## 1 Introduction
In situ spinel formation involves the expansion of the material's volume as a result of its formation and creates stresses. Additionally, microcracks may occur due to differences in the density of MgO, Al\({}_{2}\)O\({}_{3}\), and MgAl\({}_{2}\)O\({}_{4}\), which could favor slag penetration into the material [26]. Therefore, in the present tests, corrosion was carried out on previously fired castables where the spinel forming process was completed. The advantage of in situ spinel over prefabricated spinel is the lower costs of its use [30]. Another important aspect is the grain size of the spinel. Spinel must be added predominantly to the fine fraction of the castable formulation to attain the best penetration resistance [16]. The stoichiometry of spinel also has a significant influence on the chemical and physical properties. Compared with stoichiometric spinels, alumina-rich spinels are superior. The stoichiometric amount of Al\({}_{2}\)O\({}_{3}\) in spinel is 71.67 wt.%. The commercially available spinels MR66, AR78, and AR90 are distinguished by their chemical composition, containing 66%, 78%, and 90% alumina, respectively [31]. [PERSON] et al. concluded that the corrosion thickness increases with an increase in magnesia content in the castable [32]. However, there are not many publications that investigate this problem with calcium aluminate cement-free (CAC-free) refractories. Steel slag, a by-product of steelmaking processes, is a group of materials diverse in terms of composition that cause the corrosion of castables. Depending on the feedstock used in the smelting process, the slag may contain ones, impurities, fluxes, metal oxides, and residues from the coal or coke combustion process. Steel slag is formed by smelting steel in converters, and slag from the oxygen converter (LD), slag from steel refining in the ladle (VAD, VOD, LF) [33; 34; 35], slag from the electric arc furnace, and slag from the tundish from the continuous casting of steel can be distinguished. Two methods of obtaining steel in converters with bottom blowing are the most widely used. The main oxides in the converter slag are CaO and Fe\({}_{2}\)O\({}_{3}\), but there may also be oxides of MgO, Al\({}_{2}\)O\({}_{3}\), SiO\({}_{2}\), MnO, P\({}_{2}\)O\({}_{5}\) and TiO\({}_{2}\)[36]. Due to their chemical nature, we can distinguish acidic slags (the main component is SiO\({}_{2}\)) and basic slags (the main component is CaO). The CaO/SiO\({}_{2}\) (C/S) ratio in the slag determines its chemical nature and has a significant impact on the corrosion resistance of the refractory lining. The basic quaternary system of slag (VOD) is CaO-SiO\({}_{2}\)-MgO-Al\({}_{2}\)O\({}_{3}\). In this system, the slag phases that can coexist with each other include CA\({}_{2}\)-CA-C\({}_{2}\)AS, CM\({}_{2}\)A\({}_{8}\)-C\({}_{2}\)AS-CAS\({}_{2}\), CAS\({}_{2}\)-C\({}_{2}\)S, C\({}_{2}\)MS\({}_{2}\)-CS-C\({}_{2}\)AS, and CM\({}_{2}\)A\({}_{8}\)-CAS\({}_{2}\)-A [37; 38; 39; 40]. It is difficult to relate corrosion test results of castables on a laboratory scale to industrial conditions, but such attempts are made. Various methods are used to determine the corrosion resistance of refractory material. Dynamic methods are characterized by the displacement of slag in relation to the refractory working surface and static methods. Static methods include immersion, contact, pyrometric cone, and crucible methods. The rotary kiln method, induction furnace method, and finger test method are dynamic methods [41; 42]. In this study, the contact method was used. The aim of the work is to investigate the corrosive properties of spinel-forming alumina castables with different contents of MgO (3,5 i 7 wt.%) in contact with slags with different mass ratios of C/S from 1.1, 3.5, and 5.5 and to predict possibly occurring corrosion mechanisms between the tested castables and slags. Analyses were carried out on mixtures composed of 50 wt.% castables and 50 wt.% slag.