Study of Hot Corrosion Resistance of Electroless Nickel Coating with Different Content of Phosphorous in Molten Salt Deposit Na_2SO_4-NaCl at 650°C

摘要

In this research, the hot corrosion behavior of electroless nickel-(4, 7,9, and 12 wt % P) was studied in 70 wt % Na_2SO_4-30 wt % NaCl (3 and 6 mg/cm2) molten salt on a copper substrate. After being plated with a thickness of 25 ± 3 urn, the specimens were annealed at 400°C for 1 hour, and then the hardness of each was measured by the Vickers hardness test method. The hot corrosion test was performed at 650°C for 20 hours inside the furnace at a rate of 4°C/min under atmospheric conditions. The results showed that Ni-7 wt % P and Ni-12 wt % P electroless coatings included the highest and the lowest hot corrosion resistance, respectively. The difference in the weight increase of the hot corrosion of Ni-12 wt % P coating was about8 times as much as that of Ni-7 wt % P, and the difference in thermal expansion between the substrate and the coating was responsible for the cracks in the coating. Diffusion of the molten salt through these cracks expanded and reached the intersection of the coating and the substrate. The results of the XRD showed the formation of NiSO_4, NiO, and Cu on the coating surface after the hot corrosion test.