柠檬酸对镧盐转化膜耐蚀性和自愈性的影响

将热镀锌钢分别浸入添加和不添加柠檬酸的镧盐钝化液中,在镀锌钢表面获得柠檬酸改进型镧盐转化膜和常规镧盐转化膜.用中性盐雾(NSS)试验、塔菲尔极化和电化学阻抗谱研究了这些试样的耐蚀性能,并对带划痕的柠檬酸改进型和常规镧盐膜层试样进行NSS腐蚀,用扫描电子显微镜和能谱仪观察分析了腐蚀过程中划痕表面的组织形貌和化学成分.结果表明:柠檬酸的加入显著提高了镧盐转化膜的耐蚀性能,并使膜层具备自愈性;腐蚀过程中,划痕附近的柠檬酸镧溶解产生La3+和柠檬酸根离子,从膜层中扩散迁移至划痕处,形成新的由Zn、O、La、C元素组成的保护膜,从而抑制了划痕处锌的腐蚀.

Effect of Citric Acid on Corrosion Resistance and Self-Healing Ability of Lanthanum Conversion Coating

In this paper,first,hot-dip galvanized steels were immersed into passivation lanthanum solutions with or without citric acid to respectively obtain an improved lanthanum conversion coating or a traditional lanthanum conversion coating on the steel surface.Next,the corrosion resistances of the two kinds of coatings were investigated via the neutral salt spray(NSS) test,the Tafel polarization and the electrochemical impedance spectroscopy.Then,the coatings scratched with a knife edge were corroded in a NSS chamber.Finally,the microstructure and chemical composition of the scratch surface during the corrosion were analyzed by means of SEM and EDS.The results show that the addition of citric acid in the passivation solution remarkably improves the corrosion resistance and the self-healing ability of the coating,and that,during the corrosion,lanthanum ions and citric acid anions produced by the dissolution of LaCit3 at the scratch migrate from the coating to the scratch to form a new passive coating containing Zn,O,La and C,thus effectively suppressing the corrosion of zinc at the scratch.