In this research, the effect of thiosulfate ion on pitting corrosion behavior of 316 stainless steel was studied. Part I of this work deals with the influence of thiosulfate on the critical pitting temperature of 316 SS in 0.1 M chloride solution and studies the effect of this ion on the pit chemistry. Electrochemical experiments performed in absence and presence of 0.01 M thiosulfate ion revealed that thiosulfate deteriorates the CPT. Pencil electrode studies showed that thiosulfate addition decreases both saturation concentration of metal cations necessary to metal salt precipitate at the pit bottom (C S ) (the value of diffusion controlled limiting current density (i Lim ) was decreased) and the ratio of C * /C S (where C * is the critical concentration of metal cations in pit solution essential for a pit to remain stable) while it increases the anodic dissolution rate in a simulated pit solution. Furthermore, scanning electron microscopy showed that the lacy cover formed over the pits mouth has coarser pore structure in the presence of thiosulfate ion.The effect of thiosulfate in industrial environments, in particular, in the pulp and paper industries and in refinery equipment, is of importance. Moreover, the role of thiosulfate is not necessarily limited to the cases where thiosulfate exists in the media, but also formed by electrochemical dissolution of sulfide inclusions, where it might be considered as the way that pitting corrosion initiates. 1,2 However, it is well known that pits in stainless steels would not be stable in pure thiosulfate solutions, 3 but it has been proven that thiosulfate and chloride have a synergistic effect in inducing localized corrosion. In other words, small thiosulfate additions are known to enhance localized corrosion in chloride solutions, 4,5 although the effect of thiosulfate ions does not exist until the chloride ion leads to breakdown of the passive layer. 5 Using X-ray photoelectron spectroscopy, DuretThual et al. 6 showed that no reduced sulfur could be produced when a well-developed passive film was exposed to thiosulfate containing solution. They also showed that thiosulfate ions or their products of decomposition do not penetrate into the passive film. It is recognized that the internal surfaces of pits are at the pH of around zero. 7 Thiosulfate ion is quite stable in neutral solutions. 8 However, under low pH conditions, thiosulfate has thermodynamic tendency for chemical disproportionation and is reduced into adsorbed sulfur and sulfide on the bare metallic surface. 6 Marcus et al. 9 showed that formation of adsorbed sulfur (S ads ) layer on the dissolving surface of pure nickel leads to a catalytic process that lowers the metal-metal bond energy and reduces the activation energy for metal dissolution. The catalytic effect of adsorbed sulfur was demonstrated later for stainless steels. 3,9,10 In addition, it is suggested that when molybdenum is present in alloy, the sulfur is unstable as S 0 and is reduced to aqueous H 2 S. 11 Besides, reduction of S 2 O 3 2− to...

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The transition potential and the repassivation potential of AISI type 316 stainless steel was investigated in the absence and the presence of 0.01 M thiosulfate in chloride containing media. The pencil electrode method was employed to explore the key factors affecting the pit transition potential and the repassivation potential in thiosulfate containing solution. Using this method the pit chemistry at various temperatures was also evaluated. A good correlation was found between the pitting potential and the pit transition potential at various temperatures. Moreover, there was a significant decrease in the repassivation potential by addition of 0.01 M thiosulfate. The results were in accordance with the theory suggesting that the chemistry of the pit governs the pit repassivation potential.

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