脱脂对镀锌汽车板涂装性能影响

Effect of Degreasing on Coating Performance of Galvanized Automobile Sheet

目的 研究脱脂影响镀锌钢板磷化反应的机理及电泳漆膜性能。方法 使用扫描电镜、电化学工作站、X射线光电子能谱分析仪、成形试验机和循环式腐蚀试验机等设备研究了磷化膜形貌、表面元素、电化学性能以及电泳漆膜性能。结果 脱脂不良时磷化膜在镀锌钢板表面生长异常,导致磷化膜呈现为明暗相间的花斑状;表面元素和材料在磷化液中的电化学性能说明磷化膜生长异常的原因是残存油膜阻碍磷化反应进行,同时异常磷化膜相对于正常磷化膜在3.5%NaCl溶液中开路电位负、阻抗小、自腐蚀电流大,说明异常磷化膜耐蚀性能差,易腐蚀;电泳漆膜性能表明,脱脂不良对漆膜的划格附着力影响小,评级为0级,但对电泳漆膜的杯突性能影响明显,杯突高度为6 mm时,漆膜发生破裂;电泳耐蚀性能下降明显,电泳漆膜扩蚀宽度从2.4 mm增加到3.9 mm。结论 脱脂不良会导致镀锌板表面磷化膜质量异常,进而影响电泳耐蚀性能,在实际生产过程中,需要对脱脂工序有效管控,结合车身材料用油种类及油量及时调控参数,避免脱脂不良问题出现。

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 0.5~1μm,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 Zn^(2+)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%Na Cl 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 1.18×10~3 Ω·cm^(2) was smaller than3.72×10~3Ω·cm^(2) 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.