A zinc transition layer in electroless nickel plating

A new process, which is characterized with zinc transition layer obtained by combining immersing zinc in an improved zinc bath containing FeCl3 and electroplating zinc, is used to plate Magnesium alloy AZ91D. Corrosion testing and SEM images indicate that the immersion zinc layers obtained in zinc immersion baths with FeCl3 are more compact. Peel testing shows that there is good adhesion to the subsequent electrodepositing layer. The electroplating zinc can further promote the coverage of zinc transition layer on the magnesium alloy substrate before electroless nickel plating and reduce the galvanic corrosion effect between Mg alloy and Ni–P alloy. The good zinc deposit can be obtained by electroplating zinc in pH 9∼10 zinc pyrophosphate bath at 40 °C under the current density of 2∼3 A/dm2. The developed new process of electroless nickel plating greatly improves the adhesion and coverage of Ni–P coating. The zinc transition layer obtained by the developed process can supersede copper transition layer obtained by cyanide plating process in electroless nickel plating on magnesium alloy.