STABILITY OF CHROMIUM (III) SULFATE IN ATMOSPHERES CONTAINING OXYGEN AND SULFUR

The stability of chromium (III) sulfate in the temperature range from 880 to 1040 K was determined by employing a dynamic gas-solid equilibration technique. The solid chromium sulfate was equilibrated in a gas stream of controlled SO3 potential. Thermogravimetric and differential thermal analyses were used to follow the decomposition of chromium sulfate. Over the temperature range studied, the change in the Gibbs' free energy of formation of chromium sulfate Cr2O3(s) + 3SO3(g) → Cr2(SO4)3(s) can be expressed as ΔG0 = •143,078 + 129.6 T (±300) cal mole•1 ΔG0 = •598,350 + 542 T (±1250) J mole•1. X-ray diffraction analysis indicated that the decomposition product was crystalline Cr2O3 and that the mutual solubility between Cr2(SO4)3 and Cr2O3 was negligible. Over the temperature range investigated, the decomposition pressures were significantly high so that chromium sulfate is not expected to form on commercial alloys containing chromium when exposed to gaseous environments containing oxygen and sulfur (such as those encountered in coal gasification). © 1979 The Metallurgical of Society of AIME.