Molybdenum effect on the Sulfide Stress Corrosion of a super martensitic stainless steel in sour environment highlighted by Electrochemical Impedance Spectroscopy

The effect of molybdenum on sulfide stress corrosion cracking resistance has been studied for super martensitic stainless steels in sour environment at pH 3.5. Static proof rings test and dynamic slow strain rate traction measurements show that molybdenum appears to have a dual positive effect on stress corrosion resistance of super martensitic steels for contents of molybdenum higher than 2.25%. On one hand, molybdenum enhances the passive film and in the other hand, it promotes an increase in the retained austenite fraction that can, in some conditions, be beneficial regarding hydrogen embrittlement resistance. In order to separate retained austenite contribution from the straightforward molybdenum effect on the passive film, electrochemical impedance spectroscopy (EIS) tests have been performed throughout both the static and dynamic mechanical tests. EIS spectra show a correlation between molybdenum content and sulfide stress corrosion surface process kinetics. More specifically, results at intermediate frequency could be related to the onset of the failure process, thus constituting a promising tool for monitoring the threshold of load level related to the mechanical rupture of the specimens.