Corrosion carburization

Although all high-temperature corrosion is considered oxidation, there are other terms that are also encountered, such as oxidation-reduction, sulfidation, fuel ash corrosion, carburization, and nitridation, to name a few. 

Green Rot-a corrosion product particular to nickel alloys and greenish in color that normally results from carburization and oxidation of certain nickel alloys at temperatures around 1000 C (1832°F). 

Wear and corrosion protection can be provided by the well-established techniques of hard-facing and plating or by surface-modification processes such as bonding, nitriding, carburizing, and ion implantation. The protection these processes afford is adequate in most environments but may fail over a period of time if the conditions are too severe. 

The Primary Reformer is a steam-hydrocarbon reforming tubular furnace that is typically externally fired at 25 to 35 bar and 780°C to 820°C on the process side. The reformer tubes function under an external heat flux of 75,000 W/m2 and are subject to carburization, oxidation, over-heating, stress-corrosion cracking (SCC), sulfidation and thermal cycling. Previously SS 304, SS 310 and SS 347 were used as tube materials. However these materials developed cracks that very frequently led to premature tube failures (see Table 5.10)88. 

High-temperature corrosion frequently shows general corrosion, as in oxidation, sulfidation, carburization, hot corrosion and hydrogen effects, etc. It should be noted that subsurface corrosion or internal corrosion at high-temperature corrosion is a highly localized corrosion phenomenon. 

In such reactions as the tarnishing of silver in air, the oxidation of aluminum in air, or attack of lead in sulfate-containing environments, thin, tightly adherent protective films are formed, and the metal surface remains smooth. It should be mentioned that underground corrosion is frequently observed as localized corrosion. Oxidation, sulfidation, carburization, hydrogen effects, and hot corrosion can be considered as types of general corrosion.20... 

The selection of materials must also consider oxidation/reduction processes that occur in the absence of an aqueous electrolyte. Examples include sulfidation, destructive oxidation of alloys in air or steam at high temperatures, carburization, nitriding, fuel ash corrosion, and high-temperature hydrogen attack. 

J. Klower, G. Sauthoff, D. Lelzig Alloy 10 Al - A New Sulphidation and Carburization Resistant Alloy for Fuel Combustion and Conversion. In Corrosion 96. NACE International, Houston/Texas (1996) 144/1-144/15. 

The material chosen for the IHX and die steam reformer of the Japanese HTTR is HASTELLOY XR. Corrosion tests and long-term creep tests in helium atmosphere in the temperature range 800 - 1000 C have been conducted reaching 50,000 hours. No significant degradation in creep properties was observed even after carburization. Creep rupture was found to be caused by nucleation, growth, and link-up of grain boundary cavities [26]. 

Vanadium also forms a very stable carbide VC, and carburization of this metal is part of the corrosion reactions of vanadium based alloys contacted with liquid lithium as well as sodium. Vanadium alloys with contents of titanium have an even higher affinity to form solid carbides by absorbing of carbon from liquid metals. In systems in which vanadium titanium alloys and stainless steels are in contact with the same lithium or sodium, carbon migrates from the steel to the refractory metal alloy, thus passing the alkali metal serving as a transport medium The free energies of formation of the alkali acetylides are compared with the values of several metal carbides in Table V. 

Under most practical carburization conditions for steels the reaction rate is under mixed control of surface reaction and diffusion. Whereas the carburization of simple steels, for instance, is straightforward, and hardness is achieved by the formation of high-carbon martensite on heat treatment, the presence of carbon in stainless steels and superalloys results in the formation of carbides based on chromium and other alloying elements. Excessive carburization can result in the removal from soluhon of protective elements such as chromium. This can seriously reduce the corrosion resistance of the component, particularly at grain boundaries. 

Hastelloy X (HastX) is a nickel-based superalloy used in a variety of applications [Brown, 1992]. Its composition is 49% nickel, 22% chromium, 18% iron, 9% molybdenum, 1.5% cobalt and 0.5% tungsten. The melting point of HastX is about 1530 K, and it has a density of 8.22 g/cc. This material has been suggested as a possible reactor material for a variety of reasons. HastX is a material with decent high temperature characteristics. Hastelloy is also noted for excellent corrosion, oxidation and carburization resistance at the desired temperatures. Finally, Hastelloy-X is a commonly used metal whose properties are well understood. The expected peak temperature of Hastelloy-X is roughly 875 K when used for the pressure vessel of the reactor. 

As a result of the desire for a pressure vessel material compatible with the Martian environment at the desired temperatures Hastx was chosen for the pressure vessel. HastX is known for being extremely insensitive to corrosion and carburization [Brown, W., 1992]. Theoretically, the relatively small neutronic penalty associated with Hastelloy means that any comparably transparent material (from a neutronic standpoint) could be substituted, which may be necessary if HastX and NblZr are incompatible. 

The reheater tube failed due to oil ash corrosion, resulting in thinning and creep. Reducing conditions caused carburization of stainless steel. The microstructure had transformed to carbides along the austenitic grains. Carburization and sensitization contributed to a reduction in corrosion resistance that resulted in significant wall loss. The OD deposits also point to the presence of low-melting compounds that caused oil ash corrosion. 

Therefore the helium atmosphere with the impurities must be adjusted in such a way that stable protecting oxide layers form on the surfaces (without Hj-production) in order to avoid an inner oxidation as well as a carburization or a decarburization, or other corrosion attacks. 

Vanadium in a fuel forms various metal compounds with low melting points, and causes molten-salt corrosion of steel called vanadium attack. Another example of high temperature corrosion is sulfidation. Carbon monoxide, carbon dioxide, and hydrocarbons form metal carbides at high temperatures and this is called carburization. Nitriding involves chemical reaction of nitrogen with metal. 

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