Influence of Ti and La Additions on the Formation of Intermetallic Compounds in the Al-Zn-Si Bath
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TRODUCTION
HOT-DIPPED coating has been widely applied in many fields owing to its superior corrosion resistance and low production cost.[1–3] Galvalume (55Al-ZnSi wt pct) has become one of the most used coating materials on steel substrate. In the process of hot-dip coating, several solutions have been advocated to promote the mechanical and anticorrosion properties of the alloy layer. The usual additive elements include C,[4] Si,[5] Al,[6,7] Ni,[8,9] Mg,[10,11] Cu,[12] Ti,[2,13] and RE (rare earth) elements.[3,14,15] Both Ti and RE are most commonly used to enhance the performance of coatings. Ti is added as a grain refiner into the 55Al-Zn-Si bath to enhance the nucleation rate in the liquid film. When the coated strip leaves the coating pot, the Ti element causes a refining effect on the spangle size of the strip which improves the paintability of the coated product. With up to 0.5 wt pct Ti addition, the size of spangles formed on the surface of the 55Al-Zn-1.6Si (weight percent) coating decreased and the corrosion resistance, durability, JIN XU, Master Student, is with the State Key Laboratory of Advanced Special Steels & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China. QIN-FEN GU, Senior Scientist, is with Australian Synchrotron, 800 Blackburn Rd, Clayton 3168, Australia. QIAN LI, Professor, is with the State Key Laboratory of Advanced Special Steels & Shanghai Key Laboratory of Advanced Ferrometallurgy & School of Materials Science and Engineering, Shanghai University, and also with the Materials Genome Institute, Shanghai University, Shanghai 200072, China. Contact e-mail: [email protected], shuliqian75@gmail. com HU-SHENG LU, Professor, is with the Key Laboratory of Integrated Exploitation of Bayan Obo Multi-Metal Resources, Inner Mongolia University of Science and Technology, Aerding Street No. 7, Baotou 014010, China. Manuscript submitted March 13, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
and formability were also obviously improved.[13] McDevitt et al.[16] pointed that the spangle size of the coating steel would be decreased from 1.4 to 0.3 mm when the titanium content of melt reached 0.01 wt pct. Moreover, the thickness of IMC layer can be reduced dramatically with the addition of Ti, which eventually enhances the adhesion of the coating/substrate.[2,17] Rare earth (RE) elements are the other most common elements added into Al alloy-coating baths. RE elements can lower the viscosity of melt, reduce the thickness of intermetallic layer and improve the high-temperature oxidation resistance. For instance, Yang et al.[14] studied the effect of La addition (0 to 0.20 wt pct) on the corrosion resistance of the hot-dip 55Al-Zn-Si coating steel wire and found that the corrosion resistance of coating could be significantly improved by adding 0.05 wt pct La. Adding a small amount of La into the 55Al-Zn-Si bath contributed to the improvement on compactness and uniformity of s5 phase, which can help to suppress the d
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