The Determination of Phase Boundaries and Thermodynamic Functions in the Iron-Oxygen System by EMF Measurements

Abstract

Coulometric titration in a high‐temperature cell employing a stabilized zirconia electrolyte was used successfully in the determination of phase boundaries and related thermodynamic data in the iron‐oxygen system. The standard free energy of formation of oxygen‐deficient wustite was determined between 540° and 1200°C. Values agree to within 500 cal of the most critically reviewed data. The free energy of reaction for the formation of magnetite from wustite and oxygen was determined between 900° and 1200°C. The composition of the iron‐rich boundary of wustite was found to decrease very slightly in oxygen content with increasing temperature. For all practical purposes, however, the composition of the boundary can be considered constant at . The compositions of oxygen‐rich wustite agreed fairly well with existing data. Evidence was presented, however, for the existence of additional phases in the region of the Fe‐O diagram between the presently reported oxygen‐rich wustite and magnetite. The temperature for the Fe, , three‐phase equilibrium was found experimentally to be . Extrapolation of low temperature equilibrium data resulted in lower transformation temperatures.