Oxidation and electrochemical behaviors of Al2O3 and ZrO2 coatings on Zircaloy-2 cladding by thermal spraying

Alumina (Al2O3) and zirconia (ZrO2) coatings on Zircaloy-2 (Zry-2) with a thickness of 100 μm were prepared by thermal spraying. Corrosion behavior of the two coated and bare Zry-2 were evaluated by means of uniform corrosion at 360 °C and 19.2 MPa in distilled water, and by electrochemical measurements in 3.5 wt% NaCl solution. As-sprayed and corroded coatings were characterized by laser microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, nano-hardness and X-ray diffractometry. As-sprayed coated Zry-2 showed higher roughness than the as-received Zry-2. Alumina coating was observed with cracks on the surface and cross section, and it mainly consisted of γ-Al2O3, while some were transformed into γ-AlO(OH). The ZrO2 coating was observed with cracks on the surface and pores in its cross section. Corroded samples, on the other hand, were seriously degraded under corrosion test due to spallation of the ZrO2 coating. Regardless of coating material, the corrosion resistance of coated Zry-2 was mainly dependent on the substrate since the coating failed to block the diffusion of the electrolyte into the substrate.