Electroless Ni–P plating on magnesium alloy by innovative, simple and non-toxic oxalate pretreatment and its corrosion protection

Abstract

In this work, simple and non-toxic oxalate pretreatment was used for electroless Ni–P plating on AM60B magnesium alloy. The morphology and chemical composition of the alloy sample after oxalate pretreatment were studied by X-ray Photoelectron Spectroscopy (XPS) and Scanning Electron Microscopy (SEM), respectively. Also, the morphology, chemical composition, microstructure, and corrosion resistance of the electroless Ni–P coatings were investigated by the SEM, Energy Dispersive X-ray Spectroscopy (EDX), X-Ray Diffraction (XRD) and electrochemical methods, respectively. Moreover, porosity and adherence of the applied electroless coatings were examined by porosity and thermal sock tests, respectively. The SEM images revealed that the alloy surface was covered by a cracked conversion film and the cracks were extended by increasing the immersion time from 5 to 30 min. The XPS analysis showed that the conversion layer was mainly composed of the magnesium oxalate with nodular molybdenum oxide particles on the β phase area due to the presence of the sodium molybdate in the pretreatment solution as an accelerator. The formation of the nickel nucleation centers has been started through the cracks of the oxalate film. Also, cauliflower-like nodular electroless Ni–P coatings were formed on the surface of the 5, 15, and 30 min oxalate treated samples after 3 h plating and the surface smoothness of the coatings decreased by increasing the pretreatment time. The EDX analysis showed that the coatings have moderate phosphorus (P) content which slightly increases with pretreatment time. Moreover, the XRD patterns revealed that all of the electroless coatings have mixed crystalline amorphous structure. Application of the electroless Ni–P coating was significantly increased the corrosion resistance of the bare alloy and the best corrosion protection performance was observed for the 15 min pretreated sample. All of the applied coatings were pore-free and adherent to the alloy substrate. The micro-hardness value of the AM60B alloy sample was significantly increased after application of the electroless coatings.