ELECTROCHEMICAL INVESTIGATIONS OF PITTING CORROSION IN NITROGEN-BEARING TYPE-316LN STAINLESS-STEEL

Nitrogen (N) alloying was found to inhibit active dissolution and to i ntroduce a secondary loop with fluctuating currents in the anodic pola rization curve of type 316LN stainless steel (SS) (UNS S31653) in 1 N hydrochloric acid (HCl) solution. Potentiostatic tests in this potenti al range confirmed the occurrence of current transients as a result of metastable pits, resulting in secondary loop formation. Higher minimu m chloride (Cl-) concentration and low acidic pH were shown to be requ ired for stable pit formation in type 316LN SS compared to similar all oys without N alloying. Results showed no selective anodic dissolution of any of the alloying elements in actively growing pits in type 316L N SS. Although ammonium ions (NH4+) were found within pits under suita ble applied potentials in 1 N HCI and under natural corrosion in ferri c chloride (FeCl3) solutions, the more anodic the potential, the less was its yield. The formation of NH4+ ions was found to be greater at m ore active potentials under uniformly dissolving conditions, and an ap parent Tafel slope for the reduction reaction N + 4H+ + 3e --> NH+ for the dissolution of N from the steel was estimated to be 0.125 V. Sign ificant enrichments of N, chromium (Cr), and nickel (Ni) to a marginal extent were observed by Auger electron spectroscopy (AES) of the acti vely 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 observati ons, a mechanism elaborating the beneficial effect of alloyed N on pit ting corrosion resistance of SS was developed.