Evaluation of oxide layers formed during the decarburisation of grain-oriented electrical steel using a Fourier transform infrared (FTIR) technique

Electrical steels are highly specialised, magnetically soft materials, used to form the cores that carry the magnetic flux in electrical machines such as motors, generators and transformers. During the production of GO electrical steel, the strip passes through a decarburisation furnace, which promotes the formation of a thin surface oxide layer consisting of predominantly fayalite (Fe2SiO4) and silica (SiO2). During a subsequent high temperature anneal, this layer reacts with magnesia (MgO) to form a forsterite ‘glass film’ layer, which applies a tensile stress to the steel. This reduces the magnetic losses of the material on which the final product is routinely graded. Due to the effect that the oxide layer has on the quality of the final material, it would be beneficial to possess a technique that can rapidly assess its composition and/or morphology. This paper details the assessment of Fourier transform infrared (FTIR) and electrochemical potential (ECP) analysis, and a technique of combining the two. FTIR analysis of the decarburisation oxide layer exhibited evidence of just fayalite, with silica only being observed on the spectra following brief acid etching. To refine the etching process, samples were removed from the acid at various intervals based on the output of the ECP technique. It was established that there was a clear link between the position reached on the ECP profile and absorption bands observed on the corresponding FTIR spectra.