Untypical Electrochemical Corrosion Behavior of High-Nitrogen Austenitic Stainless Steel: Non-Pitting in Transpassivation

The transpassivation and pitting corrosion behavior of a high-nitrogen stainless steel (HNS), Fe18Cr15Mn3Mo0.92N, were systematically investigated by electrochemical analysis, morphology observation, and X-ray photoelectron spectroscopy surface analysis. It was surprisingly found that no pitting corrosion occurred in the transpassivation region of HNS. This electrochemical corrosion behavior is untypical for stainless steels, i.e., the traditional critical pitting potential method was invalid for HNS. Both N and Cr enrichments in the transpassivation film on HNS were found extremely higher than those in the passivation film. The N existed in the form of [CrN] complex, which could stabilize the above both films. Besides, the corrosion product of N was detected as NH₃ that exhibited an effective corrosion inhibition effect. On this basis, although the transition of Cr from 3-valent to 6-valent was confirmed, the transpassivation film on HNS still maintained it high stability and no pitting was found to occur. Therefore, the real pitting resistance of HNS should be higher than the expected before. And the stable transpassivation film played an important role in its untypical pitting corrosion resistance.