Increasing the corrosion properties of duplex steel with REM modification

Abstract

Duplex stainless steels are a modern class of materials with a unique combination of high corrosion and mechanical properties. Due to this, they can be widely used in machine parts and aggregates in fields with aggressive oil and gas production conditions. One of the disadvantages of these materials is their tendency to local corrosion damage on non-metallic inclusions, other things being equal, formed during smelting and casting. To control the purity of steel in conditions of open induction smelting, it is effective to use modification with rare earth metals (REM). Therefore, the purpose of this work was to determine the optimal content of REM in duplex steel to increase corrosion properties. Thermodynamic modeling of the formation of nonmetallic inclusions in duplex corrosion-resistant steel S32750 was carried out and the results of calculations were compared with the experimental data. It is shown that there is an optimal concentration of REM at which contamination with inclusions is minimal due to favorable conditions for their removal, and with a further increase in consumption it increases due to coagulation of a large number of refractory oxides. Electrochemical tests were performed and parameters such as corrosion potential, pitting formation potential and the basis of pitting resistance of experimental steels were determined. Therefore, the corrosion properties of the investigated duplex steel are significantly improved when treated with REM. The electro­chemical potentials of different types of inclusions are evaluated on a qualitative level. Based on the obtained results on corrosion resistance and contamination of the studied castings, the optimal amount of REM introduced for modifying inclusions is 0.05 % (0.65Ce + 0.35La).