Intergranular Stress Corrosion Cracking of Type 304 Stainless Steels Treated with Inhibitive Chemicals in Simulated Boiling Water Reactor Environments

Abstract

Electrochemical potentiodynamic polarizations, electrochemical corrosion potential (ECP) measurements and slow strain rate tensile (SSRT) tests were conducted to investigate the intergranular stress corrosion cracking (IGSCC) characteristics of Type 304 stainless steels treated with inhibitive chemicals in simulated boiling water reactor (BWR) environments. A number of thermally sensitized specimens were prepared and were pre-oxidized in a 288°C environment with the presence of 300 ppb dissolved oxygen for 360 h. Most of the specimens were then treated with various chemicals including powdered zirconium oxide (ZrO2), powdered titanium oxide (TiO2), and zirconyl nitrate [ZrO(NO3)2] via static immersion at 90°C, 150°C, and 200°C. Test environments were specifically designed in a circulation loop to create a dissolved oxygen concentration of 300 ppb. Test results showed that the corrosion current densities of all treated specimens were lower than that of the untreated, pre-oxidized specimen at ambient temperature in a solution mixed with 1 mM K3Fe(CN)6 and 1 mM K4Fe(CN)6. The ECPs of the treated specimens could be lower or higher than that of the pre-oxidized one at 288°C, depending upon the type of treating chemical and the treating temperature. In addition, IGSCC was observed on all specimens (treated or untreated) in the same environment. However, the untreated specimen exhibited lower elongation, shorter failure time, and more secondary cracks on the side surfaces. It was therefore suggested that inhibitive chemicals such as ZrO2, TiO2, and ZrO(NO3)2 did provide a certain degree of enhancement in improving the mechanical behavior of the treated specimens and in prolonging the IGSCC initiation time.