Investigation of Passive Layers Formed on Alloy 600 in Zinc Containing High Temperature Primary Water

Abstract

Nickel-based Cr-Fe alloys have been found to show very low corrosion rates in extreme environments and thus have been extensively used in industries, such as in nuclear power plants. One of the reasons that Ni-Cr-Fe alloys show good corrosion resistance is that there are oxides that prevent the bare metal from general corrosion. However, Pressurized Water Reactors (PWRs) have experienced stress corrosion cracking (SCC) on the primary side of the Alloy 600 (Ni-15Cr-9Fe) steam generator tubing. It is believed that passive layer formed on Alloy 600 is crucial for SCC initiation. Zn ions are added to primary water in industry in order to reduce SCC initiation. However, the mechanism of SCC suppression with Zn is not yet fully understood. The Zn incorporation process is of great importance in the way that it can help understand the mechanism of SCC initiation. Our research aims to understand the influence of surface oxides of Alloy 600 on SCC in primary water of PWR, focusing on the effect of Zn ions during short period of time (less than 100 hours) at different potentials. The present research examined the effect of Zn ions on the passive layer using in-situ electrochemical method as well as ex-situ characterization method. We constructed a system simulating the high temperature primary water with 100ppb Zn addition and operated the system at 320 C and 1800psi. The sample was oxidized in the autoclave for up to 48 hours at different potentials. During the oxidation period, electrochemical impedance spectroscopy (EIS) was applied at different oxide forming potentials in order to get information of how the alloy performs at different potentials. Ex-situscanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron dispersive spectroscopy (EDS) were also performed on the samples to determine the morphology and chemical profiles of the passive layer. Our EIS study indicated that there was likely a change in oxide property on Alloy 600, likely related to the incorporation of Zn ion, at different potentials for short time oxidation. It was found from the ex-situ experiments that with potential changes, both the thickness and the morphology of the passive layer changes, which agreed with the changes in the impedance results. Figure 1 shows the changes on surface morphology on Alloy 600 samples polarized at different potentials for up to 40 hours. It is indicated that both the distribution and the size of oxide particles changes with polarizing potentials. Zn was found to incorporate into the passive layer of the oxide, substituting some of the Ni content in the passive layer. As potential increased, the content of Zn incorporation changed. Figure 1