Advanced characterization of oxide films formed on Ni alloy in PWR relevant conditions (and its relation to stress corrosion cracking) (Postdoc)

Primary water stress corrosion cracking (PWSCC) is known as one of the most important degradation phenomena which occur in nickel-base alloys, exposed to harsh conditions of Pressurized Water Reactors (PWRs). For example, the PWSCC of alloy 600 is found in the primary side of actual PWRs. There are many theories of SCC and it appears unlikely that a single mechanism is capable of explaining all instances of SCC. The presence of a surface film is a common requirement in most, if not all, theories of SCC. This has lead to the hypothesis that the dependency of SCC on potential is related to potential driven changes in the identity or properties of surface films [1, 2]. According to this hypothesis, a specific surface film is necessary for SCC to occur. In steam generators made of Ni-base alloys such as alloys 600 and 690, the properties of the oxide film formed due to exposure to the primary water of the PWR are argued to be crucial for stress corrosion cracking effects. Previous studies of these alloys showed that the oxide film has double-layer structure with an inner compact layer rich in chromium, and an outer layer consisting of well-defined octahedral Ni-rich crystals. A short-time experiment on alloys 690 and 600 in primary water conditions suggested a film structure with an internal layer of Cr₂O₃ and an external layer of Ni(OH)₂. It is also reported that the structure of the oxide layer of alloy 600 can be greatly influenced by dissolved hydrogen content in PWR primary water.

Additionally, the operation of PWRs produces radionuclides, fission products and activation products. Fission products and activation products of the fuel UO₂ normally remain inside the fuel rod unless a fuel rod is defective. The range of the activation products encompasses those originating from the coolant's impurities and its additives, and those from the corrosion products formed from the primary circuit materials. Only the corrosion products' radionuclides play an important role in the contamination of the primary circuits.  ⁶⁰Co is mainly responsible for the dose rate measured at the outer surface of the primary circuit. Besides knowledge of the sources of ⁶⁰Co, the capacity of the oxide layer (formed on the structural materials due to corrosion) for the uptake of this radionuclide is a further essential value required for evaluation of the dose rate. Therefore, information on the structure of the oxide layers is significant. Activity build-up is a corrosion related problem, which causes problems during the maintenance of the nuclear power plant.

Since both corrosion and activity build up are determined by the oxide layer formed on the material in the environment, it is important to understand the properties of the oxide layer, which eventually leads to better understanding of these two processes, especially SCC. However, in spite of a detailed analysis, the reliable correlation between the characteristics of the oxide film and SCC effect is not yet fully established. In recent years, although significant field experience involving PWSCC has been observed on weld metal alloy 182 [3], the studies of oxide layer formed on it in PWR primary water conditions are still lacking.  Therefore, in this post-doc research we focus on the study of the oxide films formed on alloy 182 after exposure to simulated PWR primary water conditions.

Modern surface-analytical techniques can provide useful information regarding the nature and composition of passive oxide films and transport processes in oxide scales grown at high temperature. For better and detailed characterization of the formed oxide films, one usually has to combine different techniques. 

1.         T.S. Mintz and T.M. Devine, Influence of surface film on the susceptibility of Inconel 600 to stress corrosion cracking. Key Engineering Materials, 2004. 261-263: p. 875-884.

2.        T.S. Mintz and T.M. Devine. The role of surface films in the stress corrosion cracking of alloy 600 in PWR primary water. in 12th International Conference on Environmental Degradation of Materials in Nuclear Power System-Water Reactions, August 14–18, 2005, Salt Lake City, Utah, USA. 2005.

3.         F. Vaillant, et al., Environmental behaviour and weldability of Ni-base weld metals in PWRs. Les Materiaux Dans Le Nucleaire, 2007(6): p. 62-71.

4.         J.-H. Liu, et al., Characterization of oxide films formed on alloy 182 in simulated PWR primary water. Journal of Nuclear Materials, 2009. 393: p. 242-248.

The objective of this post-doc research is to understand the PWSCC behaviour of Ni-based alloy 182 & alloy 52 through understanding of the properties of the oxide films formed on these materials in PWR relevant conditions.  For this purpose, the formed oxide films after exposure will be thoroughly characterized. We expect to understand the dependency of SCC of these materials on potential by understanding of the potential driven changes in the identity or properties of surface films formed on these materials.

Based on our preliminary study [4], a new systematic experimental scheme has been defined. This new experimental scheme is as following:

1.    Materials: pure Ni & Cr (as reference), alloy 182; all materials were polished up to 1 µm to provide a reproducible initial condition.

2.    Exposure time: fixed at 4 weeks

3.    Temperature: 325 °C (more relevant for SCC)

4.    Water chemistry: 4 different H₂ concentrations (0 cc/kg, 2 cc/kg, 25 cc/kg & 50 cc/kg in water). 

5.    Number of samples: Ni × 4, Cr × 4, alloy 182 × 4.

Exposure tests have been finished. One of the objectives of this Post-doc study is to perform advanced characterisation on these exposed samples using techniques including SEM, (GI)XRD, XPS/Auger, TEM and Raman. We expect to obtain information including composition, thickness, elemental chemical states, microstructure etc. TEM sample preparation procedures will have to be developed for studying oxide films, building on existing knowledge.

Parallel to the characterisation of the already exposed samples, new exposure experiments with pure Fe and alloy 52 will be carried out. Alloy 52 is the weld metal for alloy 690 and is also studied as weld overlay on alloy 182, and this alloy has been reported to have better SCC resistance than alloy 182, which could lead to arrest of cracks started in alloy 182. Studying the oxide films formed on alloy 52 might allow one to understand better the crack arresting mechanism in it. Same characterisation procedure will be applied to the exposed samples. The pure elements Fe, Cr, Ni are the main alloying elements of Ni-based alloys. Changing the concentration of dissolved hydrogen influences the stress corrosion crack growth rate of the Ni-based alloys that tend to have a maximum crack growth rate at 2 ppm dissolved hydrogen, which is actually the standard concentration used in most PWRs.

Another important aspect in this research is that we would like to investigate the effect of the specific additions of the weld metal (Mn, Si, Nb) on the formation of oxide film on alloy 182. This could possibly allow us to understand better why alloy 182 cracks more than alloy 600 since both alloy have comparable compositions in terms of main elements (Fe, Cr & Ni). One possible experimental scheme will be making model alloys consisting mainly of Fe, Cr & Ni, but with varying Mn contents, for the exposure experiments. After exposure, the oxide films will be thoroughly characterized.