Thermodynamic analysis of equilibria in Fe-Cr-C alloys and evaluation of their dusting stability in aggressive carboniferous atmospheres

Thermodynamic analysis of phase transformations and reactions in Fe-Cr-C alloys is carried out with allowance made for all possible states, both stable and metastable, in the temperature range of metal dusting in aggressive carboniferous atmospheres. The results are compared with data on the corrosion resistance of chromium steel pipes under oil refining conditions. It is shown that, at high temperatures, one of the equilibrium phases in this system is the cementite solid solution (Fe1-yCry)3C. Below ~ 930 K, this phase becomes unstable to the decomposition into mixtures containing other carbides, but the tendency for chromium to stabilize cementite persists. The metal dusting mechanism changes in the composition range 0.03 < or = y *D# < 0.055 (2.8-5.1 wt % Cr). This conclusion is supported by the results of tests on chromium steel (I5Kh5M and 15Kh2MI) pipes under oil refining conditions, which indicate that the introduction of chromium into Fe alloys can only inhibit the degradation of Fe-Cr-C alloys under metal dusting conditions, without fully preventing it.