Taking a bath with basic nickel carbonate as the source of nickel,sodium hypophosphite as the reducing agent and citric acid as the complexing agent,the Ni-P coatings on AZ91D magnesium alloy were prepared. The influence of pH value of the bath on the content of phosphorus,microhardness and corrosion resistance of the deposits was examined. The XRD analysis results show that the Ni-P coatings plated under all conditions have a mixed microstructure of amorphous and nanocrystalline and undergo a phase transformation to crystalline nickel and nickel phosphide upon heat-treatment. The microhardness of the Ni-P coatings increases with the increase of pH value of the bath and has a maximum at pH=6. The potentiodynamic polarization test in 3.5%(mass fraction) NaCl solution reveals that the Ni-P coatings exhibit a very good corrosion resistance to protecting AZ91D magnesium alloy.
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