Composition Analysis and Corrosion Resistance of Different Surface Treatment Systems for Zn-Al-Mg Coating

The purpose of this study is to analyze the composition of different surface treatment systems of Zn-Al-Mg coatings, investigate the corrosion resistance of surface treatment films, clarify the anti-corrosion mechanisms of different surface treatment methods, and explore the essential reasons for the difference in corrosion resistance. In this study, the surface morphologies of Zn-Al-Mg steel plates under different surface treatments were observed by white light interferometer and scanning electron microscopy (SEM). The thicknesses, element compositions and structures of surface treatment films were characterized by glow spectrometer (GDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared reflection and absorption spectrometer (FTIR). At the same time, the corrosion resistance of the three samples was analyzed by neutral salt spray test, Tafel test and electrochemical impedance spectroscopy (EIS). The results indicate that the anti-rust oil film was only about 10 nm thick and mainly consisted of hydrocarbon base oil and corrosion inhibitor. The anti-corrosion mechanism mainly relied on the physical adsorption of the polar head of the anti-rust agent on the surface of the coating to form a single molecule or multi-molecule adsorption layer. The adsorption layer could prevent the contact between the coating surface and oxygen and water. Cr3+ passivation film was mainly composed of chromium phosphate and silica-containing organic additives (silane, etc.) with a thickness of about 50 nm. The passivation mechanism was through the dissolution of Zn on the surface to form Zn2+, and then the Cr3+ in the solution reacted with Zn2+ and OH–. A dense insoluble oxide film layer mainly composed of Cr2O3, Cr(OH)3 and ZnO was precipitated on the surface of the coating. A small amount of organic silicon additives in the passivation film formed a physical barrier to the corrosion factors, and then improved the corrosion resistance. Cr-free passivation film mainly adopted amino silane and water-based resin as film forming materials, which had a thickness of about 3 μm. XPS and FTIR results indicated that Si-O-Si, Si-O, Si-C, Si-O-Zn, C-O and other forms existed in the passivation film, which proved that silane and resin cross-link on the surface of the steel plate formed a three-dimensional network structure. It also formed strong chemical bonds between silane and Zn-Mg-Al coating. The electrochemical tests showed that compared with the oiling treatment sample, the Cr3+ passivation sample and Cr-free passivation sample had smaller self-corrosion current densities and larger electrochemical impedance. In the neutral salt spray environment, compared with the chromium-free passivation film, the Cr3+ passivation film layer was denser, so it showed better planar corrosion resistance. The white rust area was less than 5% after 360 h. When the passivation film was destroyed, the inorganic salt corrosion inhibitor in the Cr-free passivation film re-dissolved and migrated to the damaged film layer, and reacted with the substrate to form a new oxide film. Thus, Cr-free passivation film showed better self-healing ability and more excellent scratch corrosion resistance. After 360 h, the white rust in the crossing part no longer increased. These results proved that the corrosion tendency of Zn-Al-Mg coated plates with Cr3+ passivation film or Cr-free passivation film was much smaller than that with anti-rust oil film. This work provides a theoretical support for the application and popularization of Zn-Al-Mg coating materials in the home appliance board market. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.