热镀锌层柠檬酸改进型铈盐转化膜的生长机理

Growth mechanism of cerium salt conversion coating modified with citric acid on galvanized steel

将热镀锌钢板浸入含有25 g/L Ce(NO3)3·6H2O、4~6 g/L H2O2(30%)、15~20 g/L H3Cit的处理液中,在70℃下处理10 s^240 min,从而在其表面获得铈盐转化膜。采用中性盐雾试验(NSS)和电化学极化曲线来分析膜层耐蚀性能,确定最佳成膜时间范围。采用扫描电镜(SEM)观察膜层的微观形貌,利用能谱仪(EDS)、X射线光电子能谱仪(XPS)、红外吸收光谱仪(IR)分析膜层的化学组成。结果表明:处理时间为10 min左右的铈盐转化膜耐腐蚀性能最优,最佳工艺条件下得到的铈盐转化膜的耐蚀性能与铬酸盐转化膜的相当;随着处理时间的延长,膜的厚度增加,膜层的裂纹变宽;处理时间超过10 min后膜层逐步产生脱落,耐腐蚀性能也随之降低;转化膜的生长过程中,前期以柠檬酸铈吸附膜的沉积为主,后期以Ce(OH)3/Ce2O3及Ce(OH)4/CeO2的沉积占主导。

Cerium salt conversion coatings modified with citric acid were obtained on galvanized steel samples by immersing the samples into an aqueous solution containing 25 g/L Ce（NO3）3·6H20, 4-6 g/L H2O2（30%） and 15-20 g/L H3Cit at 70℃ for 10 s-240 rain. The corrosion resistance of the modified coatings was assessed by neutral salt spray tests （NSS） and electrochemical polarization curve. The micromor-structure of the coatings was observed by scanning electron microscopy （SEM）. The chemical composition of the coatings was investigated by X-ray energy dispersive spectrometer （EDS）, infrared spectroscopy （IR） and X-ray photoelectron spectroscopy （XPS）. The results indicate that the optimum treatment time is about 10 min and the corrosion resistance of the modified coating is closed to that of chromate film. The thickness of the coating increases and the cracks in the coating is wider with the treatment time prolonging. When the treatment time is more than 10 min, the coatings are easy to fall off, and the corrosion resistance of the coating also decreases. The growth process of the coating can be divided into two stages： at the initial time the complexes ions of cerium citrate are adsorbed on the whole surface of galvanized steel, while cerium citrate is deposited on surface, then the Ce（OH）3/Ce2O3 and Ce（OH）4/CeOa mixture are deposited on surface of the coatings.