The role of Borides in the Passivity and SCC of Alloy 600

Abstract

This chapter presents the effects of borides as candidate inhibitors on the passivity and SCC behavior of Alloy 600. Alloy 600 had been used as steam generator tubing materials in various nuclear power plants constructed in the late 1960s and early 1970s. However, the experience with Alloy 600 in the early pressurized water reactors (PWRs) revealed numerous corrosion problems. Polarization curves of the alloy were measured in the solutions at 315° C and the addition of the borides to the reference solution resulted in a decrease in the peak current and the active potential range, implying that passive films could be formed more easily in caustic solutions containing the borides. Samples of the alloy were exposed to the solutions for 120 hours and the surfaces of the samples were examined using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). The results showed that much thinner passive films were formed on the samples exposed to the solutions containing the borides than that in the reference solution. By using slow strain rate tests (SSRT), the SCC susceptibility of the alloy was investigated in the same solutions. The SSRT test results showed that the borides suppressed the SCC of the alloy in the caustic solution. It was concluded that the borides enhanced the passivation of the alloy in the solution, resulting in a decreased SCC susceptibility.