Effect of double zincating on microstructures and bonding strength of coatings on aluminum alloy for pulse gold plating

Gold coating is prepared by pulse electroplating on zincated aluminum alloys with electroless plated Ni-P and electroplated Ni interlayers under different zincate pretreatment conditions. The quantitative characterization of the bonding strength of coatings on aluminum alloy is conducted by bonding the coating to the face of a loading fixture with E-7 high-strength two-component epoxy adhesive in a tension normal to the bonding interface. The microstructures of coatings on aluminum alloy are investigated by means of scanning electron microscopy, energy disperse spectroscopy and X-ray diffraction. Double zincating has a minimal impact on the grain morphology and size of the Au electroplated layer. However, it does result in a reduction in the surface undulation of the aluminum alloy substrate and an improvement in the continuity of its surface platings. Double zincating is beneficial for enhancing the bonding strength of the platings on aluminum alloy. Without zincating, the bonding strength between the plated layers and the aluminum alloy substrate is only18.3 MPa. When secondary zincating for 30 s, 40 s, and 60 s respectively, the bonding strength of gold plating samples significantly increases up to above 30 MPa.