Investigation of Effect of Zn on Passive Film for Alloy 600 in High Temperature Primary Water
Due to its excellent corrosion resistance, Alloy 600 has been extensively used as the heat exchanger tubing material in the steam generator (SG) of pressurized water reactor (PWR) in nuclear industry for many years. However, the Alloy 600 is susceptible to stress corrosion cracking (SCC) on the primary side of the tubing, causing safety problems as well as increasing the cost of maintenance. Zn treatment has been applied in recent years in the industry to suppress the SCC problems, but the main mechanism for this effect is not fully understood. Since the surface film is strongly related to SCC initiation, the goal of the research is to understand how Zn affects the surface film growth on Alloy 600. This research focuses on understanding the change of kinetic parameters on oxide formation as well as the change in chemistry and structure on Alloy 600 oxide.
Electrochemical tests are performed to understand how the passive layers respond electronically to the presence of Zn. Alloy 600 samples are oxidized at various potentials from -743 mV SHE to -223 mV SHE in a lab constructed autoclave at simulated PWR PW environment with 0.1ppm Zn addition. EIS is performed in-situ to determine how the passive layer changes electrochemically over potential, and cross-section TEM-EDS is conducted to some samples to investigate the film formation.
The passive film formed on Alloy 600 consists of a (NixFe1-x)O outer layer, a Cr-rich spinel layer and a Cr2O3 inner layer. Zn incorporates into the outer and intermediate layer, and changes the morphology of the outer layer. EIS equivalent circuits based on the TEM results is constructed for each potential, and it is found that Zn largely reduces the rate of oxygen incorporation into the inner layer at potentials below -573 mV SHE. Other parameters are not affected by Zn addition in to the solution, which agrees with the TEM results.