热镀锌层上硅酸盐膜的耐蚀性和自愈性

Corrosion Resistance and Self Healing Ability of Silicate Conversion Coatings on Hot Dip Galvanized Steels

热镀锌(HDG)钢片经SiO2∶Na2O摩尔比为1.00和3.50的硅酸钠溶液中处理后,在其表面获得硅酸盐转化膜。用中性盐雾(NSS)试验、塔菲尔极化和电化学阻抗谱(EIS)研究了硅酸盐膜试样的耐蚀性,将被刀片划伤的硅酸盐膜试样进行NSS腐蚀后,用扫描电镜(SEM)和能谱仪(EDS)观察和分析了划痕处的腐蚀,以探讨硅酸盐膜的自愈性。结果表明:在较高SiO2∶Na2O摩尔比溶液中获得的硅酸盐转化膜具有较好的耐蚀性和自愈性,腐蚀过程中硅酸负离子从膜层中迁移划痕处形成新的保护膜(由Zn,O和Si组成)抑制了划痕处锌的腐蚀。AFM观察发现,在摩尔比为3.50中获得的试样的膜层表面更加致密,这有利于阻止腐蚀介质的侵入和提供充裕的硅酸负离子迁移。并对硅酸盐转化膜试样的划痕的腐蚀过程的细节进行了分析和讨论。

The silicate conversion coating was prepared by immersing hot dip galvanized （HDG） steel in sodium silicate solution with a SiO2：Na2O molar ratio of 1.00 or 3.50, respectively. The corrosion resistance of coatings was investigated by neutral salt spray （NSS） test, Tafel polarization and elec-trochemical impedance spectroscopy （EIS） measurement. NSS corrosion process of the coatings scratched by the edge of knife was analyzed using scanning electron microscopy （SEM） and energy dispersive spectroscopy （EDS）. The results show that the superior corrosion resistance and self healing ability are obtained from the silicate coating pretreated in sodium silicate solution with higher SiO2 ： Na2O molar ratio. During the corrosion process, silicate anions migrate from the coating to the scratch, on which a new passive coating composed of Zn, O and Si is formed and the corrosion is suppressed. Moreover, it is observed that by the atomic force microscopy （AFM）, the silicate coatings obtained in the sodium silicate solution with molar ratio of 3.50 is more compact, which is favor of the migration of silicate anions. The detail of corrosion process of the scratches on silicate coatings is discussed.