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

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 MoO42- 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.