The Electrochemistry of Copper Release from Stainless Steels and Its Role in Localized Corrosion

In this investigation, the role of copper in MnS dissolution was studied in a series of lab-made austenitic stainless steels (SS) with varying Cu content. The base composition of these samples was that of SS 303 and the Cu content was varied between 0.02 and 0.80 wt%. In potentiodynamic polarization experiments, it was found that Cu deposition passivated the MnS inclusions in all except the 0.02 wt% specimen. The critical potential for this passivation, from potentiostatic experiments, was found to be associated with the onset of metastable pitting. The "apparent" pitting potential in the specimen with 0.02 wt% Cu content was approximately 200 mV more negative than the other specimens. This apparent potential was attributed to MnS dissolution, due to a lack of Cu deposition/passivation, and not pitting. With respect to pit repassivation, at concentrations equal to and greater than 0.2 wt% Cu, repassivation potentials were on the order or +0.10 V SCE. In comparison, for the 0.02 wt% Cu specimen, the repassivation potential was less than the OCP (−0.125 V vs. Ag/AgCl) indicating Cu reduction inside the pit plays a role in the measured repassivation potential. Cu release was quantified using a rotating ring disk electrode. In these experiments, oxidation peaks for Cu(I) and Cu(II) were detected. In these experiments Cu was released at low potentials and low, passive, current densities but the resulting near surface concentrations of Cu(I) (0.02 mM) were insufficient to passivate MnS.