Corrosion and wear behavior of an electroless Ni-P/nano-SiC coating on AZ31 Mg alloy obtained through environmentally-friendly conversion coating

Abstract

In this paper, an environmentally-friendly pretreatment has been used for electroless NiP on AZ31 magnesium alloy. Saturated aqueous NaHCO3 solution was used to form carbonate compounds on the surface of the substrate prior to Ni-P/nano-SiC electroless plating. Various amounts of nano-SiC particles were used to enhance the hardness, corrosion and wear resistance of the coatings. The coatings were characterized by using scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS), and via X-ray diffraction (XRD). Polarization tests were carried out for investigating the corrosion resistance of the coatings. Pin on disk tests were used to study wear behavior of the coatings. The effect of heat treatment temperatures on the coatings structure, hardness and corrosion behavior were studied. However, the hardness of the substrate decreased due to the growth of Mg grains at high temperatures. Moreover, heat treated NiP coatings at 300 °C exhibited better corrosion resistance compared to other heat treatment temperatures. As a result, the temperature of 300 °C was chosen as the optimum temperature. Results also indicated that 1 g L−1 nano-SiC in plating bath provided a uniform composite NiP electroless coating with high hardness (795 HV) and corrosion resistance. Adding more nano-particles to the bath resulted in agglomeration of the particles and did not have positive effect on properties. The wear behavior of coatings were investigated using steel pin (AISI 52100) as the counterpart in pin-on-disk wear test. The optimum as-plated composite coating (Ni-P/1SiC) produced the best wear resistance at a rate of 4.2 × 10−5 mm3 N−1 m−1 wear rate and the highest corrosion resistance belonged to Ni-P/1SiC after annealing at 300 °C.