Influence of Heat-Treatment on Microstructure,Hardness and Corrosion Resistance of Pulse Electroplated Ni-P Alloy Coating

Ni-P alloy coating was prepared on the surface of Q235 steel by pulse electroplating.As-plated Ni-P alloy coating was heat-treated in a temperature range of 200～500 ℃.The phase structure of the coating was analyzed by means of X-ray diffraction.The corrosion resistance of the coating in 3.5% NaCl,10% HCl and 10% H2SO4 solution was evaluated by measuring electrochemical polarization curves.The corroded surface of the alloy coating was observed using a scanning electron microscope.Results show that the precipitating rate of Ni3P phase increases with elevating heat-treatment temperature,and the time for crystallization as well as for reaching the maximum hardness of Ni-P coating is reduced therewith.The Ni-P alloy coating heat-treated at 300 ℃ and 400 ℃ possessed the maximum hardness of 862.8 HV2 N and 887.4 HV2 N respectively.However,heat-treatment significantly reduced the corrosion resistance of Ni-P alloy coating and led to the pitting of coating in the three kinds of corrosive mediums.