Enameling of steels is widely used as coating in various products such as kitchen appliances, ovens, boilers, hot water tanks, heat exchangers or chemical reactors. Durability of enamel in these applications critically depends on its adhesion to the steel substrate. The enameling process involves several stages, including surface preparation, enamel application and high-temperature firing.
During firing in an air atmosphere, oxide scale forms on the steel surface, which must be dissolved in the liquid enamel before a good adhesion can develop. Understanding both the formation and subsequent dissolution of this oxide layer is key to elucidating the interfacial mechanisms that govern enamel-steel adhesion.
In this study, we investigated various steels grades including die-cast (DC)04, DC06, dual phase (DP)450 and open coil (OC) material. We interrupted the enameling process at specific temperatures (300°C to 820°C) to track oxide scale evolution. Oxide layer thickness and surface coverage were quantified to monitor oxidation during heating. At higher temperatures above 700°C, when a viscous enamel layer starts to form, we studied oxide dissolution into the enamel.
We developed a meticulous sample preparation route to preserve the native structure of the enamel-oxide-steel interfaces for high-resolution analysis of cross section specimens. Advanced electron microscopy techniques were employed to visualize oxide dissolution at high lateral resolution, allowing us to monitor the involved diffusion processes from the base material into the enamel. The dissolution of elements from the steel into the enamel contributes to increased surface roughness and the formation of undercuts at the steel surface, promoting mechanical interlocking and enhanced enamel adhesion.
Development of this comprehensive toolbox to analyze the exact steel-oxide-enamel interface enables a direct comparison of interfacial processes across different steel grades and firing temperatures. These insights provide a foundation for the development of improved enameling steels and optimized coating processes yielding superior adhesion performance.