TYPES OF CORROSION-ACTIVE NON-METALLIC INCLUSIONS AND METHODS FOR THEIR DETECTION IN STEELS (REVIEW)

Abstract

The article presents a systematic review of the types of corrosion-active non-metallic inclusions in steels. It is shown that according to their chemical composition, non-metallic inclusions are divided into: sulfide, oxide, nitride, carbide, carbonitride and complex (oxysulfide, oxide-nitride). Oxide non-metallic inclusions are one of the most common groups of inclusions in steels, they are the result of the interaction of dissolved oxygen with deoxidizing elements such as aluminum, silicon and manganese, and are divided into simple oxides (Al2O3, SiO2, MnO), complex (MnO·SiO2, CaO·Al2O3) and multicomponent. Modern methods of studying non-metallic inclusions, as well as methods for their detection and evaluation, are summarized. Structural and morphological methods based on the analysis of geometric parameters of inclusions (shape, size, orientation, distribution density), allow establishing potential stress concentration zones, but do not always provide information about the real electrochemical activity. Chemical and phase methods are aimed at determining the composition of inclusions and phase ratios. The methods of electron microscopy and microanalysis include: scanning electrochemical microscopy, EDX analysis, electron probe microanalysis, scanning vibrating electrode technique, scanning Kelvin probe microscopy, local electrochemical impedance spectroscopy. These methods are necessary for the identification of potentially dangerous phases, such as MnS, Al₂O₃, CaO·Al₂O₃. They provide an opportunity to comprehensively assess the impact of non-metallic inclusions on local characteristics (potential, current, impedance), which can be used to predict the durability of the material, combining structural analysis with electrochemical measurements and mathematical modeling.