The reaction between solid iron and liquid Al-Zn baths
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I.
INTRODUCTION
THE effects
of aluminum additions to galvanizing baths have been the subject of many studies. I~,2j These studies have all dealt with aluminum additions of 10 wt pct or less. Ghuman and Goldstein [21studied aluminum additions to zinc baths in the range of 0 to 10 wt pct over the temperature interval of 450 to 700 ~ They stated that at temperatures below 600 ~ aluminum sharply reduces the reaction between molten zinc and the iron panel. At temperatures above 600 ~ in Zn baths with 1 to 10 wt pct A1 the reaction between the bath and the iron panel was very violent and exothermic. It was proposed that a reaction between aluminum containing FeZn7 and the A1-Zn bath forms Fe2(A1, Zn)5 which was hypothesized to be a Brewer-Engel compound. [3] The formation of a Brewer-Engel compound is accompanied by a large heat of formation which was proposed to account for the violent and exothermic reaction observed by Ghuman and Goldstein. t:~ Gualtieri and Ficalora t4] measured the heat of formation of FeA13_xZn/as a function of Zn concentration. They found a large peak in the heat of formation near 13 wt pct Zn and attributed this to an electron transfer process similar to that expected in Brewer-Engel type compounds. The goals of this work were to determine if the reaction kinetics were diffusion or interface controlled and to explain the mechanism by which the rapid reaction between A1-Zn baths and iron sheet occurred.
II.
EXPERIMENTAL PROCEDURE
High purity Fe, A1, and Zn were used throughout this study. Isj The Si content of the Fe and AI was especially important due to its strong effect upon growth kinetics when present. 16'7'81Iron sheet was cold rolled to 0.65 mm thickJ.H. SELVERIAN, Graduate Student, A . R . MARDER, Adjunct Professor, and M. R. NOTIS, Professor, are with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Manuscript submitted June 19, 1987. METALLURGICAL TRANSACTIONS A
ness followed by a decarburizing anneal in wet hydrogen which reduced the carbon level less than 0.002 wt pct. This iron had a grain size of 20 m and was also used to saturate the A1-Zn baths prior to dipping. Three different bath compositions were investigated: 45A1-55Zn, 55A1-45Zn, and 75A1-25Zn, and all compositions are given in wt pct. Each bath consisted of 1 0 0 0 c m 3 of material, and was studied at two different dipping temperatures. The compositions and temperatures of the baths are shown in Table I. Prior to dipping, the baths were saturated with iron. This was done to prevent the iron panel from being dissolved by the molten bath which would have complicated the reaction being studied. To ensure saturation, the amount of iron used was slightly greater than that given by the Fe-A1-Zn ternary diagram of Kfster and Godecke.[gl A bath was checked for saturation by observing if any dross had collected at the bottom of the pot. This dross was determined by EMPA (electron microprobe analyzer) to be FeA13 with approximately 2 pct Zn in solid solution. The baths and panels wer
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