The characterisation of passivation and colouring of Zn-Ni electroplated 2024 aluminium alloy in the presence of Cu +

The objective of this study is to develop a black chromate conversion coating (CCC) that meets the rigorous corrosion resistance and conductivity requirements of aerospace electrical connectors. The Zn-Ni alloy was successfully electrodeposited on the surface of 2024 aluminium alloy, and a black conversion coating was achieved through Cu+-based colouring. The conversion solution consisted of 10–14 g L−1 CrO3, 5–10 g L−1 NaCl, 10–13 g L−1 CuCl, and 15–25 g L−1 65-68%HNO3. The performance of the CCC was comprehensively evaluated through various techniques, including macro-observation, SEM, EDS, EIS, polarisation curves, and conductivity testing. The experimental results demonstrated that the prepared CCC exhibited a uniform black gloss, and that the conversion coating layer was primarily composed of Zn, Ni, O, Cr, and other elements. This coating layer significantly improved the impedance of the Zn-Ni alloy, reducing the corrosion current density to 5.35 × 10−5A cm−2, thus greatly enhancing its corrosion resistance. Furthermore, the resistance of the prepared CCC on a nearly hollow cylindrical electrical connector shell specimen with a height of approximately 3 cm and an inner diameter of approximately 1.5 cm was only 1.7 mΩ, which demonstrates excellent conductivity.