Corrosion Behavior of NbMoVCrAl High-entropy Alloy Coating in Oxygen-controlled Lead-bismuth Eutectic Alloy

This research addresses the critical challenge of corrosion failure in fuel cladding and structural materials exposed to extreme high-temperature environments (500-600 ℃) within lead-bismuth cooled fast reactor (LFR). To mitigate degradation mechanisms including selective dissolution, oxidation, and liquid metal embrittlement, an innovative NbMoVCrAl high-entropy alloy coating was designed and fabricated via magnetron sputtering on HT9 ferritic/martensitic steel substrates. The coating exhibits a homogeneous single-phase BCC solid solution structure, confirmed by X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectrometry (EDS) analysis. Systematic investigations were conducted to evaluate its corrosion behavior and protective mechanisms under oxygen-controlled conditions (2×10⁻⁶%-3×10⁻⁶%) representative of LFR operational scenarios. The NbMoVCrAl coating (nominal composition: Nb-22%, Mo-22%, V-22%, Cr-22%, Al-12%) was deposited using a high-purity alloy target (99.99%) under optimized sputtering parameters: base pressure of 2.0×10⁻³ Pa, substrate bias voltage of −100 V, chamber pressure of 0.53 Pa, substrate temperature of 350 ℃, and deposition duration of 3 h. Post-deposition characterization reveals a dense, crack-free microstructure with uniform elemental distribution and a thickness of 8.71 μm. Corrosion tests were performed in static lead-bismuth eutectic (LBE) at 500, 550, and 600 ℃ for 1 000 h, with oxygen concentrations precisely regulated via Ar-5%H₂/Ar-2%O₂ gas mixtures. Preferential oxidation of Al generates a continuous, adherent α-Al₂O₃ layer that effectively suppresses LBE penetration. Cross-sectional SEM/EDS confirms corrosion depths of less than 1 μm at all temperatures, attributed to the oxide layer’s thermodynamic stability and rapid self-healing capability. In stark contrast, uncoated HT9 steel suffers severe corrosion (depth: 14.80±0.41 μm at 550 ℃) due to non-protective Fe₃O₄, Fe-Cr spinel formation, and Cr depletion at the oxide-substrate interface. In the aforementioned LBE environment, increasing temperature will accelerate the thickening of the oxide layer, surface Cr depletion, and peeling of uncoated HT9 steel; For the NbMoVCrAl coated HT9 steel sample, the NbMoVCrAl coating maintains a stable BCC solid solution structure at 600 ℃. Compared with the HT9 steel sample, the high-entropy alloy coating has good resistance to LBE corrosion, with a corrosion depth of less than 1 μm and an corrosion rate 1-2 orders of magnitude lower than HT9 steel. It has great potential for practical applications in engineering.