Analyses of defects on the surface of hot plates for an electric stove

Brown stains were observed forming after the black-oxide coating on the surfaces of grey-cast-iron hot plates made for an electric stove. These stains were metallographically examined using a scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer (EDXS). For the phase identification we also employed X-ray diffraction (XRD) analyses. The results indicated that the stained surfaces of the hot plates were covered with whiskers of hematite (Fe2O3), while on the surfaces of the non-defective hot plates only magnetite (Fe3O4) was present. The thermodynamic calculations confirmed the possibility of the formation of hematite (Fe2O3) as a result of the oxidation of magnetite (Fe3O4) if the partial pressure of oxygen is increased during the black-oxide-coating process.

► Brown stains on the hot plates for the electric stove have been analysed.
► The stains are caused by whiskers that were growing from the top oxide layer.
► Whiskers are from hematite, while non-defective surface is from magnetite.
► For proper black coating a control of the partial pressure of oxygen is necessary.