Mathematical modeling of selective dissolution of AISI 304 steel in circulating chloride-containing waters during operation of heat exchangers

Abstract

Purpose. Investigation of the effect of chloride-containing medium, chemical composition and structural heterogeneity of AiSi304 steel on selective dissolution of metals ΔCr, ΔNi and ΔFe from stable pits. Research methods. X-ray structural analysis, optical microscopy, energy dispersive microanalysis, analysis of the developed linear regression mathematical models of the second order for corrosion losses of metals from pits. Results. Based on the analysis of the developed linear regression mathematical models of the second order for corrosion losses of metals from pits, it was found that ΔFe from pits on the surface of AISI 304 steel in model circulating water with pH = 4...8 and chloride concentration of 300 and 600 mg / l decrease with an increase in it. the amount of oxides (1.98...3.95 microns), the average diameter of the austenite grain and increase when the average distance between the oxides and the volume of b- ferite in steel increases. It is proved that ΔNi from pits decreases with an increase in the amount of oxides in steel (1.98...3.95 μm), the volume of b- ferite and a decrease in the average diameter of austenite grain. It was determined that ΔCr of steel from pits mainly depends on the concentration of chlorides in the model circulating water, the amount of oxides (1.98 ... 3.95 μm), the average diameter of the austetin grain and the volume of b-ferite. It is hypothesized that this is due to the intensity of adsorption of chlorine ions on imperfections in the structure of AISI304 steel near these oxides at the intersection with the grain boundaries of austenite, where pits nucleate and grow. Scientific novelty. It was established for the first time that the coefficient of selective dissolution of Cr from pits on the surface of AISI 304 (ZCr ) steel in the circulating waters of the river decreases with an increase in the volume of b-ferite in the steel, the average diameter of austenite grain and a decrease in the amount of oxides. This can facilitate the transition of metastable pits to stable ones. Moreover, this process is autocatalytic; therefore, it does not depend on the parameters of the model circulating water. Statement of tasks. Based on the established features of selective dissolution of ΔCr, ΔNi and ΔFe from pits on the surface of AISI304 steel, to study the effect of its plastic defermation on pitting resistance in chloride-containing model circulating waters. Practical value. The developed mathematical models are used to assess the pitting resistance of AISI304 steel in model circulating waters, depending on its parameters and the environment where heat exchangers made of this steel operate.