Pulse plated Zn transition layer in electroplating Zn-Ni coatings on magnesium alloys

A novel process with an environmentally friendly Zn transition layer was used in electroplating a Zn-Ni coating onto an AZ31 magnesium alloy, and direct current (dc) and pulse current (pc) electroplated Zn coatings were compared. The surface morphology, microstructure and macroresidual stress of the specimens were investigated using scanning electron microscope and X-ray diffraction spectrometry. The electrochemical method was adopted for corrosion resistance evaluation. The SEM images indicated that the Zn transition layer was necessary in electroplating a Zn-Ni coating. For the same electroplating time, the application of the pc plating mode was superior to the dc mode when producing a low residual stress, smooth, compact, nonporous and more corrosion resistant Zn transition layer. Moreover, the grain size of the pc plated Zn transition layers was finer than that of the dc plated layers. Most importantly, an electrodeposited Zn-Ni alloy coating on a magnesium alloy could be obtained successfully using a pc plated Zn transition layer.