Pit Initiation and Growth on Stainless Alloys in Solutions Containing Sulfate and/or Chloride, and Thiosulfate
Authors: Carcea Anatolie G , Newman Roger C 
Published: February 2020

A detailed study of sulfur-activated pitting in sulfate-thiosulfate and chloride-thiosulfate solutions was recently completed [1]. The three materials studied were the nickel-rich alloys 600, 800 and 690, and the potentiostatic scratch method was used throughout. The results were different, in detail, from the behaviour of ordinary 304SS as studied in the 1980s [2-5].
In sulfate-thiosulfate solutions, pitting severity was maximized at a sulfate to thiosulfate ratio of about 40 for all three alloys. Differences in pitting current were mostly due to differences in the number of pits (600>800>690) and the 690 alloy showed a reduced pit growth rate, possibly due to a tighter cap of Cr-rich corrosion product (this alloy has 29% Cr). We cannot rule out that minor alloying elements may also have had some effect. An intriguing aspect of the sulfate-thiosulfate pitting of Alloy 600 is its ability to occur at extremely low ionic strength, with thiosulfate concentrations in the micromolar range or even less. The most severe anion concentration ratio remains close to 40.
The new result was the behaviour of these nickel-rich alloys in chloride-thiosulfate solutions. Whereas 304SS will show sulfur-activated pitting at chloride to thiosulfate ratios in the 10 to 30 range, the nickel-rich alloys only showed sulfur-activated pitting at very large chloride to thiosulfate ratios of several thousand. This suggests that it is easy to have too much thiosulfate to activate a pit in a nickel-rich alloy. In the past, the inhibiting effect of excessive thiosulfate was attributed to its electroreduction and/or disproportionation in a pit nucleus, consuming acid in either case. Possibly the kinetics of the electroreduction are different (faster) on a nickel-rich surface (but then why does this not apply in sulfate-thiosulfate solution?). It is also possible that (as in many catalytic processes) thiosulfate has an optimal concentration that generates an optimal coverage of adsorbed sulfur. We know from work of Marcus [6,7] that sulfur adsorption is purely catalytic for nickel dissolution, so even though a continuing supply of adsorbed sulfur is required when we alloy the nickel, this should be less than for iron. We should also not forget that nickel is more noble than iron, even when activated by sulfur adsorption, so the range of potential where thiosulfate reduction occurs overlaps differently with the anodic kinetics in the pit. All of these possibilities are under further investigation.
The sulfate-thiosulfate pitting, specifically, offers an opportunity to address some questions about the role of chloride in pit nucleation. An early observation, on 304SS, was that the scratching method worked particularly well for sulfate-thiosulfate pitting, because spontaneous pit initiation from the unscratched surface was very slow, though not absent. This has been confirmed for the nickel-rich alloys. By adding chloride to the sulfate-thiosulfate solution, it may be possible to detect a specific influence of chloride on pit nucleation. Of course such studies on industrial alloys need to take into account the possible influence of inclusions. Progress in this area will be reported at the meeting.
Acknowledgements:
This research was supported by NSERC (Canada) and by UNENE, the University Network of Excellence in Nuclear Engineering. The support, interest and input of K. Sedman (Bruce Power) and P.J. King (B&W Canada) are greatly appreciated.
References:

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