Duplex Ni-Zn-Cu-P/Ni-P electroless coating on magnesium alloy via maleic acid pretreatment

Abstract

In this work, the duplex Ni-Zn-Cu-P/Ni-P electroless coating was applied on the AM60B magnesium alloy via new maleic acid pretreatment for potential applications in automobile industry. Also, the binary Ni-P coating was applied using the same pretreatment in order to compare the results. Scanning Electron Microscopy (SEM) images revealed that a cracked conversion film was created on the AM60B alloy after pretreatment in maleic acid solution. The binary Ni-P and duplex Ni-Zn-Cu-P/Ni-P electroless coatings with cauliflower-like micromorphology were successfully applied on the alloy surface after the maleic acid pretreatment. The duplex coating showed more regular and compact surface morphology as compared to the binary coating. The thickness of the electroless coatings were estimated by the cross-sectional SEM images which revealed that the inhibitory action of Zn2+ ions on the plating rate was fully compensated by the addition of Cu2+ ions. The microstructure and elemental composition of the electroless coatings were assessed by the X-ray Diffraction (XRD) and Energy Dispersive X-ray Spectroscopy (EDS), respectively. The binary coating demonstrated semi-amorphous microstructure while the duplex coating showed almost fully amorphous structure due to its high phosphorus (10.25wt%) and considerable zinc (7.55wt%) contents. Large ennoblement of the corrosion potential after application of the electroless coatings was revealed by Potentiodynamic Polarization (PDP) experiments in 3.5wt% NaCl solution. Also, the corrosion current density of the bare alloy was significantly diminished after the electroless plating. The duplex coating showed relatively better corrosion protection capacity than the binary coating. The results of the PDP tests were confirmed by Electrochemical Impedance Spectroscopy (EIS) experiments in the same corrosive media. The micro-hardness of the bare alloy was remarkably increased after application of both the binary and duplex electroless coatings. The duplex coating showed relatively higher micro-hardness as compared to the binary coating due to the gain refinement. The applied electroless coatings were adherent which was revealed by the thermal shock and pull-off tests.