Dependence of critical pitting temperature on the concentration of sulphate ion in chloride-containing solutions

• Published in: Applied surface science Vol. 253; no. 18; pp. 7369 - 7375
• Main Authors: Deng, Bo, Jiang, Yiming, Liao, Jiaxing, Hao, Yunwei, Zhong, Cheng, Li, Jin
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.07.2007; Elsevier Science
• Subjects: Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Critical pitting temperature (CPT); Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; Physics; Stainless steel; Sulphate and chloride; Sulphate and chloride; Critical pitting temperature (CPT); Stainless steel; 47.55.dr; Critical temperature; Stainless steels
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2007.03.034

Effects of varying concentration of sulphate (SO 4 2−) ion on the pitting behavior of 316SS have been investigated using potentiostatic critical pitting temperature (CPT) measurements, potentiostatic current transient technique and scanning electron microscopy in NaCl solution containing 0.5% Cl − ions. The results demonstrated that when the concentration of SO 4 2− ion is less than 0.42%, the CPT is surprisingly lower than that without SO 4 2− ion, showing an accelerating effect of the SO 4 2− ion on pit initiation, which is different from the traditional concept. As the concentration of SO 4 2− ion increases beyond 0.42%, the CPT is higher than that without SO 4 2− ion, displaying an inhibiting effect of the SO 4 2− ion on pit initiation. Based on the above results, a qualitative model is proposed to explain the inhibiting and accelerating effect of SO 4 2− ion on the pit initiation using the mechanism of ions-competitive adsorption between SO 4 2− and Cl − ions. The electric charges calculated in the process of pitting corrosion indicated that the pit morphology and its dimension are dependent on the content of SO 4 2− ion in chloride-containing solutions. The higher the concentration of SO 4 2− ion, the larger the dimension of the pit, reflecting an accelerating effect on pit growth.