Mathematical Modeling of the Corrosion Behavior of Austenitic Steels in Chloride-Containing Media During the Operation of Plate-Like Heat Exchangers

Abstract

Mathematical models that describe the dependences of the critical temperatures of pitting formation of AISI 304, 08Kh18N10, AISI 321, 12Kh18N10T steels in model circulating waters with pH 4…8 and chloride concentrations from 350 to 600 mg/l on their chemical composition and structure have been developed. They are based on linear squares regressions and a feed-forward neural network for reduced feature numbers. Using the developed mathematical models, it was found that the critical pitting temperatures of these steels increase with an increase in the pH of the circulating water, the number of oxides up to 3.95 μm in size, the average distance between titanium nitrides, the Cr content and a decrease in the concentration of chlorides in the circulating waters, the average distance between oxides and average austenite grain diameter. At the same time, it was found that the geometric dimensions of the steel structure most intensively affect their pitting resistance in circulating waters, and the effect of their chemical composition is minimal and is determined by the amount of Cr, which contributes to an increase in the pitting resistance of steels, probably increasing the solubility of nitrogen in the austenite solid solution. It is proposed to use the developed mathematical models to select the optimal heats of these steels for the production of heat exchangers and predict their pitting resistance during their operation in circulating waters.

Mathematical Modeling of Pitting Resistance in 06xh28мдт Allo-Heat Exchangers Exposed to Circulating Waters

“…Автори праць [8,9] оцінювали пітинготривкість сплаву 06ХН28МДТ (аналог сталі AISI 904L) електрохімічними та гравіметричними методами, які не дають можливість прогнозувати тривкість сталей і сплавів до їх пітингування за конкретних умов експлуатації теплообмінників (температура, концентрація хлоридів та рН оборотних вод). У праці [11] пітинготривкість сплаву 06ХН28МДТ оцінювали за критичною температурою його пітингування в модельних оборотних водах, застосовуючи встановлені прямопропорційні регресійні залежності між критичною температурою пітингування (КТП) сплаву та складовими його хімічного складу і структури. Однак такий підхід має дискретний характер і потребує апроксимації отриманих результатів.…”

“…Їх критичну тмпературу пітингування, хімічний склад (табл. 1), структурну гетерогенність, визначали в працях [8][9][10][11]. За основу побудови математичної моделі використовували багатовимірну лінійну регресію [12].…”

Prediction the durability of heat exchangers made of 06KhN28MDT alloy (analogous to AISI904L steel) to crevice corrosion during their operation in recycled water

“…In particular, design features of plate-like heat exchangers and chlorides in circulating water can be the cause of their crevice corrosion between the corrugations of adjacent plates. Today, there are many studies devoted to the assessment and prediction of pitting resistance of steels AISI304 [8], AISI321 [12,13] and alloy 06KhN28MDT [7][8][9][10], which are used in the production of plate-like heat exchangers. In papers [10][11][12], the patterns and mechanisms of the genesis and growth of pitting on their surface in chloridecontaining media have been investigated, in paper…”

“…[13] a method for identifying stable and metastable pitting has been proposed, and in [11,12] it has been established that the critical pitting temperatures of the above-mentioned structural materials mainly depend on their structural heterogeneity and the parameters of the chloride-containing media. Crevice and pitting corrosion of steels and alloys have many common features [15], in particular, pitting corrosion of heat exchanger plates under sediment from circulating water on their surface can turn into crevice corrosion when the chloridecontaining media in the gap between them is acidified or alkalized with a simultaneous increase in chloride content [3].…”