EXAMINATION OF THE ROLE OF MOLYBDENUM IN PASSIVATION OF STAINLESS-STEELS USING AC-IMPEDANCE SPECTROSCOPY

The role of Mo in the passivation of stainless steels was investigated using ac impedance spectroscopy. Six steels were examined: two ferrit ic (Fe-20Cr) alloys containing 0 and 1% Mo and four austenitic (Fe-20C r-25Ni) alloys containing 0, 1, 2, and 4.5% Mo. The experiments were p erformed in 1 M HCl and, in some cases, 0.5 M H2SO4, 1 M HCl + 3 M NaC l, and 4 M HCl. Quasi-steady-state polarization curves were obtained a nd the impedance was measured at potentials in the active region and t hrough the active-to-passive transition. The impedance spectra exhibit ed up to four arcs with well-resolved time constants. Equivalent circu it parameters were evaluated in each case to examine the effect of Mo on the dissolution of the alloys. Mo appears to influence virtually al l of the principal reactions involved in the dissolution, intermediate formation, and passivation of stainless steels. Its effect is complex and extends beyond a single reaction step in the mechanism for the di ssolution and passivation of these alloys.