The effect of heat treatment on corrosion resistance of Ni-W-P-Al2O3 composite coating

Ni-W-P-Al2O3 electroless composite coating was successfully co-deposited on 45 steel substrate using electroless plating. Optical microscope (OM), X-ray diffraction (XRD) and potentiodynamic polarization were used to analyze the morphology, microstructure and corrosion resistance of the composite coating. The results show that Al2O3 particles co-deposit homogeneously, and the structure of the composite coating as deposited is amorphous and crystallite. After heat treatment, the amorphous structure of the composite coating appears a precipitation transformation. When annealing at 400 degrees C, because of the emergence of crystal defects brought out by the precipitation of crystal phases, the composite coating exhibits the lowest corrosion resistance. As the annealing temperature rising to 600 degrees C, the crystalline structure continually grows up and the precipitation transformation tends to be completed. Then the crystal defects decreases which results in an improvement to the corrosion resistance of the composite coating.