Modeling Al enrichment in galvanized coatings
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I.
INTRODUCTION
STUDIES of A1 partition in continuous galvanizing baths indicated that A1 was significantly enriched in galvanized coatings when bath AI contents exceeded a threshold level. For example, recent work by Belislem indicated that A1 in the coating remained unchanged at about 0.2 pct of the coating weight for bath A1 up to 0.14 pct; further increase in bath A1 content led to a rapid increase of A1 in the coatings. Such a trend was also noted and reported by other researchers, t2,31 It has been suggested that the mechanism of AI enrichment in coatings was the formation of an inhibition layer, consisting of Fe2A15. Research work carried out recently by the author and his co-worker~4,5~confirmed the conclusion drawn decades ago by a number of researcherst6-9] that the inhibition layer in galvanized coatings consisted of Fe2A15 containing Zn in solid solution. The Zn content in the compound was found to increase with increasing immersion time before reaching a saturation level, and the minimum bath AI content for full inhibition was found to be approximately 0.15 pct at temperatures relevant to galvanizing practice, ta,gj These observations are consistent with isotherms of the Zn-Fe-Alternary phase diagram, revised recently by the present author.t5,101 The currently prevailing 450 ~ isotherm was proposed by Urednicek and Kirkaldytl u in the early seventies, in which the upper limiting tie-line of the &liquid two-phase region was placed at 0.11 pct A1 along the liquid-phase boundary. After intensive experimentation and thermodynamic calculations, the present author found that the upper limiting tie-line of the &liquid two-phase region is actually located at 0.13 to 0.14 pct A1, depending on the bath temperature. This composition corresponds to the change of the thermodynamically stable phase from 6 to Fe2A15. It is easy to understand that the significant enrichment of A1 in coatings produced in baths with A1 in excess of 0.14 pct is due to the formation of the Fe2A15 inhibition layer in the coating/substrate interface. The revised isotherms have been proven also to be helpful in providing a better understand-
NAI-YONG TANG, Senior Research Metallurgist, is with the Product Technology Centre, Cominco Ltd., Mississauga, ON, Canada L5K 1B4. Manuscript submitted August 10, 1994. METALLURGICALAND MATERIALSTRANSACTIONSA
ing of other processes occurring in galvanizing practice.tS,m2] However, the rate of A1 uptake in coatings appears puzzling. It has been found to change rapidly with time and to be strongly dependent on the strip-entry temperature.tZ.3] Isobe t2] showed that during the first 3 seconds, approximately the time required for steel strip to pass through a bath on a continuous galvanizing line, over 80 pct of the total A1 uptake at the interface took place in the first second. Faderl et al.t3] found that for a bath containing 0.18 pct AI, the content of A1 in the interface increased approximately linearly with the difference between the strip-entry temperature and the bath temperature. On the other ha
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