EVALUATION AND PREDICTION OF PITTING RESISTANCE OF HEAT EXCHANGERS MADE OF ALLOY 06KhN28MDT (SIMILAR TO AISI 904L STEEL) IN MODEL CIRCULATING WATERS

Abstract

A mathematical model was developed, which is based on a multivariate regression dependence and establishes the relationship between the critical pitting temperature (CPT) of the 06KhN28MDT alloy (similar to AISI 904L steel) and its chemical composition and structural components in model circulating waters with pH 4...8 and chloride concentration from 350 up to 600 mg/liter, which correspond to industrial circulating water during the operation of heat exchangers. Based on the results of the analysis of the developed mathematical model, chemical composition, structural heterogeneity of the studied alloy melts and parameters of model recycled water (pH and chloride concentration), it has been found that the significant variables of the model are the parameters of the media, the mean grain diameter of the austenite of the 06KhN28MDT alloy, and the chromium content in it within the standard for its production. It is shown that the CPT of the 06KhN28MDT alloy increases by 8.3 �C (degree Celsius) with a decrease in the chloride concentration in model recycled water from 600 up to 350 mg/L, by 6.8 �C (degree Celsius) with an increase in its pH from 4 up to 8, by 5.9 �C (degree Celsius) with a decrease in the mean diameter of the austenite grain from 31 to 11 ?m, and by 4.7 �C with a decrease in its chromium content from 24.31 up to 21.84 wt %. It has been found that, under the same research conditions, the parameters of model recycled water have the most significant effect on the CPT of 06KhN28MDT alloy and stainless steels 08Kh18N10, AISI 304, 12Kh18N10T and AISI 321. However, the chromium content and mean austenite grain diameter of the alloy have an opposite effect on its CPT to these steels. This is most likely due to the fact that at such a high chromium concentration and the presence of molybdenum, metastable pittings that occur on its surface are quickly repassivated. This can contribute to the transformation of the remaining pitting into stable pitting at a lower CPT due to the redistribution of the current density between them. The paper substantiates the use of the developed mathematical model to evaluate and predict the pitting resistance of heat exchangers made of 06KhN28MDT alloy under possible changes in the parameters of recycled water (pH and chloride concentration) during their operation, depending on the chromium content and the size of the mean diameter of the austenite grain.