Pitting corrosion behaviour of UNS N08904 stainless steel in chloride-sulphide solutions

Abstract

The pitting corrosion behaviour of UNS N08904 (type 904L) stainless steel (SS) in 3 × 10^-3 M Na₂S, in 0·6M NaCl, and in 0·6M NaCl with different concentrations of Na₂S at elevated temperature was studied using potentiodynamic anodic polarisation and scanning electron microscopy (8EM). Type 904L S8 did not pit in pure 3 × 10^-3M Na₂S at 60°C, while pitting corrosion of type 904L SS in chloride-sulphide mixtures occurred at more active potentials than those obtained in pure 0·6M NaCl. The pitting potential (E_pit) of type 904L SS in 0·6M NaCl with 3 × 10^-3M Na₂S decreased with increasing temperature in the range 50–70°C. The value of E_pit did not change in acidic chloride-sulphide solutions (pH 3–6), but it became more noble in alkaline chloride-sulphide solutions (pH 8–12). Observations by SEM showed that the large pits formed on type 904L SS in 0·6M NaCl with 3 × 10^-3 M Na₂S contained some micropits and that there was intergranular attack.