The Comparative Investigation of Corrosion Behavior of a Cast Lean Duplex and Austenitic Stainless Steels in Chloride Environment

The comparative investigation of the corrosion behavior of S32304 cast lean duplex and CF3M cast austenitic stainless steels was conducted in a chloride environment. The resistance to pitting corrosion of the S32304 alloy was “one” time higher than that of the CF3M alloy because the pitting potential, passive region, and critical pitting temperature of the low Ni-low Mo S32304 alloy were higher than those of the high Ni-medium Mo commercial CF3M alloy. There are two main reasons for the enhancement of the pitting corrosion resistance of high Cr-low Mo-medium N S32304 alloy compared to the low Cr-medium Mo CF3M alloy: First, the pitting resistance equivalent number (PREN_δ+γ) value of the S32304 alloy is higher than that of the CF3M alloy. Second, the passive region of the S32304 alloy is larger than that of the CF3M alloy. It indicates that the synergistic effects of the three elements by adding high Cr and low Mo-medium N to the S32304 alloy enhance the passivity of the passive film, thereby increasing the resistance to pitting corrosion. It was verified that based on the PREN_γ and PREN_δ values calculated using an N-factor of 30, the pitting corrosion of the S32304 alloy was selectively initiated at the γ-phases and the inclusions of Mn oxy-sulfieds because the γ-phase’s PREN_γ value was smaller than the δ-phase’s PREN_δ, and finally propagated from the γ-phase to the δ-phase.