Magnesium alloys have been considered as green engineering materials in the 21st century and widely used in automobile, aerospace, electronic industry and communication equipments [1,2]. Unfortunately, magnesium alloys have poor corrosion resistance and high chemical reactivity that have limited their widespread applications [3]. Several surface coating technologies have been developed to improve corrosion resistance for magnesium alloys, such as electrochemical plating [4,5], conversion coatings [6], hydride coatings [3], anodizing [7,8], gas-phase deposition processes [9], laser surface alloying [10] and organic coatings [11]. Among various surface coating treatments, electrochemical plating was considered to be a relatively effective choice, as it can provide the alloy substrate with metal coatings that have the desired properties of good corrosion and wear resistance, solderability, electrical conductivity or decorative appearance [3]. Two more mature processes have been applied successfully for obtaining metal coatings on magnesium alloys as described in Refs. [12,13]: (1) degrease ! pickling ! activation ! zinc immersion ! cyanide copper plating ! common plating; (2) degrease ! pickling ! activation ! electroless nickel plating ! common plating. In the two methods described, cyanide copper plating and electroless nickel plating were employed as pre-plating steps to provide proper bases for subsequent common plating. However, cyanides used in copper plating are acutely toxic to human and pollutants to environment; electroless nickel plating bath is thermodynamically an unstable system, which causes a short use period and high production cost.
In order to overcome the limitations mentioned above, a new process of nickel electroplating based on zinc immersion for preplating on magnesium alloy AZ91D has been developed in our laboratory; the process flow was as follows: degrease ! pickling ! activation ! zinc immersion ! nickel electroplating. In current protocol, nickel electroplating instead of cyanide copper plating or electroless nickel plating was employed as the pre-plating step. Prior to nickel electroplating, the same zinc immersion step as reported in Ref.
[12] was adopted. It was generally thought that the zinc film obtained by zinc immersion on magnesium alloy surface prevented re-oxidation of Mg, equalized surface potential and reduced potential difference between magnesium alloy substrate and subsequent deposits [3]. The bath Applied Surface Science 255 (2009) 7773-7779