Pitting of Steam-generator Tubing Alloys in Chloride Solutions Containing Little Additions of Thiosulfate

Abstract The presence of thiosulfate ions in chloride solutions may produce pitting corrosion of UNS N06690 (alloy 690) and UNS N08800 (alloy 800). These alloys are the current options for steam-generator tubing of nuclear reactors. Two submodes of pitting corrosion in chloride plus thiosulfate solutions were observed in both alloys: low potential pitting corrosion (LPPC), close to the corrosion potential, with slow kinetic growth and hemispherical pit morphology; and high potential pitting corrosion (HPPC) with similar characteristics to pitting corrosion in pure chloride solutions. Under dilute concentrations of chloride and thiosulfate, LPPC is characterized by an induction time prior to stable pit growth. This pit induction time can be higher than 14 hours, hindering the determination of the threshold amount of aggressive chemicals required for LPPC occurrence. Therefore, in this work, scratch tests were used to study LPPC, where the mechanical action of passive film removal effectively eliminated pit induction time. The alloys were tested in the solution annealed (SA) and thermally aged (SA+A) conditions, under thermal treatments that simulate those adopted by industry. The threshold amount of thiosulfate required for LPPC occurrence decreased with thermal aging, and was lower for alloy 800 than for alloy 690. As an aid for results interpretation, pure metal specimens (Ni, Cr and Fe) were tested in chloride plus thiosulfate solutions to characterize the electrochemical behavior of the main constituents of alloys 690 and 800.