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Development of a Novel Si-Modified Zn-Al Eutectoid Alloy for Hot-Dip Batch Galvanizing

by Joshi, Abhay Vikas

Abstract (Summary)
The hot dip galvanizing process facilitates a combination of good strength and toughness of the substrate with excellent resistance to corrosion provided by the coating through a simple and economic technique. There are two broad categories to this process; the continuous hot- dip galvanizing process and the batch hot-dip galvanizing process. The batch process is particularly important for preformed members or structures. Along with improvement in the life of galvanized steel, there is a growing environmental concern about leaching of zinc into the soil, and this project was an attempt to address this issue. Intensive research has resulted in successful development of new products like Galvanneal®, Galfan® and Galvalume® in recent years, which are produced through the continuous galvanizing process. These products have varying degree of aluminum in the galvanizing bath. There has not been much development in the batch process, mainly due to the restrictions imposed by the process itself, and due to the incompatibility of the conventional zinc-ammonium-chloride flux with baths containing high aluminum content. A novel Cu-Fe and Cu-Sn chloride-free fluxes were therefore developed first, which work satisfactorily with baths containing Al as high as 50%. Prior research work on the Zn-22.3 wt% Al alloy showed that the galvanized coatings obtained from the binary Zn-Al eutectoid bath were an order thicker than the commercially acceptable norm of approximately 80 ìm and the coating surface was rough and the coatings showed a lot of trapped porosities and outbursts. The ternary additions of Bi and RE in the Zn-Al eutectoid bath were made in order to control the thicknesses of the coatings, but with limited success. Ternary addition of 0.2-0.4 wt% Si in the Zn-Al eutectoid bath had beneficial influence on the growth rate and morphology of the coatings. A coating as thin as 10-40 ìm could be achieved. The porosities in the coatings were totally eliminated but the outbursts although, greatly reduced, were still a cause of concern. However, the Si-treated Zn-Al eutectoid bath emerged by far as the most promising alloy for hot-dip galvanizing of steel. Dr. W. van Ooij on behalf of the University of Cincinnati and Weert Groep, requested the assistance and galvanizing expertise of Teck Cominco Ltd. Product Technology Center (PTC) in completing the development of this alloy, understanding the underlying principles and scaling-up the development work from small laboratory furnaces to larger crucibles for dipping trials. Following the agreement between the three parties a joint project was started at PTC from 22nd June, 2005 until 20th September, 2005. The novel Cu-Sn and Cu-Fe fluxes were found to be ineffective during the initial stages of the work. A modified Zn Ammonium Chloride flux developed by PTC and Ferrotech Inc. was used and it was found to be working with this high Al bath. The thesis work was focused towards development and optimization of the experimental matrix to assess the effects of pretreatment chemistries, bath temperature, bath composition, immersion time and withdrawal rates on the coating quality. Evaluation and understanding of the coating properties was accomplished by doing in depth characterization of the coatings, produced at PTC, in the Advanced Materials Characterization Center at The University of Cincinnati. The thesis traverses through the study of the working mechanism of the Si modified Zn-Al alloy in an attempt to completely eliminate the outbursts observed on the coating surface and to produce a commercially acceptable coating quality.
Bibliographical Information:

Advisor:

School:University of Cincinnati

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:hot dip batch galvanizing zn al eutectoid alloy

ISBN:

Date of Publication:01/01/2006

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