The Effect of Water Vapor on Selective Oxidation of Fe–Cr Alloys

Abstract

Binary Fe-Cr alloys containing 10 and 20 mass% Cr were studied with respect to isothermal oxidation behavior at 900 and 1,050°C in Ar-20%O 2 , Ar-7%H 2 O and Ar-4%H 2 -7%H 2 O. 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.

Effect of Element Diffusion Through Metallic Networks During Oxidation of Type 321 Stainless Steel

“…Internal oxidation has been studied for several decades (Ref 8,(26)(27)(28)(29)(30)(31)(32)(33)(34)(35)(36)(37)(38)(39)(40). The mechanism is extensively discussed in the literature.…”

“…However, the concept has been widely used for analyzing oxidation in concentrated alloys in many Fe-and Ni-based alloy systems with high concentrations of active elements ( Ref 7,39,[41][42][43][44][45]. Although internal oxidation is always possible during high-temperature oxidation, the question is whether internal oxidation is the primary cause for breakaway oxidation.…”

Influence of Gas Composition and Exposure Cycle on the Formation of Surface and Subsurface Oxides in Iron-Aluminum-Based Alloys at High Temperatures

“…Whereas the formation of pure iron oxides is thermodynamically expected in dry nitrogen/oxygen as well as in reheat furnace gas, [14] the huge differences in the oxidation behavior suggests that either the presence of water vapor, which is known to cause massive corrosion, [17,18] or carbon dioxide [16,19] has a significant impact on corrosion kinetics. [3,20] Furthermore, oxygen transport from the atmosphere into the sample has a highly transient character.…”

“…[24] It is well known that CrO 2 (OH) 2 evaporation will affect the thickness of the formed surface oxide layers and the corrosion behavior of the alloy. [17,20] Similarly, it is suggested that the formation of volatile aluminum (oxy-)hydroxides could at least play a minor role in pore formation, albeit to a much lesser extent. The proposed mechanisms from literature, leading to volatilization of aluminum oxide [23] , are shown in Eqs.…”

Chromia Evaporation in Advanced Ultra-Supercritical Steam Boilers and Turbines

“…lead to scales with less Cr 2 O 3 formation and thus increased tendencies to breakaway oxidation (Essuman et al, 2008).  Rapid, or "breakaway" oxidation from the depletion of Cr in the scale resulting from reactive evaporation of Cr 2 O3 in the presence of O 2 and H 2 O (Essuman et al, 2008;Holcomb, 2008Holcomb, , 2009Young & Pint, 2006).…”