Microstructure and Corrosion Behavior of the Modified Layers Grown In Situ by Plasma Nitriding Technology on the Surface of Zr Metal

Abstract

Preparing protecting coatings on the surface of Zr claddings has been regarded as one of the accident tolerant fuel (ATF) strategies. In this study, a series of nitride-modified layers were in situ grown by hollow cathode plasma nitriding on the surface of Zr metal. The influence of nitriding currents and time on the phases, composition, microstructure and corrosion resistance of the modified layers was investigated by X-ray diffraction (XRD), X-ray Photoemission Spectroscopy (XPS), transmission electron microscope (TEM), scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS) and potentiodynamic polarization curves. The ZrO2 layer with loose microstructure and cracks prefers to form under low nitriding current of 0.4 A, which also causes poor corrosion resistance. The high temperature caused by high nitriding currents (0.6 A and 0.8 A) promote the formation of compact nanocrystalline layers, made up of nitride and oxynitride. Below the nanocrystalline layer, it is Zr2N caused by N penetration. Besides this, a double-layer structure of the nanocrystalline layer, i.e., an equiaxed crystal zone with a grain size of ~10–50 nm on the surface and a long strip grain region beneath it was observed. The compact nitride/oxynitride layer with excellent interface bonding can improve the corrosion resistance effectively.