Effect of Cold Work on High Temperature Electrochemical Performance and Stress Corrosion Cracking Behavior of 321 Stainless Steel in Simulated PWR First Circuit Environment

Abstract

321 stainless steel (SS) is one kind of austenitic stainless steel, in which Ti is added as stabilizing element. Due to its excellent corrosion resistance and comprehensive mechanical properties under high temperature, 321SS is an important construction material in light water reactor (LWR). It is prone to cold work due to various factors in the whole cycle, which changes its working performance, especially corrosion resistance and stress corrosion cracking (SCC) property, while corrosion and SCC are main failure forms of 321SS in LWR. The mainstream view is that the transformation of grain boundary type, the increase of slip lines, the formation of voids and the phase change from austenite to martensite caused by cold work will increase the probability of failure. However, the dominant factors and main mechanisms are still controversial. The purpose of this study is to investigate the effect of cold work on corrosion and SCC of 321SS and preliminarily explain the mechanism. Samples with different work rate and martensite fraction were acquired through different extent of cold work and hot work. The as-received specimen was remarked as SA, the cold worked specimen was remarked as CW20 and the hot worked HW20. The corrosion behavior was studied by high temperature electrochemical tests. The electrochemical impedance spectroscopy (EIS) of SA and CW20 was measured in a simulated pressurized water reactor (PWR) first circuit environment, and HW20 was used as a comparison. The SCC performance of specimens was tested by stress corrosion test under slow strain rate tensile (SSRT) loading mode. After tensile tests, micro cracks initiated on the surface were counted and the average crack length was calculated to evaluate SCC sensitivity. The microscopic characteristics of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). XRD analysis shows that barely no martensite exists in SA, the martensite volume fraction in CW20 is 22% and 11% in HW20. In EIS tests, the charge transfer resistance rises both in CW20 and HW20 compared with SA. SA has the highest film resistance, HW20 the second, and CW20 has the lowest. After SCC test, the most cracks initiated on the surface of HW, and the least on SA. The conclusions can be drawn as following: The cold work causes the transformation from austenite to martensite in the matrix, and the high temperature inhibits this process. With the increase of strain degree, the charge transfer resistance increases, and the film resistance decreases with the increase of martensite content. A Cr-depleted zone is caused by martensite at the interface between the oxidize layer and the matrix, thus leads to the reduction in protective effect of the passivation film. With the increase of martensite content, the film resistance decreases, and the corrosion resistance of the passivation film decreases. Micro cracks tend to initiate more easily after either cold work or hot work compared with primary solution annealed 321SS. The martensite phase distributed in the matrix is oxidized preferentially and suppresses the corrosion of austenitic phase. Under the conditions of this research, strain-induced martensite inhibits SCC crack initiation.

Keywords

• pressurized water reactor
• 321 stainless steel
• cold work
• high temperature electrochemical test
• stress corrosion cracking