| LONG Yuan,HAO Yu-lin,CAI Ning,ZHOU Ji-ming,YAO Shi-cong,CAO Jian-ping,LIU Hua-sai,LI Xue-tao,YANG Jian-wei.Effect of Degreasing on Coating Performance of Galvanized Automobile Sheet[J],52(10):259-266 |
| Effect of Degreasing on Coating Performance of Galvanized Automobile Sheet |
| Received:September 26, 2022 Revised:February 24, 2023 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2023.10.021 |
| KeyWord:automobile sheet degreasing phosphating electrophoresis corrosion resistance |
| Author | Institution |
| LONG Yuan |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China;Beijing Key Laboratory of Green Recyclable Process for Iron & Steel Production Technology, Beijing , China |
| HAO Yu-lin |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| CAI Ning |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| ZHOU Ji-ming |
Manufacturing Department, Shougang Jingtang United Iron and Steel Co., Ltd., Hebei Tangshan , China |
| YAO Shi-cong |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| CAO Jian-ping |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| LIU Hua-sai |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| LI Xue-tao |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
| YANG Jian-wei |
Technology Research Institute of Shougang Group Co., Ltd., Beijing , China |
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| Abstract: |
| Degreasing is one of the important processes of coating pretreatment. Its main function is to clean and remove oil stains on the surface of steel sheet, ensure the quality of subsequent phosphating process, and then ensure the film properties of steel sheet after electrophoretic coating, such as film adhesion and corrosion resistance. Since most of the previous studies focus on the composition of phosphating solution and the surface properties of steel sheet, there are few studies on degreasing. Therefore, the work aims to study the mechanism of the effect of degreasing process on the quality of phosphating film and the performance of electrophoretic film. Hot-dip galvanized steel sheet (CR4+Z) was used in this study. Under laboratory conditions, the discontinuous water film on the surface of the steel sheet indicated poor degreasing when the steel sheet was degreased for 4 min. The results of electron microscopy of phosphating film showed that when the degreasing was poor, the phosphating film grew abnormally and appeared flat, resulting in the appearance of bright and dark patches of phosphating film. The cross-section results showed that the thickness of the flat phosphating film was 0.5~1 µm, which was obviously thinner. After removal of the phosphating film according to ISO 3892:2000, XPS results showed that metal oxides and phosphate mainly existed on the surface of the steel sheet, and the content of metal oxides on the surface of the steel sheet was 39% when the phosphating film was abnormal, which was significantly higher than that when the phosphating film was normal. According to the change of open circuit potential of the steel sheet in phosphating solution, the first, second and third stages of the phosphating reaction were obviously inhibited when the degreasing was not good, which reduced the production efficiency of Zn2+ on the surface of the steel sheet, left more metal oxides, weakened the growth rate of the phosphating film, and resulted in the poor corrosion resistance of the phosphating film. The open circuit potential, impedance and polarization curves of phosphating films obtained under different degreasing states in 3.5% NaCl solution showed that the open circuit potential of phosphating films under abnormal degreasing condition was –1.10 V, which was negative to –1.06 V of normal phosphating films. The impedance of 1.18×103 W×cm2 was smaller than 3.72×103 W×cm2 of normal phosphating film and the self-corrosion current density of 6.31×10–5 A×cm–2 was larger than 7.94×10–6 A×cm–2 of normal phosphating film, indicating that abnormal phosphating film would lead to poor corrosion resistance and easy corrosion. The electrophoretic film properties showed that poor degreasing to a certain extent had little effect on the scratch adhesion of the film, which was rated as 0. However, it had a significant impact on the performance of the cupping of the electrophoretic paint film. When the cupping height was 6 mm, the paint film was broken, which could not meet the quality requirements of the automobile factory. The corrosion resistance of electrophoretic paint film decreased obviously, and the corrosion width of electrophoretic paint film increased from 2.4 mm to 3.9 mm. In a word, poor degreasing will lead to the abnormal quality of phosphating film on the surface of galvanized sheet, which will affect the electrophoretic corrosion resistance. In the actual production process, it is necessary to effectively control degreasing process temperature, alkalinity, surfactant parameters and spray pressure, and timely control parameters based on the type and amount of oil used in body materials to avoid poor degreasing problems |
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