Abstract
Scaling rates and scale compositions of Co‐Cr alloys were determined in the temperature range 900°–1200°C. At any given temperature the scaling rate increased with low chromium additions, then dropped precipitously with further additions reaching a minimum at about 25% chromium. Thereafter the scaling rate again increased approaching the scaling rate of Cr as the upper limit. Above a critical concentration of about 25% chromium, the scale consisted exclusively of 
. Below this critical concentration, complex scales consisting of the oxides of both cobalt and chromium were formed. The best scaling resistance was associated with a scale consisting predominantly of 
, not spinel. Schematic isothermal sections of the deduced Co‐Cr‐O phase diagram were applied as an aid in interpreting the scaling behavior.