4种典型热镀锌板表面钝化膜耐腐蚀性能研究

Corrosion Resistance of Passivation Films on Four Typical Hot-dip Galvanized Plates

目的分析4种热镀锌板钝化膜体系成分,考察不同钝化膜的耐腐蚀性及防腐机理。方法通过SEM、XPS、GDS、红外光谱、润湿角,分析了4种钝化膜的微观结构和化学成分。通过电化学、中性盐雾、循环盐雾试验,验证了镀锌板钝化膜的耐腐蚀性。结果耐指纹膜、自润滑膜、全无铬钝化的主体均含Si和O,膜厚约1μm,耐指纹膜中还含有Sn和P,自润滑膜和全无铬钝化中含有较多的P和Sn,三价铬钝化膜主要含有Si、O、Cr、C、P,膜厚仅为0.05μm。4种钝化膜的成分分布与光整坑有较强的对应关系,光整坑中成膜较厚,光整坑外部出现明显的Zn元素强度,说明该处钝化膜较薄,三价铬具有最优的疏水性,全无铬和耐指纹也具有较好的疏水性,自润滑板疏水性较差,三价铬钝化的自腐蚀电流密度为0.97µA/cm^(2),全无铬、自润滑、耐指纹的自腐蚀电流密度依次为1.6、2.3、2.6µA/cm^(2)。全无铬中树脂交联密度较高,三价铬钝化存在致密的氧化物膜,疏水性较好,对去极化剂的阻隔能力也更强,电荷在界面转移阻力较大,而耐指纹板和自润滑板存在部分孔隙,所以其平面耐腐蚀性稍差。在循环腐蚀过程中,由于存在干湿交替,高盐环境对钝化膜的持续攻击时间较短,故4种钝化膜在循环腐蚀过程中平面处的耐腐蚀性均优于中性盐雾环境中,盐雾结果与电化学结果有较好的一致性。结论不同表面膜耐腐蚀性有一定区别,三价铬和全无铬具有较优的耐平面腐蚀性和划叉腐蚀性。

The work aims to analyze the composition of four passivation films for hot-dip galvanized plates,and investigate the corrosion resistance and corrosion protection mechanism of different passivation films.The microstructure and chemical composition of four passivation films were analyzed by SEM,XPS,GDS,IR and wetting angle.The corrosion resistance of the passivation film of a galvanized plate was verified by electrochemical,neutral salt spray and cyclic salt spray tests.The fingerprint resistant film,self-lubricating film and total chromium-free passivation all contained Si and O,and the film thickness was about 1μm.The fingerprint resistant film also contained Sn and P,while the self-lubricating film and total chromium-free passivation film contained more P and Sn.The trivalent chromium passivation film mainly contained Si,O,Cr,C and P,and the film thickness was only 0.05μm.There was a strong correspondence between the composition distribution of the four passivation films and the smooth pit.The combination forms of Si in the four passivation films mainly existed in the form of Si—O—Si(102.4 eV)and Si—C(102.8 eV)and there was also Si—O—Zn(100.7 eV)in chromium-free,fingerprint-resistant and trivalent chromium.The Si—O—Si bond was the hydrolysis of silane coupling agent in passivation solution to form silanol.The hydrolysis-condensation rate was affected by silane concentration,pH value and solvent type,etc.The Si—O—Si bond in chromium-free passivation was obviously more than that in other passivation films,which might be due to the fact that the type and concentration of silane in chromium-free passivation solution were different from others,and it was hydrolyzed into more silanol in passivation solution.After curing into a film,its cross-linking density was higher and the film was denser.The Si—O—Zn bond proved that partially hydrolyzed silanol formed with zinc coating.The film thickness in the smooth pit was thick,and the apparent Zn element intensity appeared outside the smooth pit,which indicated that the passivation film here was thin,trivalent chromium had the best hydrophobicity,all chromium-free and fingerprint-resistant had better hydrophobicity,and the self-lubricating plate had poor hydrophobicity.The self-corrosion current density of trivalent chromium passivation was 0.97µA/cm^(2),and all chromium-free,self-lubricating and fingerprint-resistant were 1.6,2.3,2.6µA/cm^(2) in turn.All-chromium-free resin had higher cross-linking density,dense oxide film in trivalent chromium passivation,better hydrophobicity,stronger barrier ability to depolarizer,and greater resistance to charge transfer at interface,while fingerprint resistant plate and self-lubricating plate had some pores,so their plane corrosion resistance was slightly poor.In the cyclic corrosion process,due to the alternation of wet and dry,the sustained attack time of passivation film in high-salt environment was short,so the corrosion resistance of the four passivation films in the cyclic corrosion process was better than that in neutral salt spray environment,and the salt spray results were in good agreement with the electrochemical results.The corrosion resistance of different surface films is different.Trivalent chromium and total chromium-free film have better plane corrosion resistance and cross corrosion resistance.