Patterns and Mechanisms of Pitting of 06xh28mдt Alloy (Analogue of Aisi 904l Steel) in Model Recycled Water of Enterprises

Abstract

Purpose. The aim of the study is to establish the patterns and mechanisms of pitting of the 06ХН28МДТ alloy in model recycled waters with a pH of 4-8 and a chloride concentration of 350 up to 600 mg/l, which are most often used in industrial enterprises.
 Research methods. A mathematical model based on regression trees to establish the relationship between the critical pitting temperatures (CPT) of the 06ХН28МДТ alloy and its chemical composition, structure components, and parameters of model recycled water (pH, chloride concentration) has been developed. Metallographic analysis, energy dispersion analysis, regression analysis.
 Results. Based on the results of the analysis of the developed mathematical model, chemical composition, structural heterogeneity of the alloy, and known literature data, it was found that in model recycled waters with a pH of 4.5 up to 7.5, the CPT of the 06ХН28МДТ alloy has a constant value of about 48.3 °C, which does not depend on its chemical composition, structural components, and chloride concentration in the model recycled waters. In model recycled waters with a chloride concentration of 350 up to 600 mg/l, the alloy’s CPT increases, on mean, to 59 °C, with an increase in their pH from 7.5 up to 8, but it decreases to 39 °C, with its decrease from 5 to 4. It has been found that the pH of the media has a more significant effect on the CPT of the alloy than the concentration of chlorides in it. It was found that the pH of the media has a more significant effect on the CTE of the alloy than the concentration of chlorides in it.
 Scientific novelty. Based on the established patterns between the critical pitting temperatures of the 06ХН28МДТ alloy and its chemical composition, structure components, and parameters of recycled water, the mechanisms of its pitting has been determined, in particular, in low acidic media with pH 4.5, its CPT is minimal, which is due to a decrease in the hydrogen ionisation overvoltage on the carbide phase (Cr, Mo, Ti, Si, Mn, C) in the vicinity of titanium nitrides, where pitting occurs, and in close to neutral model recycled waters, the alloy’s CPT is higher in a wide pH range, which is associated with the formation of an oxide film from a monolayer of Cr atoms on its surface due to its solid-phase diffusion under the influence of Mn, but in low alkaline model recycled waters, the alloy’s CPT is the highest, which is due to a decrease in the activity of chlorine ions and their adsorption on the imperfections of the alloy structure.
 Practical value. The developed mathematical models are proposed to be applied to the selection of the most resistant melts to pitting in recirculating industrial waters of the alloy 06KhN28MDT melts and prediction of its pitting during the operation of heat exchangers.