Mass-transport processes at the steel-enamel interface
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I. INTRODUCTION
VITREOUS enameling refers to the application of a thin layer of glass onto a metal substrate in order to provide a protective layer against chemical corrosion from the surrounding environment. For the enameling of hot-rolled steel with carbon content in the range of 0.05 to 0.2 wt pct C, current commercial practice is to use a two-stage process consisting of a precoat followed by a further coating (top coat) of glass frit, which melts to form a uniform layer of enamel. The precoat is wet sprayed on the steel surface, which is dried prior to top coating with a silica-based slip. This top coat, which is also wet sprayed over the dried precoat layer, is subsequently dried in a temperature range of 100 °C to 150 °C. After drying, the sheet is fired at approximately 850 °C for 3 to 4 minutes to form a homogenous glass-forming liquid of mixed oxides, which is allowed to vitrify on cooling. The structure of the enamel thus consists of a thin layer of glass dispersed with gas bubbles and metals as a product of interfacial reaction. Occasionally, chunks of unreacted silica/oxides are also found. The precoat was applied as a very thin (20 to 25 m), porous film onto the surface of steel. The two most important constituents of the precoat are nickel oxide (NiO) and barium metaphosphate (BMP). The NiO acts both to promote adherence[1,2] and to reduce fish scaling,[3,4,5] and the BMP causes nickel oxide to be evenly dispersed over the steel surface and enhances wetting between the glass and the steel. At high temperatures, adhesion occurs between the glass and the steel as a consequence of the galvanic reaction: Fe Ni2 Fe2 Ni
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It is well known that a rough surface is more desirable than a smooth surface in the enameling industry. However,
X. YANG, Research Fellow, A. JHA, Professor, and R.C. COCHRANE, Professor, are with the Institute for Materials Research, University of Leeds, Leeds LS2 9JT, United Kingdom. Contact e-mail: [email protected] S. ALI is with Permastore Ltd., Eye, Suffolk IP23 7HS, U.K. Manuscript submitted December 2, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS B
King et al.[6] also demonstrated that, in the absence of iron oxide saturation, good adhesion could not be ensured on a rough surface. By comparison, a glass composition saturated with FeO promoted adhesion on an electropolished iron. The results on the effect of FeO6 indicated that iron oxide plays a more important role in promoting adhesion between the glass and the steel and the key to strong adhesion is to attain a sufficiently high concentration of iron oxide at the interface regardless of the surface condition. At the initial stage of enameling, a sufficiently high concentration of iron is provided by dissolution of surface oxide, present either before enameling or formed during the initial stages of firing prior to fusion of the enamel. Once this oxide layer has dissolved, a critical concentration of iron oxide can only be maintained via the chemical reaction at the interface, which oxidizes the base metal and i
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