CORROSION RESISTANCE OF AUSTENITIC STAINLESS STEELS WITHOUT NITROGEN AND NITROGEN-ALLOYED IN OXIDIZING AND CHLORIDE ENVIRONMENT
The corrosion resistance of the developed austenitic steel 03Cr20Ni9Mn3N0.30, alloyed with nitrogen in comparison with steel 03Cr18Ni11 (AISI 304L) was studied. The analysis of corrosion resistance to intergranular corrosion in 65 % boiling nitric acid was carried out according to the “DU method” of GOST 6032-2017. The studied steels resistance in 42% boiling MgCl2 solution was also evaluated. It has been established, that the corrosion rate in 65 % boiling nitric acid for steel AISI 304L and nitrogen-alloyed is comparable and amounts to about 0.2-0.4 mm / year. However, it was shown that the incubation period to achieve a sensitization state in steel 03Cr20Ni9Mn3N0.30 is about 4 times longer than of steel AISI 304L. Additionally, the conditions for the studied steels resistance to intergranular corrosion in boiling 65 % nitric acid after heating at temperatures from 500 to 850 °C and holdings from 1 hour to 100 hours are determined. Based on the test results, a Rollason diagram was constructed. The microalloying effect with boron and rare-earth metals on 03Cr20Ni9Mn3N0.30 corrosion resistance (according to the DU method of GOST 6032-2017) was also investigated. The permissible concentrations of microalloying elements (boron up to 0.0025 wt. %, and rare-earth metals up to 0.04 wt. %) are determined, which provide high technological plasticity without reducing its corrosion resistance. When tested by the DU method, the corrosion metal loss in both of these cases was not more than 0.35 mm/year. However, with the boron introduction in an amount of 0.01 wt. %, the corrosion rate increases sharply to 1.328 mm/year, while reducing technological plasticity. As a result of compared steels testing samples for stress corrosion in a 42 % boiling MgCl2 solution. The superiority of 03Cr20Ni9Mn3N0.30 steel over AISI 304L steel at a strain of 280 MPa was more than 8 times higher, and at a strain of 100 MPa almost 10 times higher. It should be noted that the failure nature of steel 03Cr20Ni9Mn3N0.30 and AISI 304L is identical, but the time required for cracks initiation by almost differs.