Spectroscopic study of anion–cation interactions in molten sulphates

Abstract

The anion–cation interactions in those molten sulphates thought representative of and participating in the high-temperature corrosion of stainless steels, and the nature of dissolved corrosion products, sulphato complexes of NiII, CrIII and FeIII, have been investigated using electronic spectroscopy of melts and vibrational spectroscopy of quenched melts, and have been compared with recent X-ray diffraction and infrared data of sulphate melts. The addition of Al3+ ions to (Na,K) SO4 lowers its melting point to within the range important for corrosion by sulphate melts, and C3v and C2v(bridging and chelating) sulphato complexes are then observed: addition of Li+ only produces C3v sulphato complexes. The ligand-field spectra of CrIII and FeIII are reported for the first time in sulphate melts and the species [Cr(SO4)3]3–(Oh) and [Fe(SO4)4]5–(Td) identified. The spectrum of nicke(II) in sulphate melts is reinterpreted as comprising two species, [Ni(SO4)3]4–(Oh) and [Ni(SO4)4]6–(Td). The presence of these complexes, and chloride ions, reduce the strong Al–SO4 interactions, and the build-up of corrosion products may reduce the aggressive nature of this corrosive melt. The much weaker Li–SO4 interactions found suggest that previous corrosion studies using (Li,Na,K) SO4 eutectic should be reassessed, and future studies should employ aluminium-containing melts.