Insight into the structure and protective performance of Mo/FeCrAl bilayer coatings on Zry-4 substrates in hydrothermal corrosion and high-temperature steam environment

Abstract

Coatings serve as critical safeguards against environmental degradation. This study assesses the performance of Mo/FeCrAl bilayer and FeCrAl single-layer coatings, which have been deposited onto zircaloy-4 by magnetron-sputtered. The study involved a comparative analysis of the coatings' corrosion behavior in high-temperature and high-pressure (HTHP) water at 360 °C and 18.6 MPa, as well as their oxidation resistance at temperatures between 1000 °C and 1200 °C. The Mo sublayer can only function as a barrier layer under certain conditions, effectively exhibiting the material characteristics of a Mo alloy at high temperatures. Under HTHP water conditions for 14 days, a mixed oxide layer of Fe₂O₃ and Cr₂O₃ formed on the surface of the FeCrAl coating, with numerous cracks. In contrast, the Mo/FeCrAl bilayer coating demonstrated superior protection due to the formation of a dense, continuous FeCr₂O₄ spinel layer, effectively shielded the zirconium alloy substrate from corrosion. When exposed to high-temperature steam, the FeCrAl coating underwent a severe Fe-Zr eutectic reaction, leading to coating failure. Meanwhile, the Mo/FeCrAl coating underwent structural evolution. A continuous Al₂O₃ layer formed on the surface, and the MoFe₂ Laves phase precipitated at the grain boundaries of the FeCrAl layer. This significantly hindered the diffusion of Fe and O ions and further reinforced the coating's structure. Additionally, a thermally stable FeMo layer was present at the Mo/FeCrAl interface, and a ZrMo₂ layer formed continuously at the Zr/Mo interface, creating a diffusion barrier and preventing detrimental low-melting-point eutectic reactions. These findings corroborate the crucial role of the Mo barrier layer in enhancing the protective capabilities of the FeCrAl coating. Empirical evidence suggests the potential adoption of Mo/FeCrAl coatings for zirconium alloy cladding, offering outstanding corrosion resistance and high-temperature oxidation resistance crucial for extending the lifespan of materials in extreme environmental conditions.