Chromium vaporization from high-temperature alloys I. Chromia-forming steels and the influence of outer oxide layers

The vaporization of chromium species from chromia scales limits the applicability of chromia-forming steels at high temperatures and is one of the major reasons for degradation in the development of planar solid oxide fuel cells (SOFCs). Cr(VI) vaporized from the interconnector is reduced at the cathode and deposits in the form of solid Cr(III)-oxide, thereby inhibiting the electrochemical processes. This work presents the first systematic study on the Cr vaporization of Cr-, Fe-, Ni-, and Co-based alloys in air and in H-2 atmospheres at high temperatures. The influence of outer oxide layers of (Cr,Mn)(3)O-4, (Fe,Cr)(3)O-4, Co3O4, TiO2, and Al2O3 on the Cr vaporization is investigated. It is shown that the Cr vaporization of chromia-forming steels can be reduced by more than 90% by alloying. An estimate of the expected degradation effects on planar SOFC designs for the use of uncoated interconnector materials is used to show that in order to achieve the desired lifetimes for SOFC systems, additional Cr-retention coatings are necessary. Additionally, equilibrium vaporization measurements are carried out for pure Cr2O3(s) in humid air in order to elucidate controversies in the literature concerning the thermodynamic data of CrO2(OH)(2)(g).