Patterns and Mechanisms of Pitting of Heat Exchangers Made of AISI 321 and 08Kh18N10T Steels During their Operation in Recycled Eater

Abstract

The article examines the patterns and mechanisms of pitting corrosion in AISI 321 and 08Kh18N10T stainless steels under conditions simulating recycled waters used in heat exchanger operation. It was established that the critical pitting temperature (CPT) of these steels increases significantly with increasing pH and decreasing chloride concentration in model solutions. The influence of the structural components of the steels was found to be less significant than that of the chloride-containing environment and to depend strongly on its parameters (pH and chloride concentration). In acidic model recycled waters (pH 4), AISI 304 steels – which contains minimal amounts of titanium nitrides, aluminum oxides, and carbon – exhibits the highest CPT values, regardless of the chloride concentration in the range of 350–600 mg/L. In contrast, AISI 321 and 08Kh18N10T steels, characterized by the highest volume fractions of titanium nitrides, as well as melt 2 of AISI 321 steel and 12Kh18N10T steel with elevated carbon content, show the lowest CPT values. Based on an analysis of the literature, it is hypothesized that this behavior may be attributed to the increased overvoltage of hydrogen ionization on titanium nitrides and carbonitrides, which act as initiation sites for pit formation and propagation. Additionally, the corrosion rate was found to correlate with the specific paramagnetic susceptibility χ₀ of austenite, a parameter reflecting the atomic magnetic state and determined by the chemical composition and phase structure of the alloy.