FLOW EFFECTS ON THE STRESS CORROSION CRACKING OF SENSITIZED STAINLESS STEELS UNDER BWR ENVIRONMENTS

摘要

One of the major degradation mechanisms for the stainless steel components and piping of the Boiling Water Reactor is the intergranular stress corrosion cracking (IGSCC). In order to mitigate the deleterious effect of IGSCC, hydrogen water chemistry is adopted to suppress the electrochemical corrosion potential of stainless steel, which is viewed as a measure of IGSCC susceptibility. When corrosion potential is lowered below -230 mV SHE as a result of hydrogen injection, stainless steel is considered to be immune from IGSCC, whereas stainless steel becomes more prone to cracking when the environment is more oxygenated and the corrosion potential moves higher in the anodic direction. The effect of hydrogen injection is to reduce the oxidizing species by accelerating their recombination reactions with higher concentration of hydrogen. The depletion of oxidizing species of the stainless steel surface and the resulting diffusion-limited cathodic reduction reaction is the main cause for the lowering of corrosion potential. However, the increase of flow rate can substantially raise the rate of oxygen supply to the metal surface and electrochemical potential shifts anodically, lessening the intended effect of hydrogen. On the other hand, the stronger flushing action of the increasing flow will wash out the more aggressive environment present at the crack tip and IGSCC can be mitigated to a less extent. Whether the flow effect will be beneficial or detrimental may be dependent on crack geometry, flow rate, water oxygenation, and crack tip environments. An experimental study is conducted to determine the flow effect on the intergranular stress corrosion cracking behavior of stainless steels under simulated BWR environments using both compact tension specimens and thumbnail surface crack specimens. Results have indicated that increasing flow rate generally has a beneficial effect on the IGSCC. Under most conditions including cases where the corrosion potentials become more anodic, the crack propagation rates are generally lowered due to better mixing of the crack tip environment with the bulk solution. Only at excessively high stress corrosion crack propagation, high flow rate fails to slow the crack propagation and even exhibits some accelerating effect at times.