Phase composition of steel–enamel interfaces: Effects of chemical pre-treatment

Mössbauer spectroscopy in transmission geometry, performed simultaneously with conversion X-ray, and conversion electron Mössbauer spectroscopy, X-ray powder diffraction and electron microprobe analysis (EMPA), have been used to monitor the effects of a chemical pre-treatment on the phase composition of a “steel–enamel interface” of a hot-rolled steel surface formed during a thermally induced coating process. In this comparative study, we have analysed both an untreated sample and a treated sample of a steel sheet with the surface pickled by dilute H2SO4 and passivated in Na2CO3 solution. A continuous magnetite layer covering the steel surface as a result of the chemical pre-treatment was identified by backscattering Mössbauer spectroscopy and EMPA. During a fusion process, well crystalline fayalite (Fe2SiO4) is formed at the phase boundary by the solid-state reaction of magnetite with the SiO2-based enamel matrix. Wüstite (FeO) was detected as a product of oxidation of iron under the magnetite layer of the chemically pre-treated sample, whereas hematite represents a product of oxidation of magnetite. This wüstite–magnetite–(hematite)–fayalite layered interface is rimmed by isometric metallic intricate network-forming particles with a composition of about Fe0.95Ni0.05 and which are related to a diffusion–reduction process of iron and/or to a reduction of Fe3+ in glass. In the case of chemically untreated steel, EMPA revealed a sharp phase boundary, rimmed again by Fe-based metallic particles. Magnetite covered by fayalite was occasionally observed in a few areas of the phase boundary. The presence of magnetite at the interface of the chemically untreated sample is due to a thermally induced air-oxidation of the α-Fe steel surface during an enamelling procedure. The surprising effect of the chemical pre-treatment, resulting in a lowering of adhesion of enamel on the steel surface, is discussed.