Observations on the Fe‐Cr‐O System

An isothermal phase diagram for the Fe‐Cr‐O system is presented for 1300°C which, with a minor modification, holds for 1000°C. It is observed that Fe‐Cr alloys containing over 13% Cr are in equilibrium with containing a little dissolved . Alloys of lower chromium content are in equilibrium with iron chromite of variable composition of the type . Evidence is presented indicating that when the spinel forms next to the metal a pronounced internal oxidation occurs. However, higher chromium alloys with next to the metal show no tendency for internal oxidation. An oxygen pressure‐composition diagram for the entire system is also presented. Marker experiments made during the oxidation of 20% Cr and 30% Cr alloys suggest that is formed almost entirely by cation (Cr⁺³) diffusion.