Microstructure and corrosion behavior of hafnium coatings on AZ91D magnesium alloys by magnetron sputtering

Hf coatings are fabricated on the AZ91D Mg alloys by magnetron sputtering with bias voltage ranges from 0 to -125 V. Both potentiodynamic polarization test and neutral salt spray test reveal that the Hf coating deposited at -100 V exhibits the best protective performance. It possesses the lowest corrosion current density of 1.032 mu A cm (-2) and the highest protection rate of 6, respectively. This perfect anticorrosion property is due to the dense structure and low porosity induced by applying the appropriate bias voltage. Various types of corrosion sites after corrosion tests are examined in detail. The results indicate that the coating failure is strongly dependent on the coating defects and the random phase distribution in the substrate.