STRUCTURE AND CORROSION RESISTANCE OF ELECTROLESS PLATING Ni-P AND Ni-W-P COATINGS ON AZ91D MAGNESIUM ALLOY

Ni-P and Ni-W-P coatings were prepared on AZ91D magnesium alloy by a direct electroless plating processing.The microstructure,phases and corrosion resistance of the prepared and post-annealed coatings were studied by using scanning electron microscopy,X-ray diffraction technique and electrochemical test.The results show that the Ni-P coating is of amorphous structure while the Ni-W-P coating is of nanocrystalline structure with a grain size of about 7 nm,and they have similar corrosion resistance against 3.5%NaCl aqueous solution. The corrosion potential of the Ni-W-P coating after annealing at 250℃for 50 min positively shifts by 150 mV as compared to that of as-prepared Ni-W-P or Ni-P coating,implying the corrosion resistance of the annealed Ni-W-P coatings has been much improved.