Assessment and Prediction of the Pitting Resistance of Plate-Like Heat Exchangers Made of AISI304 Steel and Operating in Circulating Waters

We develop a mathematical model of dependences of the critical pitting temperature of AISI304 steel on its chemical composition, structural heterogeneity, pH value of circulating water, and the concentration of chlorides. It is shown that the influence of model circulating waters on the critical pitting temperature of steel is twice more intense than the influence of the other characteristics. In particular, it is shown that the critical pitting temperature of steel increases by 13.4°C as the concentration of chlorides decreases from 600 down to 350 mg/liter. At the same time, it increases only by 5.8°C as the Cr content of steel increases from 17 up to 19.5 wt.%. This is connected with the formation of a dense chromium-containing oxide film on the steel surface, which efficiently prevents pitting and decreases the rate of solid-phase diffusion of Fe atoms to the surface of pits, which may promote their repassivation. It is shown that the critical temperature of pitting of steel increases by 5.2°C as the mean distance between oxides increases from 151 up to 172 μm. Thus, the larger the mean distance between oxides in steel and the mean diameter of austenite grains, the lower the probability of its intersection with grain boundaries, where the formation of stable pits is most probable. It is proved that the higher the Cr content of steel and the finer the oxides formed there, the higher the pitting resistance of steel. We recommend to use the developed mathematical model for the prediction of pitting resistance of heat exchangers made of AISI304 steel operating in circulating waters and for the choice of the optimal parameters of melting.