Investigation of the impact of coatings on corrosion and hydrogen uptake of Zircaloy-4

• We tested the effectiveness of coatings on Zircaloy-4 samples to reduce corrosion and hydrogen ingress.
• High temperature aqueous and steam-phase corrosion tests were performed.
• Two of the coatings tested showed good corrosion resistance.
• No localized corrosion events or clear damage to the coatings were observed.
• Once oxidized, the coatings reduced hydrogen ingress into Zircaloy-4.

In this study, commercially available coatings of CrN, TiAlN and AlCrN were applied by physical vapour deposition to obtain 2–4 μm thick coatings on Zircaloy-4. The corrosion resistance, as well as the impact on hydrogen permeability of these coatings, was investigated. The latter is important with respect to hydrogen embrittlement of reactor components exposed to water with a certain dissolved hydrogen content. The corrosion resistance was investigated under pure water at 350 °C for 24 h and under CANDU conditions (300 °C and pH 10.5) for 30 days. Limited experiments were also performed in superheated steam (atmospheric pressure) at temperatures up to 1100 °C. Surface imaging (scanning electron microscopy) was performed before and after the exposure of the samples, with additional transmission electron microscopy analysis on a limited selection of samples. The composition of the coatings was determined by energy dispersive x-ray spectroscopy and depth characteristics were determined by secondary ion mass spectrometry. The CrN-coated samples have shown the best corrosion resistance to date, under pressurized water reactor, CANDU, and superheated steam conditions. Additionally, oxidation of the CrN and TiAlN coatings caused a significant reduction in the hydrogen uptake.