Comprehensive Evaluation of Aluminum Diffusivity in Liquid Zinc
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is often coated with zinc for corrosion protection.[1] A small amount of Al, up to 0.3 pct, is added into continuous galvanizing baths to inhibit the formation of a Zn-Fe intermetallic layer in the coating, because the intermetallic compounds negatively affect the processability of the coated strip. An interface layer consisting of Fe2Al5–xZnx forms immediately upon contact of the incoming strip with the molten Zn-Al alloy. The inhibition layer significantly slows the interdiffusion of Fe and Zn, thereby inhibiting the Fe-Zn reaction in the coating.[2] The addition of small amounts of Al to a Zn bath can control the excessive reactivity induced by the Si contained in the steel in general galvanizing.[3] In both types of galvanizing operations, Al is involved in the formation of the intermetallic compounds in the coating and in the bath metal. It is also involved in the corrosion and erosion of pot hardware submerged in galvanizing baths and the pots holding the molten alloys. A recent investigation of the corrosion of submerged hardware in a molten Zn-Al bath showed that the supply of Al is frequently the rate controlling step in the chain of corrosion reactions.[4] SUI YANG, Research Associate, is with the Faculty of Materials, Optoelectronics and Physics, Xiangtan University, Xiangtan, 411105 Hunan, People’s Republic of China, and Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University. XUPING SU, Professor, is with the School of Materials Science and Engineering, Changzhou University, Changzhou 213164 Jiangsu, People’s Republic of China and Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University. Contact e-mail: [email protected] JIANHUA WANG, FUCHENG YIN, and ZHI LI, Professors, XINMING WANG, ZHONGXI ZHU, HAO TU, and XIAOQIN LI, Research Associates, are with the Key Laboratory of Materials Design and Preparation Technology of Hunan Province, Xiangtan University. NAI-YONG TANG, Manager, is with Zinc Metallurgy and Customer Technical Services, Teck Metals Ltd., Product Technology Centre, Mississauga, ON L5K 1B4, Canada. Manuscript submitted August 19, 2009. Article published online December 14, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
In order to gain a deeper understanding of all mechanisms at work in these processes, an accurate knowledge of Al diffusion in molten Zn is essential. Although the knowledge of liquid metal diffusion coefficients is of great significance in a variety of fields, including material science, physics, chemistry, etc.,[5] reliable data are lacking and understanding of the mechanism of liquid metal diffusion is much less developed. Liquid diffusivity can be measured using the conventional capillary reservoir technique. However, a buoyancy-driven convective flow in the reservoir introduces a ‘‘lid-driven’’ flow in the capillary, resulting in an artificially enhanced ‘‘apparent’’ liquid diffusion coefficient.[6] As a result, diffusivities in liquid metals were mostly evaluated using the shear cell technique. Howe
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