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Preparation and Properties of Rare Earth Modified Electroless Plating Coating on Titanium Alloy Surface

SUN Jie;ZHANG Xing-wei;MING Ting-yun;TAN Yong;School of Environmental and Chemical Engineering, Shenyang Ligong University;  
The work aims to prepare electroless Ni-P coating on titanium alloy surface and study effect of rare earth on properties. SEM and EDS were used to test thickness and composition of chemical plating; XPS was adopted to analyze valence state of rare earth element in plating coating; XRD was taken to analyze composition of plating coating through different heat treatments; microhardness tester was used to test hardness of plating coating through heat treatment; and electrochemical test was conducted to characterize corrosion resistance of titanium alloy and plating coating through heat treatment. Rare earth-modified thick coating was composed of a 70 μm Ni P layer and a 30 μm Ni-P-Ce layer. The coating was formed by three elements including Ni, P and Ce with respective mass fraction of 89.94% , 10.03% and 0.03% . The element Ce was composed of three valence states of +4, +3 and 0. With the increase of heat treatment temperature, the coating gradually changed from amorphous to crystalline. The transformation occurred at 200~300 ℃ and the composition also changed. The hardness of coating increased and reached 1000 HV after heat treatment at 400 ℃. The coating could effectively improve the corrosion re-sistance of titanium alloy substrate, but the corrosion resistance dropped dramatically as the heat treatment temperature increased. The optimum heat treatment temperature was 200 ℃(the corrosion current density and polarization resistance were 0.2445 μA/cm~2 and 155.464 k? respectively). The coating modified by rare earth was 100 μm thick plating coating. Rare earth cerium, nickel andphosphorus are co-deposited on the titanium alloy due to reaction. The heat treatment temperature has obvious influence on crystallization mode, composition and hardness, especially corrosion resistance.
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