Ryan et al. reply
Frankel et al. find no depleted zone adjacent to MnS inclusions in stainless steels, which contradicts our results1 and raises important issues about pitting mechanisms. We have also used high-resolution electron microscopy and secondary-ion mass spectrometry mapping to study these systems, and our results are similar to those of Frankel et al., giving a consistently large scatter in the analysis. With SIMS mapping in particular, we find that the flux required to generate a significant signal means that too much material has to be removed; in addition, the sputter rate of the inclusion is higher than that of the matrix, and sulphur 'smears' are observed that blur the interfaces. We conclude that this method is not useful in these systems.
By using the SIMS probe1, we were able to analyse 25 inclusions within a reasonable time — in contrast to transmission electron microscopy, which involves complex sample preparation. Of these inclusions, some showed a large scatter in the data, and sputtering of the inclusion complicated interpretation of the results in a few other cases. Some 20% of the inclusions showed no depletion, consistent with the observations of Frankel et al., but over 20% showed the type of depletion we reported earlier1. As with all pitting studies, there is a strong statistical consideration to be taken into account — although the initiation of pitting corrosion is confined to sulphide inclusions, not all inclusions nucleate pits. We are working to develop a method that correlates inclusions that have chromium depletion with subsequent pit nucleation.
Frankel et al. show an inclusion connected to an oxide particle. We did not analyse sites that appeared to be multiple or connected inclusions to avoid complications arising from such oxide particles2. The inclusion was always sampled to ensure that it was a sulphide particle.
As we pointed out1, the final attack on the steel that leads to an observable pit is triggered through dissolution of the inclusion. The mechanism by which this unusual dissolution occurs is the key step in the chain of events. There is further evidence indicating that the interface between inclusion and matrix is critical3,4, and that there may be several precursor stages in which the electrochemical current is small5,6, and we have directly investigated these aspects of corrosion.
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Ryan, M., Williams, D., Chater, R. et al. Stainless-steel corrosion and MnS inclusions. Nature 424, 390 (2003). https://doi.org/10.1038/424390a
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DOI: https://doi.org/10.1038/424390a
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