Effect of Heat Treatment Temperature on Microstructure and Properties of Electrodeposited Ni-Sn-Cu Alloy Coating

The Ni-Sn-Cu alloy coating was prepared on galvanized iron sheet by electroplating process, and then the coating was heat treated at 300-500℃ for 1 h under nitrogen protection. The microstructure, phase composition, bonding state with matrix, hardness and corrosion resistance of the coating after heat treatment at different temperatures were studied. The results show that with increasing heat treatment temperature, the coating structure gradually changed from amorphous structure to crystalline structure, and Ni, Cu₃Sn and Ni₃Sn₂ phases precipitated. The bonding between the coating and the matrix was good with increasing heat treatment temperature from 300℃ to 400℃ and was poor above 450℃. With increasing heat treatment temperature, the microhardness of the coating increased first and then decreased, the free corrosion current density decreased first and then increased, the free corrosion potential increased first and then decreased, the capacitance arc radius of alternating current impedance increased first and then decreased, and the charge transfer resistance increased first and then decreased. When the heat treatment temperature was 400℃, coating had the best overall performance with the best surface quality, the good bonding with the matrix, the highest microhardness of 328.7 HV, the lowest free corrosion current density of 10.9 μA·cm^-2, the highest free corrosion potential of -0.689 V, the largest alternating current impedance capacitance arc radius and the largest charge transfer resistance of 1.031 kΩ·cm².