Steam Generators inevitably contain crevices at tube surfaces where they contact tubesheets and support plates, in which solids accumulate. Within this matrix there are elusive effects of minor impurities including sulfur and lead. NiCrFe alloys are protected by a Cr-rich oxide film. Exposure to environments containing reduced sulfur species can result in rapid corrosion or cracking, mediated by adsorption of sulfur mainly on the Ni component. This catalyzes the dissolution of Ni, removing whatever inhibiting effect it had on the dissolution, and undercutting the Cr-rich oxide. The important aspect of the lead effect is the adsorption of a zero-valent species on the metal. Coupled with the high surface mobility of the lead, this peptizes the surface oxide network and facilitates its undercutting. This paper describes the effects of sulfur and lead on the dissolution of Ni and undercutting of the protective oxide layer via electrochemical measurements and surface science techniques.
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