SYMPOSIA PAPER Published: 27 February 2019

Status of Pitting Corrosion Prediction in Mixed Solutions Containing Reduced Sulfur Species


Pitting of stainless steels and nickel-base alloys in solutions containing reduced sulfur species has features that are distinct from ordinary chloride-induced pitting. In particular, very severe pitting of molybdenum-free alloys occurs in neutral sodium sulfate solution containing thiosulfate ions for a certain range of sulfate to thiosulfate concentration ratio and a certain range of potential. Such pitting is hard to initiate without mechanical damage of the metal surface in situ; thus, the potentiostatic scratch test is particularly appropriate in this case. For chloride-thiosulfate mixtures, mechanical damage is not mandatory, and the quantification of pitting susceptibility is more complex because pitting can also occur in solutions containing only chloride anions. The signature of catalyzed (thiosulfate-induced) pitting is a large drop in pitting potential compared with the pure chloride case. The chloride-thiosulfate case is also notable in that the favorable ratio of chloride to thiosulfate concentration is very large for nickel-base alloys but much smaller for stainless steels. Reaction-transport modeling has been used to elucidate two particular features of pitting in these systems—the B value in Galvele's pitting potential equation and the enrichment of anions in pits as a function of their charge. It is found that the B value for chloride solutions is around 100 mV, in agreement with experiments, if one assumes that chromium cations exist mainly in the form of chloro-complexes. When thiosulfate is present, it is much more strongly enriched in pits than chloride. For sulfate-based solutions, the B value is around 50 mV, in line with the double charge of sulfate (and thiosulfate). Sulfide, not being an anion in the acidic conditions of a pit, shows different behavior in that deeper pits are less stable than shallow pits because the limiting flux of sulfide to the bottom of the pit decreases.

Author Information

Nguyen, Van Anh
Dept. of Chemical Eng., University of Toronto, Toronto, ON, CA
Carcea, Anatolie, G.
Dept. of Chemical Eng., University of Toronto, Toronto, ON, CA
Ghaznavi, Mahmoudreza
Dept. of Chemical Eng., University of Toronto, Toronto, ON, CA
Newman, Roger, C.
Dept. of Chemical Eng., University of Toronto, Toronto, ON, CA
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Developed by Committee: G01
Pages: 280–293
DOI: 10.1520/STP160920170245
ISBN-EB: 978-0-8031-7664-5
ISBN-13: 978-0-8031-7663-8