High Nitrogen Steels. In-situ Observation of Dynamic Reacting Species at Pit Precursors of Nitrogen-bearing Austenitic Stainless Steels.

Abstract

Anodic steady state polarization and potentiodynamic polarization studies were carried out for nitrogen-bearing austenitic stainless steels in 0.6 kmol·m–3 NaCl solution at room temperature. The pitting potential measurement indicated that the pitting resistance increased with increase in nitrogen addition. In-situ distribution and imaging of H+ and Cl– at the pit precursors were identified using scanning electrochemical microscopy (SECM) in the chronoamperometric mode, and the observation of dynamic reacting species at the pit initiation stage was made using laser Raman spectroscopy. SECM study gave useful information about the distribution and imaging of H+ and Cl– at the pit precursors. Using in-situ Raman spectroscopy, nitrate ions at the pit precursor region were observed for the first time. There was no evidence for the presence of ammonia/ammonium ion at the pit precusor stage. The formation of nitrate ion was attributed to the combination of nitrogen and oxygen ions in the passive film. It is concluded that nitrate ions were incorporated into the passive film along with hydrated chloride ions, and help in the self-repair of the passive film during pit initiation.