Effect of deformation and stress-induced precipitations on the corrosion resistance of silicon alloyed stainless steel X 2 CrNiSi 18 15 in nitric acid

The corrosion resistance of silicon alloyed X 2 CrNiSi 18 15 stainless steel with about 4 wt.-% Si in nitric acid depends on both deformation degree of the material and number and distribution of precipitations in the microstructure. After cold deformation and subsequent heat-treatment at 700deg C (1292deg F), the corrosion rate in 5 M HNO3 is higher than that of non-deformed material. This result was established with specimens isolated from each other but was particularly pronounced with specimens being in galvanic contact. By deformation and subsequent annealing at 700deg C, precipitations are formed at the grain boundaries the number of which increases with increasing degree of deformation. With higher degree of deformation, the precipitations are preferably arranged at slip-lines. They were identified as chromium carbide, M23C6, and silicide of the Cr5Ni3FeSi2 type. It is shown by electrochemical measurements that these precipitations stimulate the cathodic partial reaction, i.e., the reduction of nitric acid. Hence, the corrosion resistance of the silicon alloyed stainless steel is impaired by deformation and stress-induced precipitations formed during annealing at 700deg C. (orig.)