Zn–Ni Alloy Plating with Trivalent Chromate: Effects of NaF Additive Concentration and Treatment Time on Film Color, Thickness, and Electrochemical Properties

Zn–Ni alloy plating is widely applied in manufacturing of automobile and construction material components because it provides better corrosion resistance and wear resistance than Zn plating. Furthermore, chromate coating treatment is gaining attention with respect to improving the corrosion resistance of Zn–Ni alloys. In this study, we investigated the effects of NaF additive concentration and treatment time on trivalent chromate coating, which has been developed as an alternative to hexavalent chromate coating. The chromate post-treatment solution used in this study comprises Cr(NO₃)₃·9H₂O (360 g/L), CoSO₄·7H₂O (40 g/L), and HNO₃ (35 mL/L), to which NaF is added in the concentration range of 0–30 g/L. The as-formed coating films at 1.6 pH and 60 °C treatment temperature for deposition times ranging from 30 to 120 s demonstrated a decreasing corrosion rate as the NaF concentration increased. The electrochemical and morphological analyses inferred that NaF acted as a catalyst, enhancing the rate of film formation. Furthermore, the film thickness increased with the treatment time, and the film color changed in the order of yellow, purple, and green.