Effects of rare earth modifying inclusions on the pitting corrosion of 13Cr4Ni martensitic stainless steel

Abstract

• ASPEX PSEM, SKP, SEM, TEM and electrochemical methods are employed to study corrosion behavior of 13Cr4Ni steel modified with rare earth metals. • Rare earth metals change the composition, size and quantity of inclusions in steel. • The addition of rare earth metals has a significant effect on both metastable and stable pitting processes, the optimum content of rare earth elements (La, Ce) added is 0.021%. • The mechanism of pitting corrosion induced by rare earth inclusions is revealed. In this study, the pitting corrosion behavior of 13Cr4Ni martensitic stainless steel (BASE) and that modified with rare earth (REM) in 0.1 mol/L NaCl solution were characterized. Techniques such as automatic secondary electron microscope (ASPEX PSEM detector), scanning electron microscope (SEM), transmission electron microscope (TEM), scanning Kelvin probe force microscope (SKP), potentiodynamic and potentiostatic polarizations were employed. The results obtained indicate that BASE steel contains Al 2 O 3 /MnS, Al 2 O 3 and MnS inclusions, while REM steels contain (La, Ce, Cr, Fe)-O and (La, Ce, Cr, Fe)-O-S inclusions. Compared with BASE steel, REM steel is more susceptible to induce the metastable pitting nucleation and repassivation, whereas it restrains the transition from metastable pitting to stable pitting. Adding 0.021% rare earth element to BASE steel can reduce the number and area of inclusions, while that of 0.058% can increase the number and enlarged the size of inclusions, which is also the reason that pitting corrosion resistance of 58REM steel is slightly lower than that of 21REM steel. In the process of pitting corrosion induced by Al 2 O 3 /MnS inclusions, MnS is preferentially anodic dissolved, and also the matrix contacted with Al 2 O 3 is subsequently anodic dissolved. For REM steels, anodic dissolution preferentially occurs at the boundary between inclusions and matrix, while (La, Ce, Cr, Fe)-O inclusions chemically dissolve in local acidic environment or are separated from steel matrix. The chemically dissolved substance (La 3+ and Ce 3+ ) of (La, Ce, Cr, Fe)-O inclusions are concentrated in pitting pits, which inhibits its continuous growth.