Carburization of Fe-Cr Alloys During Oxidation in Dry Carbon Dioxide

Abstract

Iron‐chromium alloys (1–15 w/o [weight per cent] Cr) oxidized in dry carbon dioxide at 700°, 900°, and 1100°form duplex scales which are similar in structure to those generated in H2O‐normalAr atmospheres, i.e., a detached but continuous outer layer of iron oxides (predominantly normalFeO ) and a porous inner layer of normalFeO and Fe‐Cr spinel particles. Carburization accompanies this oxidation at all temperatures, and the resulting microstructural changes, the carbides formed, the hardness increases and the carbon content of homogeneous specimens are related to the equilibria of the Fe‐Cr‐C system. The carburization and the continued high oxidation rates, in spite of the detached duplex structure of the scale, require that a gaseous transfer of oxygen and carbon,occur within the voids separating the outer and inner oxide layers. This requirement suggests a carbon permeation of the dense outer scale and the generation of a CO2‒CO atmosphere in these voids which is carburizing as well as oxidizing to the alloy.

Carburisation of ferritic Fe–Cr alloys by low carbon activity gases

“…This structure, illustrated in Fig. 2, is typical of dilute Fe-Cr alloys and has been observed by others after exposure to CO 2 [5,13] and CO 2 -H 2 O [1,14] in the temperature range 550-900°C.…”

“…Reaction of pure CO 2 with an 18Cr (304) steel at 704, 816 and 927°C [3] and with an Fe-15Cr alloy at 900°C [4] also led to internal carbide precipitation. Fujii and Meussner [5] reported simultaneous oxidation and carburisation of Fe-Cr alloys containing 1-15 wt.% Cr in pure CO 2 at temperatures of 700, 900 and 1100°C. All alloys remained ferritic at 700°C and produced (Fe,Cr) carbides, the nature of which varied with alloy chromium content from (Fe,Cr) 3 C to (Fe,Cr) 7 C 3 and (Fe,Cr) 23 C 6 , in accordance with the Fe-Cr-C phase diagram.…”

“…Fujii and Meussner [5] proposed that carbon was generated on the oxide surface, transported through a dense outer oxide layer and produced a CO-CO 2 atmosphere within voids in a porous inner oxide layer. However, subsequent experiments [6] showed that the solubility of carbon in iron oxides is extremely small, and its solid-state diffusion through iron oxide scales is not feasible.…”

“…However, subsequent experiments [6] showed that the solubility of carbon in iron oxides is extremely small, and its solid-state diffusion through iron oxide scales is not feasible. Transport could occur instead by means of molecular CO 2 , but the scale microstructures are reported [5] to appear impermeable to gas. Furthermore, these scales grow at rates controlled by solid-state diffusion, and continuous pathways for gas phase transport through the scale therefore cannot exist during steady-state oxidation.…”

Sulfur Effect on Corrosion Behavior of Fe-20Cr-(Mn, Si) and Fe-20Ni-20Cr-(Mn, Si) in CO2-H2O at 650°C

“…With the goal of limiting CO 2 emissions and mitigating global warming, advanced combustion technologies have been developed for CO 2 collection and sequestration, one of which is the oxyfuel process. In this process, coal is burnt in pure oxygen, so the flue gas is mainly CO 2 and H 2 O, making CO 2 capture feasible. Unfortunately, the corrosive nature of this gas, together with the increased operating temperatures adopted for higher efficiencies, provides severe operating conditions for materials of power plant construction, such as steel tubing for a boiler.…”