ABSTRACT

Alloys UNS N06600, N06690 and N08800 used for steam generator tubes are prone to pitting corrosion in aqueous chloride-thiosulfate solutions. Pitting corrosion was studied in 0.1 to 1 M chloride solutions with additions of thiosulfate ranging from 10-4 M to 10-2 M, at room temperature. The alloys were tested in solution annealed and thermally aged conditions. Potentiodynamic curves showed an anodic peak at potentials slightly higher than ECORR in N06690 and N08800 for both metallurgical conditions. This peak results from a localized corrosion process that was named low potential pitting corrosion (LPPC), because pits repassivated when potential was scanned in the noble direction, and also to distinguish it from the conventional chloride pitting process observed at higher potentials. Cyclic potential scans up to potentials in the LPPC zone showed a hysteresis loop and repassivation potentials (ERP, LPPC) were very close to ECORR measured under deaerated conditions. According to the measured values of ERP,LPPC, N06690 was more resistant to pitting corrosion than N08800. Immersion tests in aerated 1 M NaCl with [Cl-]/[S2O32-] = 2000 led to pitting corrosion in all alloys, while in deaerated conditions, only N08800 was pitted.

INTRODUCTION

Thiosulfate (S2O32-) may promote pitting corrosion in alloys UNS N06600, N06690 and N08800 used in steam generator tubes of nuclear reactors.1 In those components, thiosulfate may appear as a byproduct of the redox reaction between sulfate (SO42-) impurities and the hydrazine (N2H4) commonly added to secondary water.2 Thiosulfate is a metastable3 anion and it can reduce on metal surfaces forming an adsorbed sulfur (Sads) or sulfide layer. Sulfur-rich layers have a broad stability region in a potential vs. pH diagram4 that overlaps with the passivity domains of Fe, Ni and Cr, the main constituent elements of alloys used for steam generator tubes. Therefore, these layers can prevent passivation or repassivation4,5, thus affecting pit stability and growth.

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