Journal of The Electrochemical Society, 163 (6) C275-C281 (2016) C275 0013-4651/2016/163(6)/C275/7/$33.00 © The Electrochemical Society Pit Transition Potential and Repassivation Potential of Stainless Steel in Thiosulfate Solution M. Zakeri, a M. Naghizadeh, a D. Nakhaie, a,b, * and M. H. Moayed a, z a Metallurgical and Materials Engineering Department, Faculty of Engineering, Ferdowsi University of Mashhad, Mashhad 91775-1111, Iran b Department of Materials Engineering, The University of British Columbia, Vancouver, BC V6T 1Z4, Canada 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. © 2016 The Electrochemical Society. [DOI: 10.1149/2.0381606jes] All rights reserved. Manuscript submitted January 6, 2016; revised manuscript received February 23, 2016. Published March 4, 2016. The wide application of thiosulfate in industrial environments, par- ticularly in the pulp and paper industries and in refinery plants makes the study the role of thiosulfate on corrosion of alloys necessary. 1–4 Moreover, the thiosulfate leaching process has been introduced as an alternative approach to cyanide leaching for gold and silver extraction. 5 The action of thiosulfate on pitting corrosion has been extensively studied. 1–3,6–13 Localized corrosion of stainless steels and nickel alloys in thiosulfate containing solutions strongly depends on the thiosulfate to chloride ratio. 6,7,14 The presence of a small amount of thiosulfate enhances the localized corrosion in chloride solutions. 11,13,14 However, thiosulfate addition does not affect the pit- ting corrosion until the chloride ions lead to breakdown of the passive layer. 14 Addition of thiosulfate promotes both the initiation and the growth of metastable pitting on the surface of stainless steels. 13,15,16 Thiosulfate ions enhance the frequency, growth rate, pit stability prod- uct, and pit current of metastable events on 316 SS. The survival probability of stable pits is also found to be decreased by thiosulfate addition to chloride solution. 13 It has been reported that thiosulfate promotes both metastable pit occurrence and growth on 304L and LDX2101. 15 At low thiosulfate to chloride concentrations, adsorbed sulfur may have combined effects with chloride ions on increasing oxygen vacancies in the passive layer. 17 The catalytic effect of ad- sorbed sulfur on lowering the activation energy of metal dissolution of stainless steels has been reported in the literature. 3,16,18,19 In addi- tion, when molybdenum is present in the alloy composition, sulfur is unstable as S o , and is reduced to aqueous hydrogen sulfide. 20 Thio- sulfate ion is quite stable in neutral solutions. However, in the pit environment, which is under low pH conditions, 21 thiosulfate has a thermodynamic tendency for chemical disproportionation and is re- duced into adsorbed sulfur and sulfide on the bare metallic surface. 22 The disproportion of thiosulfate ions to sulfur ions may also give hy- drogen sulfide (H 2 S or HS − ), which has been demonstrated to be a catalyst for dissolution of iron. 23 The adsorbed sulfur (or sulfide) is produced on the bare metal surface thus hinders the repassivation of metal. Following the sulfur adsorption, the pH decreases to a value low enough to sustain the very high rate of anodic dissolution. 2 Some particular experiments have been reported for the develop- ment of a single corrosion pit in order to understand the mechanisms involving the pitting corrosion. 12,24–27 Recently, the pencil electrode has been employed to study the pitting phenomenon and to extract the kinetic parameters required for determining the critical conditions for pit stability and repassivation. 28,29 Assessment of the anodic current density required to maintain acidity via the metal ions concentration in the pit solution (C S ) provides a valuable interpretation of the transition from metastable to stable pitting. Using a single corrosion pit, it has been shown that the thiosulfate may enhance the pit stability either ∗ Electrochemical Society Student Member. z E-mail: mhmoayed@um.ac.ir through the presence of a salt film or through a cathodic side-reaction. 6 In our related work, 12 evaluation of pit anolyte by the pencil electrode method revealed that thiosulfate causes a decrease in both saturation and critical concentration of metal salt within the pit solution. It is believed that the presence of salt film is necessary for a pit to sustain stable growth after the pit cover ruptures. 30 For an open cavity, when the pit cover collapses, a model developed by Laycock and Newman 31 describes the transition from metastable to stable pitting with regard to the pit transition potential, E T , which is the potential between bare and salt covered state. In other words, the E T is a potential above which diffusion control dominates. 31,32 At potentials above E T , the change of current density with time is representative of diffusional control current density. 33 In this stage, the dissolution rate is controlled by diffusion of metal cations from the salt/pit solution interface into the bulk solution. In this stage of pit growth, the relation between the square of current density and reciprocal of time, i 2 ∼ t −1 , which is an indication of the anodic diffusion control, was confirmed. 34,35 Back scanning however, causes the salt film thickness to decrease. 36 Once the potential reaches the period at which the cation concentration decreases below the saturated concentration (C S ), precipitated salt would no longer be stable. Thereby, the metal salt film would be dissolved and afterward the pit bottom will be salt film free. This event results in an ohmic/activation control regime to be established. 37 In this region, current density decreases linearly with potential. Based on the model proposed by Laycock and Newman, 31 the pit transition potential (E T ) is the summation of E corr in the pit solution, activation overpotential (η act = b a log( i lim icorr )) relative to E corr in the saturated pit, and IR drop. This potential is expressed in Equation 1. E T = E corr + b a log  i lim i corr  + I lim R S [1] where b a is the anodic Tafel slope in the pit solution, i corr is the cor- rosion current density in the pit solution, i lim is the limiting current density, and I lim is the limiting current. In this equation, R S represents the total solution resistance. Measurement of the pit transition poten- tial as a function of the anodic limiting current density, aids to precise prediction of the pitting potential. By an extrapolation of this potential to very small length scales, Laycock and Newman 32 have predicted the variation of pitting potential with chloride concentration, Mo alloying, etc. The repassivation potential, E rep , is a characteristic potential below which, no pit could grow. In other words, at potentials less than the repassivation potential, pitting corrosion, once begun, will stop. After the passage of large charge densities, E rep is practically independent of the amount of charge passed in a localized corrosion process. 38–40 At high pit depths, the repassivation potential for pitting and the repassi- vation potential for crevice corrosion coincide. Thus, the repassivation potential for deep pits or crevices could be utilized as a conservative threshold for the occurrence of localized corrosion. 39,40