Pitting Corrosion Initiated by Al2O3–CaO–CaS Inclusions in a 304 Stainless Steel

The effect of the calcium treatment on the pitting corrosion of a 304 stainless steel (SS) was investigated. The number density, the area fraction, the composition, and the size of non-metallic inclusions in the SS after calcium treatment were analyzed. When the total calcium content in the SS increased from 3 to 54 ppm, the evolution route of inclusions was Al 2 O 3 → (Ca, Al)O → (Ca, Al, S)O. The corrosion index of the single Al 2 O 3 inclusion, the clustered Al 2 O 3 inclusion, the (Ca, Al)O inclusion, and the (Ca, Al, S)O inclusion was 0, 0.59, 0.74, and 1.09 pct min −1 after immersion for 120 min, respectively, indicating that the severity of the pitting corrosion of the SS induced by inclusions was (Ca, Al, S)O > (Ca, Al)O > clustered Al 2 O 3 > single Al 2 O 3 . When the T.Ca content in the SS was 3, 8, 46, and 54 ppm, the pitting potential of the SS was 0.39, 0.31, 0.21, and 0.12 V, respectively. The transient current density and the transient current lifetime increased with varied T.Ca contents of steel samples, indicating the metastable pitting corrosion resistance of the SS decreased with more calcium in the steel.