Black Passivation and Corrosion Resistance of Zn-Fe Alloy Coatings

In order to improve the corrosion resistance of Zn-Fe alloy coating and Zn-Fe-TiO_2 composile coating, a black passivation technology was developed for Zn-Fe alloy coating containing 0.2% -0.8% Fe. In the process, phosphatization was incorporated to increase the thickness of the coatings, and transition element salt was used as a blackening reagent to take the place of the soluble silver salt, aside the adequate additives and assistant film torming agent were added. The optimal electrolyte components and process conditions were obtained. The obtained black chromate film has the properties of black and shining, uniform hue, high corrosion resistance, good adhesion and wear resistance. The corrosion resistance of black passivation fihn of Zn, Zn-Fe and Zn-Fe-TiO_2 electroplating were studied. The corrosion test (in 5% NaCI) shows that being black passivated, the corrosion resistance of Zn,Zn-Fe and Zn-Fe-TiO_2 coating make a good progress. Among them, the corrosion resistance of Zn-Fe-TiO_2 composite coating with black chromate fihn is more than two times of Zn-Fe coating,and 5 times of pure Zn coating. The composite coating reaches the domestic advanced level, and is an ideal substitute for cadmium coaling.