The effect of iron oxide as an inhibition layer on iron-zinc reactions during Hot-Dip galvanizing
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I.
INTRODUCTION
THE annealing conditions for the substrate steel sheet prior to hot-dip galvanizing are conducted in atmospheres that are reducing for Fe, but are usually oxidizing for elements such as Mn, P, Ti, and Al which may be present in the steel.[1] The oxidation of these elements leads to the formation of internal oxides as well as surface oxide particles. The location of some of these oxide particles at substrate steel grain boundaries has led some investigators[2] to conclude that oxide particles may cause Fe-Zn outburst growths to form. The proposed oxide mechanism relies on the fact that the oxide particles located at substrate steel grain boundaries can be reduced by Al in the Zn bath to form Al2O3, thereby causing a local depletion of Al concentration in the bath in the vicinity of the newly formed oxide.[3] Because oxide particles are found to be concentrated at steel substrate grain boundaries, it has been proposed that the Al depletion near the substrate steel grain boundaries is significant, which may lead to localized Zn attack and the formation of Fe-Zn growths or outbursts at the grain boundary.[2] Guttmann et al.[4] proposed that Zn is able to diffuse along the oxide particle/Fe2Al5 interface (in Zn baths containing Al), and this interface provides a fast diffusion path for Zn. In order to evaluate the proposed liquid Zn/oxide mechanism, oxidized steel samples were prepared for galvanizing reaction studies. The purpose of the substrate iron oxide experiments was to simulate the effect of a ‘‘stable’’ inhibition barrier layer on Fe-Zn reaction kinetics during hot-dip galvanizing in 0.00 and 0.20 wt pct Al-Zn baths. In the experiments described here, the oxide was present as a continuous layer, instead of discrete oxide particles; thus, the effect of a uniform oxide layer on galvanizing reactions was studied.
II.
EXPERIMENTAL PROCEDURE
The substrate material used to study the effect of substrate surface oxide on galvanizing reaction kinetics was a low-carbon steel alloy initially produced in ingot form by BHP Steel (Port Kembla, Australia). This laboratory-produced materials had a chemistry given as alloy LC in Table I. The ingot was rolled to a final sheet thickness of 0.4 mm, which corresponded to an 84 pct cold-worked condition, and given a decarburization anneal at 850 7C for 6.5 hours in an 18 pct H2/N2 gas bubbled through water. The LC sheet samples were sectioned to 50.8 3 228.6 3 0.4 mm and cleaned in acetone. A set of thermocouple wires was attached to the center of each specimen to monitor thermal processing while the sample was heat treated in the Gleeble HAZ 1000. Using electric resistance heating, each sample wa heated up to 650 7C at a rate to 500 7C/s. Samples were then held at 650 7C for either 1, 5, 10, 30, or 60 seconds. After the isothermal hold was complete, the samples were cooled at 25 7C/s using an air and water mist spray. The entire heat treatment procedure was performed under atmospheric conditions to allow the steel substrate surface to oxidize. The samples were
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