Research Of Microstructure, Properties And Rare Earth Modification Of Hot-dipped Aluminum On Carbon Steel

Carbon steel has been widely used in petrochemical, metallurgy, machinery, architecture and traffic engineering due to its good mechanical properties and formability. However, poor high temperature anti-oxidation and corrosion resistance of carbon steel lead to a serious impediment to its further application. Among various surface coating techniques, hot dip aluminum is an effective and inexpensive method to modify the surface of carbon steel. It can greatly improve high temperature anti-oxidation and corrosion resistance of carbon steel, while ensuring its mechanical properties. Currently, the researches on hot dip aluminum mainly focused on hot-dip aluminizing process and its applications for industrial production. However, few researches have been conducted on the mechanism of coating formation, RE modification and effect of diffusion treatment on microstructure and properties of the hot-dipped aluminum coating. Therefore, specific studies on mechanism of coating formation, RE modification and effects of diffusion treatment on microstructure and properties of hot-dipped aluminum coating have important theoretical and practical values for development and application of hot-dipped aluminum process.In the paper, the composition and microstructure of hot-dipped aluminum coating were analyzed. The effect of processing parameters on microstructure, composition, high temperature anti-oxidation and corrosion resistance before and after diffusion annealing was studied. The influence of RE on microstructure and properties of hot-dipped aluminum coating was investigated, as well. The corrosion resistance was evaluated by both full-immersion corrosion test and electrochemical techniques. The following results were obtained.The hot-dipped aluminum coating is consisted of two layers, an external layer (defined as'Al-riched layer') consisting of Al and FeAl₃, and an internal layer (defined as'alloy layer') consisting of Fe₂Al₅ and FeAl₃. Metallurgical bond is observed between coating and substrate. Moreover, the external layer is affected by both dipping temperature and pulling rate, while the internal layer is affected by dipping temperature and time. After 850℃×5 h diffusion annealing, the coating is also consisted of two layers, an external layer consisting of Fe₂Al₅ and an internal layer mainly with FeAl. After 800℃×100 h anti-oxidation test, microstructure of hot-dipped aluminum coating completely turned into FeAl,Al₂O₃ and Fe2O3, The results indicate that anti-oxidation property of coating after the diffusion treatment is better than that of coating without diffusion treatment, and is obviously superior to that of 45 steel as well, and corrosion resistance of hot-dipped aluminum samples treated by diffusion annealing is greatly improved compared with samples without diffusion annealing revealed by potentiodynamic polarization curves and full-immersion weight loss test.The discontinuous, slender and tooth-shaped internal layer gradually developed into continuous and ribbon-shaped layer with increase of RE. External layer becomes thinner with increasing the rare earth content from 0.0 % to 0.5 %, then it gets thicker with increase of RE when the RE content exceeds 0.5 %. But the thickness of interlayer begins obviously increased when the content of 0.1 % RE is added, then it gets thinner gradually when the content of RE is more than 0.1 %. It is also found that FeAl₃ phase formed in hot?dip aluminum coating is refined by RE. RE content gradually increased from external to internal layer, and then finally permeates into substrate. However, the phase composition of hot-dipped aluminum coating is almost not changed after RE is added. Moreover, RE distributes uniformly in coating surface while gathered at defects such as grain boundaries and subgrain boundaries after diffusion treatment.Tests of high temperature anti-oxidation and corrosion resistance suggested that diffusion annealing could improve high temperature oxidation property of samples without RE. RE has no effect on improving high temperature anti-oxidation property of hot-dipped aluminum coating. Samples with 0.3% RE show the best corrosion resistance in OCP test before diffusion annealing. However, the results obtained from potentiodynamic polarization and full-immersion corrosion test show that the corrosion resistance in NaCl aqueous solution of the hot-dipped aluminum samples with 0.5% RE is 2 times as much as that of without RE respectively. Diffusion annealing can reduce corrosion resistance of samples with RE.