Oxidation Studies on the Iron‐Chromium‐Aluminum Heater Alloys

Abstract

The oxidation behavior of three iron‐chromium‐aluminum alloys was studied using sensitive weight gain techniques. In addition, x‐ray diffraction methods were used to determine the crystal structure and composition of the oxide films. The adhesion characteristics of the oxide film to the metal were tested by introducing strains into the metal oxide system during oxidation.Rate studies show a change in rate for the three alloys near 900 °C. Between 900° to 1050°C the rate of oxidation remained nearly constant; above 1050°C it increased again. X‐ray diffraction studies suggest that a crystal structure transformation was occurring in the oxide. At 1050 °C, was the main oxide in the scale.The parabolic rate law was used to interpret the rate data. Deviations occurred during the initial reaction period as a result of crystal structure transformations in the film. Heats of activation of 76 to 77 kcal/mole were calculated for the range 700°–900 °C.Strain oxidation studies showed the oxide scale on the high aluminum alloy to be less susceptible to damage than the lower aluminum content alloys. These studies were correlated directly with practical performance tests.Thermodynamic analyses of the several types of solid‐solid and solid‐gas reactions were used to interpret the rate and structure studies. Preliminary design principles were proposed for heat resistant alloys.