2012 IR&D Annual Report

Electrochemical Studies of the Effect of Solution Impurities on the Onset of Stress Corrosion Cracking of Austenitic Stainless Steel in PWR Primary Water, 18-R8202

Principal Investigator
Florent Bocher

Inclusive Dates:  01/01/12 – Current

Background — Existing nuclear reactor fleets in the United States are fast approaching the end of their operating license period. The Nuclear Regulatory Commission (NRC) and utilities are in the process of extending the 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 since 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 over 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. The objective of the project is to combine electrochemical and fracture mechanics techniques in high-temperature and high-pressure solution to identify the impact of passive film properties on SCC initiation.

Approach — The approach for this project was:

  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.

Accomplishments — This project was a partial success. A high pressure and temperature flowing test setup to perform electrochemical testing under realistic conditions was successfully assembled. Electrochemical measurements were successfully performed at high pressure and temperature.

The electrochemical data could be related to the semiconductor properties of the surface oxide film. However, it was not possible to link those data to mechanical properties because no crack initiation was observed over the relatively short test duration (one month). Samples were statically loaded and successfully instrumented in order to make potential drop measurements under high temperature and pressure conditions. Mott-Schottky and potentiodynamic scans have been successfully correlated at ambient pressure and temperature when assessing the oxide film properties of 316 stainless steels. Furthermore, the effect of polarization on mechanical failure was measured using SSRT at ambient temperature. The oxide film properties were measured using both Mott-Schottky and potentiodynamic scans. As with high temperature testing, no cracking was initiated. In both cases, the oxide film displayed n-type semiconductor properties.

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