Formation and hot corrosion behavior of MoSi2/NbSi2 composite coating on Nb-based alloys by combined electrodeposition and HAPC processes

Highlights: • The electrodeposited Ni-Mo layer was firstly used as the transition layer. • The increase of Mo content in Ni-Mo layer increased the thickness of MoSi₂ layer. • The MoSi₂/NbSi₂ compound coating well protected the Nb-based alloy. • Thick MoSi₂ greatly improved hot corrosion resistance of the compound coating. -- Abstract: MoSi₂/NbSi₂ composite coating was prepared on Nb-based alloys through electrodeposition and halide activated pack cementation (HAPC) process. Preferred electrodeposition process, the effect of electrolyte with different concentrations of MoO₄^2- on the electrodeposition behavior and coating formation were investigated. Besides, the microstructures of the composite coatings, and the hot corrosion behavior of the MoSi₂/NbSi₂ composite coated Nb-based alloys in the mixture of Na₂SO₄/NaCl (3:1 wt%) salt at 900 ℃ were studied. The results had shown that the co-deposited Ni-Mo layer was low oxygen, crack-free, and denser. Correspondingly 55.87 at% O and many cracks were detected in the Mo layer. With the increase of MoO₄²⁻ concentrations in the electrolyte, the Mo content of the Ni-Mo layer increased and the cellular structures also became prominent. The high Mo content of the Ni-Mo layer was silicified to form a composite coating with thick MoSi₂ outer layer. During the hot corrosion process, the composite coating exhibited stronger resistance. A SiO₂ scale formed in the MoSi₂ outer layer blocked the diffusion of oxygen so it could isolate the NbSi₂ layer from the corrosion salt. The SiO₂ scale got denser as the Mo content increased, which further improved the corrosion resistance of the composite coating. After the MoSi₂ layer was exhausted, the NbSi₂ sub-layer turned into the source of silicon dioxide and the second protective layer of the substrate. The satisfactory hot corrosion resistance of the MoSi₂/NbSi₂ composite coatings was attributed to the compactness and liquidity of the SiO₂ scale.