Electrochemical Studies of the Effect of Solution Impurities on the Onset of Stress Corrosion Cracking of Austenitic Stainless Steel in Pressurized Water Reactor Primary Water, 18-R8202
Todd S. Mintz
Steve T. Clay
Inclusive Dates: 12/20/11 – Current
Background — Existing nuclear reactor facilities in the United States are fast approaching the end of their operation license period. The Nuclear Regulatory Commission (NRC) and utilities are in the process of extending these licenses. This requires a better understanding of long-term deterioration of materials used in the power plant. Stress corrosion cracking (SCC) in primary water is of special interest because it has been the cause of failure of nickel-based alloys and stainless steels. SCC is a delayed fracture process divided into three stages: (1) initiation, (2) steady-state propagation and (3) final failure. A majority of the work carried out in pressurized water reactor (PWR) environments has focused on the second and third stages for Ni-based alloys. However, as the lifetime of the current PWR fleet increases, a better understanding of the initiation processes has become necessary. SCC of stainless steel in PWR is becoming more of a concerned because more than 130 SCC events have been reported since the 1980s. As a result, EPRI has classified SCC of stainless steel in primary water of PWR as a high priority issue.
Approach — The objectives of this project are: (1) to measure the effect of contaminants (chloride, sulfate and oxygen) in primary water on the electrochemical properties of austenitic stainless steels, (2) to measure the effect of those compounds on the semiconductor properties of the passive film, (3) to assess the impact of applied potential on crack initiation and growth, (4) to correlate the changes in electrochemical and semiconductor properties with the measured occurrence of crack initiation and propagation, and (5) to successfully develop a new technique to assess the early stages of stress corrosion cracking in passive materials.
Task 1 – Assemble the Experimental Setup: The experimental equipment is a once-through system based on preexisting equipment that was modified to accommodate the current program. The experimental setup was completed during the second quarter of the project.
Task 2 – Determine the impact of impurities on the electrochemical properties of the passive films. Cu/Cu2O reference electrodes were assembled, tested and used during the high-pressure, high-temperature electrochemical testing. Electrochemical polarization scans of stainless steels 304 and 316 were performed in Cl- (5 ppm) and Cl- +SO42- (5 ppm each) contaminated borated water solution 3 and 4, respectively. The passive current was lower for stainless steel 304 in both solutions. Both stainless steels in solution 3 display a clear passive behavior, while their open circuit potentials and current densities in solution 4 are significantly higher. It has been reported that combinations of chloride and sulfate had dramatic impact on the SCC resistance of stainless steels in the field. The potentiodynamic scans suggest that the combination of impurities has a similar impact on the electrochemical behavior of stainless steels.
Task 3 – Effect of applied potential on crack growth rate of stainless steels. The design, manufacture and calibration of the CT specimens have been completed. Non-instrumented CT specimens loaded at 85, 90 and 95 percent of yield stress have been added to the autoclave during the electrochemical tests performed in solution 4.