Effect of Different Passivation Treatments on Alkali Zn-Ni Coatings: Corrosion Resistance and Adhesion Performance of Geomet 321 and ML Black Coatings

This study aims to improve the corrosion properties of iron materials used in the automotive industry. For this purpose, two different coating techniques were applied to the same surface. As part of the research, different passivation processes (transparent, blue, yellow, and black) were applied to alkaline Zn-Ni coatings. Geomet 321 and Geomet ML Black coatings were deposited on the passivation layer to form a double-layer coating. In order to investigate the adhesion and corrosion effects of these coatings, a dry adhesion test, a water test, a humidity test, and a salt spray/spray test were carried out, and cross-cut adhesion tests were carried out after each corrosion test. After all these tests, rust formation was analyzed by visual analysis, and atomic weight percentages and coating thicknesses were examined by X-ray. Geomet 321 and ML Black coatings, which were tested using different passivation processes, successfully passed. The transparent passivation after the Zn-Ni coating fully satisfied both adhesion and corrosion protection requirements for the 321 and ML Black coatings. No red rust formation was observed on the coating samples after the test; only partial white rust formation was observed, and no peeling of the coating layer was detected in the adhesion tests. As a result, the most favorable results were obtained for the transparent, blue, and black passivated coatings.