The corrosion behavior of intermetallic compounds Ni_3(Si,Ti) and Ni_3(Si,Ti) + 2Mo in acidic solutions

摘要

The corrosion behavior of the intermetallic compounds homogenized, Ni_3(Si,Ti) (Ll_2: single phase) and Ni_3(Si,Ti) + 2Mo (Ll_2 and (Ll_2 + Ni_ss) mixture region), has been investigated using an immersion test, electrochemical method and surface analytical method (SEM; scanning electron microscope and EPMA: electron probe microanalysis) in 0.5 kmol/m3 H_2SO_4 and 0.5 kmol/m3 HC1 solutions at 303 K. In addition, the corrosion behavior of a solution annealed austenitic stainless steel type 304 was studied under the same experimental conditions as a reference. It was found that the intergranular attack was observed for Ni_3(Si,Ti) at an initial stage of the immersion test, but not Ni_3(Si,Ti) + 2Mo, while Ni_3(Si,Ti) + 2Mo had the preferential dissolution of Ll_2 with a lower Mo concentration compared to (L1_2 + Ni_ss) mixture region. From the immersion test and polarization curves, Ni_3(Si,Ti) + 2Mo showed the lowest corrosion resistance in both solutions and Ni_3(Si,Ti) had the highest corrosion resistance in the HC1 solution, but not in the H_2SO_4 solution. For instance, it was found that unlike type 304 stainless steel, these intermetallic compounds were difficult to form a stable passive film in the H_2SO_4 solution. The results obtained were explained in terms of boron segregation at grain boundaries, Mo enrichment and film stability (or strength).