UC Berkeley

Alloy 600 is found to be susceptible to stress corrosion cracking (SCC) in Pressurized Water Reactor (PWR) primary water environment. In this study, in-situ Surface Enhanced Raman Spectroscopy (SERS) is used to characterize surface films formed on Alloy 600 and Alloy 690 in PWR PW at 320oC. A simulated PWR PW system is constructed. In addition, a titanium autoclave is designed for high-temperature electrochemical tests.

In this study, we have also measured SERS spectra of surface films formed on unalloyed nickel, unalloyed chromium, Ni-5Cr-8Fe, Ni-10Cr-8Fe, Ni-10Cr and Ni-20Cr. The surface films were formed at different potentials, from -0.9V to -0.5V vs SHE.

A formation of NiO film is observed on the surface of nickel at -0.65V, which shows a Raman peak at 540 cm-1 due to crystal defects. For pure chromium, Cr2O3 passive film is formed on its surface. The Raman peaks are located at 540 cm-1 and 610 cm-1. The thickness of Cr2O3 passive film is about several nanometers, depending on the growth time in PWR PW. A peak at 610 cm-1 would likely indicate Cr2O3 and the absence of this peak, in conjunction with a peak at 540 cm-1, would indicate NiO. It is found that the surface films on Ni-5Cr-8Fe and Ni-10Cr-8Fe alloys are composed of Cr2O3 and FeCr2O4 spinel. No spinel oxide peak is observed in the SERS spectra for Ni-10Cr and Ni-20Cr binary alloys, and the surface film on both alloys is Cr2O3. An analysis of SERS spectra indicate that, as alloy's chromium concentration increases, the amount of Cr2O3 included in the alloy's surface film would increase.

The surface film of Alloy 600 is composed of Cr2O3 inner layer (IL) and FeCr2O4 outer layer (OL). For short-time growth (10 min), the inner Cr2O3 layer is about 2 - 4 nm thick, and the average size of FeCr2O4 crystallites is about 10 nm. The actual film thickness depends on the growth time and aqueous environment. The surface film formed on Alloy 600 in PWR PW has a minor change if the sample is removed from the autoclave and exposed to air. The surface film of Alloy 690 is entirely a single layer of Cr2O3 (~ 2 nm thick). According to Diffusion Path Analysis, the difference on surface films formed on Alloy 600 and Alloy 690 is a consequence of a change in the relative diffusivity of O and Cr. Our in-situ SERS spectra measured for surface films on Alloy 600 and Alloy 690 are different with most of other people's findings. Three aspects have been considered and discussed. The first, there might be a difference between in-situ and ex-situ characterization techniques. The second, the film growth time in PWR PW is significantly different. The third, different autoclaves are used. The recirculating titanium autoclave is used in this study, while stainless steel autoclave is used in most of other studies, which might cause a change of the concentration of cations in the vicinity of metal surface and a precipitation of cations on the surface of oxides. It is found that the exposure time and testing autoclave might be responsible for the discrepancies between this study and the earlier studies.

By using in-situ Surface Enhance Raman Spectroscopy (SERS), we have investigated the surface oxides formed on Alloy 600, Alloy 690, Ni-Cr, and Ni-Cr-Fe alloys in PWR primary water at 320oC. Our in-situ SERS spectra results suggest that SCC susceptibility of Ni-based alloys might be correlating with a formation of surface films formed in PWR primary water.