Abstract
Nitrogen (N) alloying was found to inhibit active dissolution and to introduce a secondary loop with fluctuating currents in the anodic polarization curve of type 316LN stainless steel (SS) (UNS S31653) in 1 N hydrochloric acid (HCl) solution. Potentiostatic tests in this potential range confirmed the occurrence of current transients as a result of metastable pits, resulting in secondary loop formation. Higher minimum chloride (Cl−) concentration and low acidic pH were shown to be required for stable pit formation in type 316LN SS compared to similar alloys without N alloying. Results showed no selective anodic dissolution of any of the alloying elements in actively growing pits in type 316LN SS. Although ammonium ions (NH4+) were found within pits under suitable applied potentials in 1 N HCl and under natural corrosion in ferric chloride (FeCl3) solutions, the more anodic the potential, the less was its yield. The formation of NH4+ ions was found to be greater at more active potentials under uniformly dissolving conditions, and an apparent Tafel slope for the reduction reaction N + 4H+ + 3e → NH4+ for the dissolution of N from the steel was estimated to be 0.125 V. Significant enrichments of N, chromium (Cr), and nickel (Ni) to a marginal extent were observed by Auger electron spectroscopy (AES) of the actively growing pitted surface. The iron (Fe) component, however, showed considerable depletion. The unattacked surface region surrounding the pit maintained normal passive film characteristics. Based on observations, a mechanism elaborating the beneficial effect of alloyed N on pitting corrosion resistance of SS was developed.
