In-situ synchrotron studies of the effect of nitrate on iron artificial pits in chloride solutions: I. On the structures of salt layers

The effect of nitrate on the salt layers in iron artificial corrosion pits in acidic chloride solutions has been studied using in-situ synchrotron X-ray diffraction. During dissolution in 1 M HCl, there is a salt layer of FeCl₂.4H₂O on the electrode surface, which is isotropic. With addition of trace nitrate, the salt layer remains FeCl2.4H2O and no nitrate phase is observed, but the diffraction pattern becomes anisotropic, consistent with the formation of platelets with (1 2 0) planes settling horizontally. In nitrate solution containing trace of chloride (0.1 M HNO₃ + 10 mM HCl), a salt layer is formed that is isostructural with Co(NO₃)₂.6H₂O, and therefore assumed to be Fe(NO₃)₂.6H₂O. This is the first reported crystal structure of ferrous nitrate. The salt layer is also found to give an anisotropic diffraction pattern, consistent formation of platelets with (0 2 0) planes settling horizontally.