Mg - Gd - Y - Zr Magnesium Alloy Surface Electroless Ni - P - P And Ni W Coating Research

In order to electroless Ni-P and Ni-W-P coating on Mg-10Gd-4.8Y-0.6Zr magnesium alloy, a new and suitable pretreatment process for this magnesium alloy was explored in this paper. The morphology, component, chemical composition and structure of the pretreatment layers and electroless nickel coating were analyzed using scanning electronic microscopy, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction spectroscopy. The microhardness, wear resistance and corrosion resistance of the Ni-P and Ni-W-P coating were evaluated by means of friction and wear monitor, a diamond indenter, potentiodynamic polarization. Experiment results are obtained as follows:(1)the Ni-P coating is uniform,compact and dense which was plated onMg-Gd-Y-Zr megnesium by using the new pretreatment process.But the traditional electroless nickel process would not deposited nickel on Mg-Gd-Y-Zr alloy perfectly.This can be explained that there was a metal Cr produced during the pretreatment process, which can provide the active point for depositing nickel. All of that is benefical for nickel deposition.(2) In the initial stage of EN deposition process, there was replace reaction occurred, the preferential nucleation on the second phase, grain boundaries and zinc layers. Then, the nucleation at the early stages of coating, the Ni crystallites act as catalytic sites to absorb the hydrogen atoms and the autocatalytic reaction for EN deposition is inatiated by the dehydrogenation of the reducing agent. During the levelling process, more Ni is deposited on the resessd areas than the protruding areas.This leads to the roughness of the coating to be reduced and a uniform deposite is obtained.(3) The structure of the as-plated Ni-P and Ni-W-P coating is amorphous; the thickness of the Ni-P and Ni-W-P coating is50μm and15μm,respectively; the P content of the Ni-P and Ni-W-P coating is9.43wt.%and7.61wt.%(W,3.48wt.%),respectively; the microhardness of the Ni-P and Ni-W-P coating is6.2times and6.5 times than the microhardness of the magnesium ally substrate, respectively,which indicated that the microhardness of later coating is higher than former. The results of the electrochemical tests shown that: an activation-controlled anodic process was observed in the anodic region of the Ni-P and Ni-W-P coating. The corrosion potential Ecorr of the Ni-P and Ni-W-P coating shifted by1090mV and1298mV positively, respectively, and the corrosion current density decreased by one order and two order respectively. The results indicated the ternary Ni-W-P coating should high corrosion resistance in accordance with the more positive corrosion potential and much lower corrosion current density.