The peculiarities of the process of forming complex compounds, namely, spinels, in flux-cored arc welding for the slag TiO$_{2}$–MnO–SiO$_{2}$–MgO–CaO–FeO–Al$_{2}$O$_{3}$ system are studied. An experimental investigation of the mechanism of spinel formation in conditions of electrode arc welding and cladding under flux is carried out. Based on the analysis of microimages of the slag surface obtained using scanning electron microscopy, the mechanism of spinel formation, namely, the growth of microcomplex compounds into macrospinels, is presented. The main stages and mechanism of formation of the complex compounds, namely, spinels, are described. As shown, the structure of spinels is determined by the temperature conditions of the crystallization of the slag melt. The study reveals that one of the determining factors of high-temperature separation of the slag crust during multilayer cladding with overlap of the previous layer is the formation of spinels at the interphase boundary between the slag and the metal. Based on modern ideas about the structure of slag melts, a mechanism for the formation of complex compounds at the interphase boundary between slag and metal is proposed. The main factor determining the process of high-temperature separation of the slag coating for the TiO$_{2}$–MnO–SiO$_{2}$–MgO–CaO–FeO–Al$_{2}$O$_{3}$ system is the formation of complex compounds based on spinel-forming elements and slag oxides at the interphase boundary between the slag and the metal.