在常规稀土镧盐钝化液中添加以柠檬酸添加剂,从而在热镀锌层表面获得镧盐转化膜.采用中性盐雾实验(NSS)分析膜层耐蚀性能,确定最佳成膜时间范围.通过扫描电镜(SEM)、能谱仪(EDS)、X射线光电子能谱(XPS)、红外光谱(IR)分析了膜层的结构、微观形貌和化学成分.结果表明,转化膜耐蚀性能与铬酸盐钝化膜相当;膜层微观上存在裂纹,且裂纹随着成膜时间的延长而变宽;膜层主要由柠檬酸镧和La(OH)_3等组成;转化膜的生长过程中,前期以柠檬酸镧吸附膜的沉积为主,中期La(OH)_3与柠檬酸镧相互渗透沉积,后期La(OH)_3沉积逐渐占据主导.
Rare earth conversion coating was proposed as one of the most prospective alternatives to toxic chromate conversion coatings on some metals and alloys. In this work, the lanthanum salt conversion coating was obtained by immersing a galvanized steel in a lanthanum salt passivation solution modified with citric acid. Scanning electron microscopy (SEM), X-ray energy dispersive spectrometer (EDS), X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (IR) were used to investigate its structure and composition. Neutral salt spray (NSS) tests were used to evaluate the anti-corrosive performance of the coating and the optimum range of treatment time. The results show that the lanthanum salt conversion coating modified with citric acid mainly consists of LaCit, La(OH)_3/La_2O_3 and a trace of Zn(OH)_2/ZnO. When treatment time is more than 10 min,the modified coating on the samples consists of two layers: the inner layer is compact and the outer lanthanum-riched layer is thin and poriferous. Cracks occur in the coating when treatment time within 10 s and are evolved during further treatment. The thickness of the coating obtained on galvanized steel that the immersion time is 1 min is approximately 250 nm, and increasing with treatment time within 60 min. The growing rate of the coating is faster than that of conversion coatings without modified with citric acid. The growth process of the coating can be divided into three stages: at the initial time complexes ions of lanthanum citrate were adsorbed on the whole surface of galvanized steel, then lanthanum citrate and lanthanum hydroxide are co-deposited on surface, and in the final stage there is lanthanum hydroxide mainly deposited on surface of the coating. The corrosion resistance of the coating is close to that of a chromate conversion coating. The optimum treatment time is 10 to 30 min.
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