In a recent paper, pitting corrosion around manganese sulfide (MnS) inclusions in stainless steel 303 (303) was investigated using in situ Atomic Force Microscopy (AFM), Scanning Kelvin Probe Force Microscopy (SKPFM) and Scanning Electron Microscopy (SEM).[1] In situ AFM experiments in 0.1M sodium chloride solution at voltages near the pitting potential combined with post immersion energy dispersive x-ray spectroscopy (EDS) found copper (Cu) deposition on MnS inclusions. SKPFM images before and after Cu deposition found that Cu ennobled the inclusion. That is, prior to immersion the SKPFM potential of MnS inclusions was more negative than the matrix, however, post immersion the potential was more positive. It was conclude that pit propagation near MnS inclusions occurs in five steps: 1) Preferential corrosion of the MnS inclusion, 2) Formation of a critical pitting solution at the dissolution site, 3) Oxidation of SS and the release of Cu(II), 4) MnS passivation and ennoblement by Cu deposition and 5) Accelerated oxidation of the SS owing to galvanic coupling with the ennobled MnS particle.

In the current study, the role of Cu deposition during localized corrosion at MnS inclusions in stainless steel 303 was investigated by means of potentiodynamic polarization and RRDE (ring disk electrode) experiments. Potentiodynamic polarization curves were conducted as a function of alloy Cu content. Although the pitting potential in these alloys did not increase with Cu content, it was found that below a threshold concentration, a step decrease in the pitting potential was observed. For specimens above the threshold Cu concentration, SEM/EDS analysis after the experiments found intact MnS inclusions passivated by a layer of Cu. In comparison, no intact MnS inclusions were found on specimens below the threshold Cu concentration at the end of the experiment. These observations reinforce our previous findings that Cu passivation of MnS inclusions plays an important role during localized corrosion. To study the electrochemical release of Cu from 303 we use RRDE experiments. In these experiments a 303 disk in the RRDE was held at a potential that was in the passive region of the polarization curve but below the Cu oxidation potential at this pH. The Pt ring in the RRDE was held at a much more negative potential. As such, any Cu(II) released from the 303 disk would be reduced onto the ring. After the collection period, the ring was stripped using cyclic voltammetry and those results are presented in Figure 1. As can be seen in this figure, a large oxidation peak is observed at 0.25V SCE indicative of Cu oxidation. From the passive dissolution current density and ring current density we have used Levitch equation to calculate the concentration of Cu(II) in solution. It was found that the concentration of Cu in solution due to passive dissolution of 303 was on the order of 0.02mM during the RRDE experiment. In the static experiment, as was the case in our polarization experiments, the concentration would be lower, potentially by orders of magnitude. Therefore, the concentration of Cu(II) in solution necessary to passivate MnS inclusions is exceedingly small.

1. R.S. Lillard, M.A. Kashfipour, W. Niu, "Pit Propagation at the Boundary between Manganese Sulfide Inclusions and Austenitic Stainless Steel 303 and the Role of Copper," Journal of the Electrochemical Society, 163 (8), C44051, 2016.

Figure 1 Stripping Voltammetry results for 303 specimen containing 0.57wt% Cu in 0.1M NaCl solution (after 5 hours of rotating ring disk collection procedure and deaeration).