The Features of the Chemical Composition and Structural-Phase State Determining a Decrease in the Corrosion Resistance of 18Cr–10Ni Steel Parts

Abstract

The features of the chemical composition and structural-phase state of the metal samples of 18Cr–10Ni steel (AISI 304) are investigated which could promote the appearance of general corrosion damage and formation of pittings on parts made of this steel under the action of an aggressive environment. It is established that the sulfur content in steel exceeds the level established by the standard for this steel (<0.03% S) almost tenfold. Therefore, it contains about 3 vol % manganese sulfides with a size of 1–~50 μm which form stitches and accumulations along the direction of rolling. According to the published data, manganese sulfide particles (MnS) in particular are the most corrosive in corrosion-resistant steels and alloys. They manyfold decrease the ability of Fe–Cr–Ni steels to passivate in a corrosive environment. The formation of FeSH+ ions requires a high concentration of S2– ions. The larger is the inclusions of sulfide particles, the higher is their ability to decrease the corrosion resistance of steel. Therefore, the large size of the MnS particles found in the steel plays an important negative role. It is shown that an additional factor promoting the decrease in the corrosion resistance of the steel under study is the presence of deformation martensite in the surface layer of the studied steel which is formed in the machining process during manufacturing by cutting and grinding parts from a billet. The appearance of this martensite is due to the low concentration of austenite-forming elements (0.01–0.04% C, 7.96–8.23% Ni). In the modified Scheffler–Delong diagram, steel is in the region, where the formation of martensite is possible, the calculated value of Md(30/50) for it was 28°C. According to the published data, deformation martensite in steels of the 18–10 type causes a decrease in their resistance to pitting corrosion in solutions of acids and salts. It is shown that the presence of an electric potential activates the corrosive action on the samples of 18Cr–10Ni steel in an acidic environment. It is concluded that the corrosion damage of the parts made of the studied steel is facilitated by the presence of accumulations of sulfide particles in the individual areas of the metal combined with the presence of deformation martensite in these areas.