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Kong G.,South China University of Technology | Wu S.,South China University of Technology | Lin D.-X.,Guangdong Anthen Iron Tower and Steel Structure Co. | Wang X.,Guangdong Anthen Iron Tower and Steel Structure Co. | And 2 more authors.
Zhongguo Youse Jinshu Xuebao/Chinese Journal of Nonferrous Metals | Year: 2012

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)3·6H2O, 4-6 g/L H2O2(30%) and 15-20 g/L H3Cit at 70°C for 10 s-240 min. 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/CeO2mixture are deposited on surface of the coatings. Source


Kong G.,South China University of Technology | Huang W.,South China University of Technology | Lin D.-X.,Guangdong Anthen Iron Tower and Steel Structure Co. | Wang X.,Guangdong Anthen Iron Tower and Steel Structure Co. | And 3 more authors.
Huanan Ligong Daxue Xuebao/Journal of South China University of Technology (Natural Science) | Year: 2012

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. Source


Kong G.,South China University of Technology | Liu R.-B.,South China University of Technology | Che C.-S.,South China University of Technology | Lu J.-T.,South China University of Technology | And 3 more authors.
Huanan Ligong Daxue Xuebao/Journal of South China University of Technology (Natural Science) | Year: 2011

The dissolution behavior of solid iron in liquid zinc at a high temperature under natural convection condition was investigated, and the corresponding dissolution mechanism was discussed. The results show that, when solid iron is immersed into liquid zinc, the saturated concentration of Fe in liquid zinc gradually increases with the temperature, that the dissolution rate greatly increases with the temperature of liquid zinc, that the dissolution rate constants of liquid zinc at 520, 560 and 600°C are respectively 2.1×10-6, 5.06×10-6 and 7.15×10-6 m/s, that the dissolution of solid iron in liquid zinc at 520°C is mainly controlled by the chemical reaction between solid iron and liquid zinc and the diffusion of iron atoms in Fe-Zn alloy layer, while that at 560 and 600°C is controlled by the diffusion of iron atoms across the concentration boundary layer in liquid zinc. Source

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