X‐ray Diffraction Characterization of the Enamel–Steel Interface

In the enameling of steel, the oxide is generally regarded as being completely dissolved by the fusing enamel, with the enamel–metal bond forming directly between oxide‐saturated glass and metal. According to this model, the adherence of the oxide layer present on the surface of the steel as the enamel begins to fuse is irrelevant, because none of the original oxide layer remains in the matured enamel–steel bond. This model has not been completely verified, however, and some researchers have presented evidence for the presence of a layer of wüstite (FeO) at the enamel–steel interface on the order of 1 to 4 μm in thickness. Whether such a layer exists has important implications regarding the mechanism of enamel–steel adherence. In the present study, a method was developed to concentrate whatever crystalline material might be present in the interfacial zone to make it more amenable to detection by X‐ray diffraction. Through the use of wüstite standards, the present technique was shown to be capable of detecting a layer of wüstite at the enamel–steel interface as thin as 0.3 μm. However, in neither one‐coat nor two‐coat enameling could a layer of wüstite be demonstrated at the enamel–steel interface. Hence, there does not appear to be a 1‐to‐4‐μm‐thick wüstite layer at the enamel–steel interface. If a layer of iron oxide is present at the interface, it must be thinner than 0.3 μm.