Corrosion and Mechanics of Materials
R&D 100 AWARD
The “Materials resistant to metal dusting degradation” project by Ken Natesan and Zuotao Zeng, made it into the list of the world’s 100 most technologically significant products introduced into the marketplace during 2005, as judged by R&D Magazine.
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Metal dusting is a catastrophic corrosion phenomenon that leads to the disintegration of structural metals and alloys into dust composed of fine particles of the metal/alloy and carbon. This phenomenon has been observed in the chemical and petrochemical industries, in reformer and direct-reduction plants, in processes that generate syngas, and in other processes where hydrocarbons or other high-carbon-activity atmospheres are present. Failures have been reported in ammonia plants as reduced energy requirements result in a lower steam/H2 ratio while CO/CO2 ratios have tended to increase. Even though metal dusting is widely prevalent, the general approach to minimize the problem in industry is to avoid the temperature/process conditions that are conducive for the attack, usually at a penalty in production, efficiency, and cost. Also, fixes such as sulfur poisoning of surface sites and preoxidation of alloy to stabilize chromia on high-Cr alloys are applied case-by-case, primarily based on experience with performance of materials in such environments. DOE’s Office of Industrial Technologies is supporting a three-year project in cooperation with several industrial partners to address the metal dusting problem from a fundamental scientific base using laboratory research in simulated process environments and subsequently field testing materials in actual process environments with participation from the U.S. chemical industries.
An extensive program has been in progress at Argonne to establish the mechanisms for metal dusting degradation in metallic materials exposed to carbon-bearing gaseous environments, to identify the key parameters that influence the onset of metal dusting and propagation of degradation, to establish the metal wastage under a variety of exposure conditions, to characterize the morphology of degradation by a wide variety of analytical techniques, and to assess the effect of alloy chemistry in the role and the extent of metal dusting. Several conclusions can be drawn from the study to date.
There are two major issues of importance in metal dusting. First is formation of carbon and subsequent deposition of carbon on metallic materials. Second is the initiation of metal dusting in the alloy and subsequent propagation of the degradation. The first is influenced by carbon activity (aC) in the gas mixture and the availability of the catalytic surface for carbon-producing reactions to proceed. There may be a threshold in aC (>>1) for carbon deposition. Metal dusting of the alloy in the reformer environments is determined by a competition between the oxide scale development and access of the virgin metal surface to the carbon deposit.
A new metal dusting mechanism was proposed in this study. Mechanisms for degradation of both Fe- and Ni-base alloys are related to the catalytic crystallization of carbon that deposits from the gaseous environment. The only difference is that iron carbide acts as a catalyst in Fe-base alloys, whereas nickel metal instead of nickel carbide acts as a catalyst in Ni-base alloys. To achieve good crystallinity, carbon dissolves, diffuses through, and precipitates at defects of iron carbide or nickel metal. The accumulation of carbon leads to separation of carbide or nickel grains into nano-size particles. The growth of carbon nano-filaments is the successive step in metal dusting. The free energy difference between poorly and well crystalline carbon is the driving force for both metal dusting and the growth of carbon nano-filaments. We believe that the proposed mechanism can better explain the experimental observations made on Fe- and Ni-base alloys subjected to metal dusting degradation.
The local nature of dusting (initiated by pits on the alloy surface) on structural alloys shows that defects in the oxide scales play a large role in initiation. Oxide scaling may not occur if aC is >>1 and/or if the H2O content in the environment is very low. Laboratory experiments have clearly indicated the effect of gas chemistry (in particular, H2O content) in the scaling, carbon deposition, and dusting initiation. It is evident that the environment in reformers is high enough in oxygen partial pressure that a Cr-rich alloy can develop a chromia scale (given enough exposure time) before carbon deposition. The presence of an oxide scale may not prevent metal dusting but can delay its initiation, thereby slowing the overall attack.
Raman spectra show the existence of spinel, Cr2O3, and disordered chromium oxide in the scale grown on high-chromium Fe-base alloys. All three phases act, to different degrees, as protective layers to prevent alloys from metal dusting corrosion. The spinel phase is not as stable as Cr2O3. It could be reduced by the deposited carbon, and metal dusting corrosion would initiate from the reduced defects.
The composition of the oxide scale is important in metal dusting corrosion. Cr2O3 is a better phase than spinel to resist metal dusting since spinel can be reduced. If alloys can generate more Cr2O3 and less spinel on the surface, their abilities to resist metal dusting will increase. Therefore, alloys with more Cr and less Fe content perform well in carburizing atmospheres. The composition of the oxide scale also changes with exposure time: spinel content increases and chromium oxide content decreases. For that reason, alloys are easily attacked by metal dusting after long time exposure since spinel content increases.
Metal dusting degradation involves two steps, namely, the incubation period and the propagation. The incubation period is determined by the carbon activity in the gas phase, alloy chemistry, system pressure, and probably the exposure temperature. For the same exposure conditions, the incubation period for the onset of metal dusting is significantly greater for the Ni-base alloys when compared with that for Fe-base alloys.
At present, the continuing work will focus on developing materials, coatings, and claddings that are resistant to metal dusting attack under high carbon environments at elevated temperatures and pressures.
Last Modified: Mon, February 15, 2010 5:13 PM