Influence of chemical composition and structural heterogeneity of 06ХН28МДТ alloy on mechanisms of its pits formation in return waters

Abstract

Purpose of the work. Determine the influence of chemical composition and structure of 06ХН28МДТ alloy on mechanisms of its pits formation in model return waters with pH = 4... 8 and chloride concentration 180 mg/l.Research methods. X-ray spectral analysis on СРМ-25 spectrometer, metallographic analysis on optical micro­scope МРР-2Р. Corrosion losses of metals in pittings were determined by photometric analysis of model chloride-containing recycled waters after holding samples therein. Straight-line regression relationships between corrosion losses of metals in pittings ΔCr, ΔNi, ΔFe and alloy parameters were determined by regression analysis by least squares method. In order to determine the influence of alloy parameters on the mechanisms of its pitting formation, previously established characteristic features of selective dissolution of metals in pittings were used.Results obtained. It has been found that in model return waters with pH = 4–6.8 06ХН28МДТ alloy pits to form stable, and with pH = 7 and chloride concentration 180 mg/l – metastable pittings. In stable pittings Cr contributes to the increase of ΔFe from them, and in metastable – vice versa. This is due to the direction of solid-phase diffusion of Fe atoms from the alloy volume to the surface of stable pittings and from the surface of metastable in its volume. Therefore, the increase in Cr content in the alloy contributes to the growth of stable pittings and the repassification of metastable ones due to the decrease in their growth intensity. However, it has been found that the smaller the sizes of titanium nitrides near which pittings are formed, the lower the ΔFe in metastable and stable pittings. This is due to the lower degree of defect of the structure of the alloy austenite solid solution about small titanium nitrides.Scientific novelty. Cr promotes the growth of solid phase diffusion of Fe atoms to the surface of stable and from the surface of metastable pittings into the volume of the 06ХН28МДТ alloy. This promotes the repassification of metasta-ble and, accordingly, the growth of stable pittings. Increasing the size of titanium nitrides in the alloy contributes to increasing the number of austenite structure defects near them and to the growth of stable and metastable pittings.Practical value. Manufacturers of heat exchangers are advised to use 06ХН28МДТ alloy smelts with minimum Cr content, as well as those having titanium nitrides size of not more than 5 mcm.