Year

2005

Degree Name

Doctor of Philosophy

Department

Faculty of Engineering

Abstract

Zn-55%Al-1.5%Si coated steel strip is manufactured by the continuous hot dipping process. An important difference in the processing of Zn-55%Al-1.5%Si coated steel when compared with galvanised steel is the higher temperature of the molten alloy bath –600ºC for Zn-55%Al-1.5%Si coatings versus 450ºC for galvanised. This increase in temperature leads to an accelerated rate of evaporation of zinc into the pretreatment furnace, leading in turn to various processing difficulties. Zinc vapour in the pre-treatment furnace has long been implicated with defects known within the industry as pinholes and bare patches - uncoated areas where wetting does not occur between the strip and the molten alloy. Despite the association with zinc vapour, the exact mechanisms of pinhole and bare patch formation have not been clearly established.

One possible mechanism of pinhole and bare patch formation is the direct reaction of zinc vapour with the strip surface. It has been suggested by industry experts that zinc vapour could condense on the strip, leaving a deposit that prevents wetting by molten Zn-55%Al-1.5%Si alloy. It remains that little is understood of the fundamental nature of the interaction between zinc vapour and a steel substrate. The purpose of this research was to examine the rates and mechanisms of zinc vapour condensation on clean and oxidised steel substrates. Such information is essential for understanding the reactions that occur in the metal coating line furnace. It is intended that this research will strengthen the fundamental knowledge base upon which a solution to the problem can be developed.

The experimental work conducted in this study was centred on condensing zinc vapour onto substrates under specific gaseous atmospheres at atmospheric pressure. A major part of the work involved the development of a technique for depositing zinc vapour onto a steel substrate and the design and construction of an experimental apparatus. Preliminary studies were undertaken to define the design criteria for an apparatus in which the rate of condensation of zinc vapour could be measured accurately and the interaction between zinc vapour and clean and oxidised steel substrates could be examined. In this probing exercise, modifications were made to a proven design of an apparatus designed to measure evaporation rates of metals in inert gas atmospheres was used. The preliminary experiments provided a wealth of essential knowledge required to design an experimental facility in which it was possible to accurately measure the rate of condensation of zinc vapour and to study the interaction between zinc vapour and the steel substrate, as well as their potential chemical reactions.

This new apparatus was specifically developed for a quantitative assessment of zinc vapour deposition, and allowed the substrate and zinc vapour to be heated in independent, but interconnecting chambers. The exposure of the substrate to the zinc vapour could be controlled with precision, and it was possible to not only measure the zinc vapour condensation rates, but also to assess the effect of using oxidising and reducing gasses during preheating of the substrate.

Under conditions of high undercooling, zinc vapour deposited by the island plus continuous thin film mode, while at higher substrate temperatures, close to that expected in the industrial process, the zinc deposited more slowly, and zinc islands did not form on the substrate within the first 60 seconds of exposure.

Deposition of zinc vapour on an oxidised substrate occurred at a much higher rate than on a clean steel substrate. This increase in deposition rate has been attributed to a direct reaction occurring between the zinc vapour and the iron oxide without the formation of any liquid condensate. The reaction is:
Fe3O4(s) + 4Zn(v) ↔ 4ZnO(s) + 3Fe(s)

This reaction will proceed to the right under sufficiently high partial pressures of zinc vapour, and at substrate temperatures both above and below the saturation temperature of the zinc vapour. It was observed that under sufficiently low partial pressures of zinc vapour, the above reaction is driven to the left. Zinc oxide has a determining influence on the wetting of the strip by the molten alloy.

Immersion tests, wherein substrates deposited with zinc vapour were dipped into molten Zn-55%Al-1.5%Si alloy, were carried out to examine the effect of various zinc vapour reactions on the quality of the coatings. Deposition of zinc vapour on both clean and oxidised steel surfaces had a detrimental effect on the coating quality. In cases where zinc was deposited onto a clean steel substrate prior to immersion in the coating alloy, pinholes resulted, while zinc vapour deposition on an oxidised surface prior to immersion led to large scale uncoated areas.

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