated and results in a severe degradation of the pitting corrosion resistance. The addition of both Ce and Si results in an inhomogeneous distribution of Ce through the formation of Ce-bearing oxysulfides, and it accelerates the precipitation of the s phase. It is concluded that the addition of Ce alone is effective in improving the corrosion resistance and in retarding the precipitation of secondary phases, but the addition of both Ce and Si formed Ce-bearing oxysulfides, induce the pitting initiation sites and promote the formation of the secondary phases; accordingly, this significantly deteriorates the pitting corrosion resistance.