The effect of water vapor on selective oxidation of Fe-Cr alloys

Binary Fe-Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050 degrees C in Ar-20%O-2, Ar-7%H2O and Ar-4%H-2-7%H2O. Thermogravimetric analyses in combination with analytical studies using SEM/EDX and Raman Spectroscopy revealed, that in atmospheres in which water vapor is the source of oxygen, Cr exhibits a higher tendency to become internally oxidized than in the Ar-O-2 gas. Contrary to previous studies which showed the presence of water vapor to affect transport processes in the scale, the present results thus reveal that the presence of water vapor also affects the transport processes in the alloy. This mechanism is an "easy" explanation of the frequently observed effect that Fe-Cr alloys with intermediate Cr contents (e.g. 10-20%, depending on temperature) exhibit protective chromia-rich scale formation in dry gases but breakaway type Fe-rich oxides in wet gases, provided the oxygen partial pressure is sufficiently high for Fe to become oxidized.