Crystal structure of an unusual corrosion deposit, bis(ethylammonium) tetrachloroiron(II)

Abstract

An Fe-containing deposit isolated from a commercial plant was identified by indexing its powder diffraction pattern, using the NIST Crystal Data Identification File to locate an isostructural Mn compound, and carrying out a successful Rietveld refinement. The principal phase in the deposit was confirmed to be bis(ethylammonium) tetrachloroiron(II). This compound crystallizes in the orthorhombic space group Pbca, with a=7.3291(6), b=7.2603(6), c=21.7544(8) Å, V=1157.59(14) Å, and Z=4. The structure consists of single perovskitelike layers of corner-sharing FeCl6 octahedra parallel to the ab plane, separated by bilayers of ethylammonium cations. The octahedra are tilted 4.4° with respect to the c-axis, and the layers are ruffled. The compound was apparently formed by reaction of the ethylamine corrosion inhibitor with Fe from the steel process vessel and chloride in the process stream.