Mechanisms of Pitting Corrosion of Austenitic Steels of Heat Exchangers in Circulating Waters and its Prediction

Mathematical models of the dependence of the critical pitting temperature (CPT) of AISI 304, AISI 321, 12Kh18N10T and 08Kh18N10 steels on their chemical composition, structural heterogeneity, as well as pH and chloride concentration of circulating water were developed. Models were based on quadratic regressions with first order partial derivatives and on polynomials with a reduced number of signs. By applying mathematical models, it was established that the pitting resistance of these steels increases with an increase in the average distance between oxides, the average diameter of austenite grain, specific magnetic susceptibility, the decrease in the volume of δ-ferrite and the number of the smallest oxides up to 1.98 μm in size. The positive effect of Cr and Mn on the pitting resistance of the studied steels was studied. Probably this occurred due to the increase of the solubility of N in austenite, reduction of the Fe atoms diffusion intensity to the surface of stable pittings and the increase of Cr to metastable pittings, contributing to their repassivation, which increased the pitting resistance of steels. The developed mathematical models are recommended for choosing optimal grades of austenitic steels and predicting their pitting resistance during operation of heat exchangers in circulating waters.