Corrosion resistance of nanocrystalline titanium zirconium nitride layers deposited on ZIRLO

Zirconium (Zr) alloys are used as nuclear fuel cladding in water-cooled nuclear reactors. Being in contact with steam, the Zr-alloy is oxidised and it absorbs hydrogen. The hydrogen pickup in Zr-alloys induces embrittlement of the tubes due to hydride formation. In this study, effectiveness of corrosion resistant nanocrystalline titanium zirconium nitride (nc-TiZrN) coatings on the surface of a Zr-alloy for the reduction of corrosion and hydrogen pickup is investigated. The nc-TiZrN layers were deposited onto the surface of ZIRLO® using the cathodic arc vapour deposition (CAVD) technique. The influence of nc-TiZrN coatings on the corrosion performance of ZIRLO® was investigated under the following conditions: (a) In an oxygen atmosphere at various temperatures from 400 to 900 ℃ for 30 minutes, (b) for various exposure times from 6 – 24 hours with the temperature kept constant at 500 ℃ in a tube furnace, and (c) in pure steam for 30 days at 360 ℃ and 18.7 MPa in an autoclave. The corrosion properties of the nc-TiZrN coated ZIRLO were compared to uncoated ZIRLO for all experiments. Following corrosion testing, the TiZrN coatings and oxide layers were investigated by scanning and transmission electron microscopy and energy dispersive X-ray spectroscopy. The TiZrN coating had a thickness of ~3.8 μm with a small number of Ti-rich and Nb-rich inclusions. The coating exhibited a single phase solid solution of Ti0.42Zr0.58N with a NaCl-type crystal structure and columnar nanocrystals. These columnar nanocrystals showed a strong {111} preferred orientation and the {111} planes of the TiZrN crystals are oriented parallel to the (001) planes of α-ZIRLO grains with direction perpendicular to the coating/ZIRLO interface. The corrosion testing at various temperatures from 400 to 900 ℃ for 30 minutes showed that the TiZrN coating oxidises slowly and retained its microstructure, composition and crystallinity up to 600 ℃. At higher temperatures above 600 ℃, the TiZrN layer completely oxidised to a non-protective TiZr-oxide layer and resulted in rapid corrosion of the underlying ZIRLO substrate. The rapid corrosion was not observed in the uncoated ZIRLO sample for all temperatures. The results of samples subjected to annealing treatment at 500 ℃ in an oxygen atmosphere at various times up to 24 hours revealed that the TiZrN layer oxidises faster than ZIRLO. This was confirmed by sub-cubic and sub-parabolic oxide growth kinetics observed in the uncoated ZIRLO which are slower than the parabolic kinetics of TiZrN coated samples. In all coated samples, the spallation was observed near the edges of the samples. The spallation was caused by rapid corrosion of the underlying ZIRLO substrate which resulted from the penetration of oxygen ions between the TiZrN coating and ZIRLO substrate along the edges of the samples. In pure steam for 30 days at 360 ℃, a complete loss of the TiZrN layer occurred but the results show that the coating might have delayed the corrosion of ZIRLO. In summary, corrosion resistance of nc-TiZrN coated ZIRLO was lower than that of uncoated ZIRLO. The study shows that the failure of TiZrN coatings leads to rapid corrosion of the underlying ZIRLO substrate which was not observed on the uncoated ZIRLO. It is not yet understood why the presence TiZrN coatings reduced the oxidation resistance of ZIRLO