EFFECT OF WATER VAPOR ON THE OXIDATION BEHAVIOR OF MODIFIED LOW ALLOY STEELS AT HIGH TEMPERATURES

Oxidation behavior of P92 and E911 alloys was investigated at 600 degrees C to 800 degrees C in air, oxygen, and O-2 + 70% H2O water vapor environments. Oxidation resistance of these alloys has been explained by the formation of a protective oxide FeCr2O4 at 600 to 700 degrees C. At 800 degrees C, the kinetics are parabolic with fast growing oxide leading to spalling of oxide for P92 alloy in air. While the scale formed in air and oxygen was protective with a chromium rich scale at 600-700 degrees C, the scale formed at 800 degrees C was a double layer with iron oxide an outer layer and inner Cr-rich spinel FeCr2O4. In water vapor, the scale was quite different. It was found that the presence of water vapor enhances the diffusion of selective elements such as Cr, which in turn depletes the substrate matrix from Cr, resulting in faster diffusion of Fe, which forms inferior iron oxides and, hence, enhances oxidation rate, leading to spalling. The higher oxidation rate was found in water vapor compared to air and oxygen. The higher rate of oxidation in water vapor has been explained with existing theories. The kinetics results were correlated with the results of the post oxidation analyses using various techniques such as scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDAX), and x-ray diffraction (XRD).