In-situ synchrotron studies of the effect of nitrate on iron artificial pits in chloride solutions II. On the effect of carbon

The effect of nitrate on the iron dissolution in artificial corrosion pits in chloride-containing solutions has been studied with in-situ synchrotron X-ray diffraction and ex-situ Raman spectroscopy. A black layer containing Fe₃C and carbon was found in the pit in pure 1 M HCl at 0.1 V(Ag/AgCl), but at higher voltages up to 1 V(Ag/AgCl), the black layer was not observed and Fe₃C was not detected. The dissolution of Fe₃C at low potentials may be inhibited by a carbon layer. In the presence of nitrate, the black layer containing Fe₃C was present at all the voltages tested, and current fluctuations were observed and found to be potential-dependent, with greater fluctuations at higher voltages. After dissolution in 1 M HCl, the metal surface appeared slightly roughened with some small cavities, but the surface became crystallographically etched after dissolution in 1 M HCl and 50 mM NaNO₃.