Carbon erosion

One may observe that in plasma deposition of a-C(N) H films, as the nitrogen precursor gas partial pressure is increased, the bombardment of the film surface by N2 ions will certainly occur. As this situation was shown to generate carbon erosion, and evolution of CN molecules, there is no reason to believe that CN fragments from the plasma would survive the bombardment, resting attached to the film. 

In ion beam experiments, carbon materials show an unexpected additional erosion mechanism which dominates the carbon erosion in the temperature range 1200 K 2200 K, called radiation enhanced sublimation (RES) [35,36]. There are doubts whether this effect also exists with very high particle flux densities as they are typical for tokamaks. Test limiter experiments in TEXTOR have demonstrated that RES is not important under such conditions. However, other devices reported a carbon influx by RES and recent measurements indicate also an enhanced erosion of metals at high temperatures under low energy particle impact [34]. Further R D is needed to clarify these issues. 

For all three regimes of carbon erosion outlined above [44] the eroded species were investigated intensively. At room temperature and energies in the keV range, physical sputtering occurs with carbon atoms being eroded, predominantly. At elevated temperatures, chemical erosion increases the erosion yield... 

Fig. 11.12. Energy dependence of the erosion yield Y(Ar+) of physical sputtering of a C H film by Ar+ ions (open symbols) and the yield Y(Ar+ H) for chemical sputtering by a simultaneous flux of Ar+ ions and H atoms (full symbols). The dash-dotted and solid lines are carbon erosion yields from TRIM.SP calculations for the sputtering of carbon by argon ions using a carbon-surface-binding energy of Esb = 0.1 eV and of EBb = 4.5 eV, respectively. The dotted line gives the absolute erosion rate by the applied flux of H atoms only...

Co-deposition of tritium with carbon is potentially the major T repository for ITER even if the use of carbon is minimized to the divertor strike plates. Retention by other mechanisms is expected to be low and to contribute only marginally to the in-vessel tritium-uptake (Sect. 12.4). For this reason the focus here is on some recent experimental findings associated with ( ) carbon erosion and deposition patterns in existing tokamaks, ( ) hydrocarbon film formation in areas of the divertor hidden from the plasma, and (in) mixed-material effects. 

As predicted by the classical equation, intervals when organic carbon burial exceeds sedimentary organic carbon erosion (i.e. when/, > (pk) would have higher 5 C values preserved in carbonate minerals. The prediction goes further, however. Assuming/, and (Pk are equal, intervals where Ab > Aw would also have elevated 5 C values preserved in carbonate minerals according to the expression ... 

A typical large three-phase ferroalloy furnace using prebaked carbon electrodes is shown in Eigure 4. The hearth and lower walls where molten materials come in contact with refractories are usually composed of carbon blocks backed by safety courses of brick. In the upper section, where the refractories are not exposed to the higher temperatures, superduty or regular firebrick may be used. The walls of the shell also may be water-cooled for extended life. Usually, the furnace shell is elevated and supported on beams or on concrete piers to allow ventilation of the bottom. When normal ventilation is insufficient, blowers are added to remove the heat more rapidly. The shell also may rest on a turntable so that it can be oscillated slightly more than 120° at a speed equivalent to 0.25—1 revolution per day in order to equalize refractory erosion or bottom buildup. 

Wear owing to corrosion and/or erosion can be particularly dangerous. For example, as carbon steel corrodes, the reduced wall thickness can eventually lead to a stmctural failure. This problem can be compounded through erosive wear of the silo wall. 

The main mechanisms of hearth bottom wear are high heat load, chemical attack, erosion from molten Hquids, mechanical and thermal stress, and penetration because of ferrostatic and process pressure. A variety of special purpose carbons have been developed to minimize or eliminate the damage caused by these wear mechanisms. 

Basic to establishing whether power recovery is even feasible, let alone economical, are considerations of the flowing-fluid capacity available, the differential pressure available for the power recovery, and corrosive or erosive properties of the fluid stream. A further important consideration in feasibihty and economics is the probable physical location, with respect to each other, of fluid source, power-production point, and final fluid destination. In general, the tendency has been to locate the power-recoveiy driver and its driven unit where dictated by the driven-unit requirement and pipe the power-recoveiy fluid to and away from the driver. While early installations were in noncorrosive, nonerosive services such as rich-hydrocarbon absorption oil, the trend has been to put units into mildly severe seiwices such as amine plants, hot-carbonate units, and hydrocracker letdown. 

Corrosion was caused by carbonic acid. A film of condensed moisture and dissolved carbon dioxide formed the acid. The erosion was caused by high-velocity movement of air across the tubes. Attack occurred intermittently. Deepest metal loss was 33% of the 0.040 in. (0.10 cm) wall thickness. 

Figure 11.10 Effect of pH of distilled water on erosion-corrosion of carbon steel at 122°F (50°C) (velocity, 39 ft/s, 12 m/s). (SOURCE M. G. Fontana and N. D. Greene, Corrosion Engineering, 2d ed., 1978, p. 75. Reprinted with permission from McGraw-Hill, Inc.)...

The kinetics of this reaction, which can also be regarded as an erosion reaction, shows die effects of adsorption of the reaction product in retarding the reaction rate. The path of this reaction involves the adsorption of an oxygen atom donated by a carbon dioxide molecule on die surface of the coke to leave a carbon monoxide molecule in the gas phase. 

Maddox shows how the major process concerns of corrosion, erosion, and column instability must be met in the design and operation of a hot carbonate process. These items will impact the capital and operating/main-tenance costs. 

For erosive wear. Rockwell or Brinell hardness is likely to show an inverse relation with carbon and low alloy steels. If they contain over about 0.55 percent carbon, they can be hardened to a high level. However, at the same or even at lower hardness, certain martensitic cast irons (HC 250 and Ni-Hard) can out perform carbon and low alloy steel considerably. For simplification, each of these alloys can be considered a mixture of hard carbide and hardened steel. The usual hardness tests tend to reflect chiefly the steel portion, indicating perhaps from 500 to 650 BHN. Even the Rockwell diamond cone indenter is too large to measure the hardness of the carbides a sharp diamond point with a light load must be used. The Vickers diamond pyramid indenter provides this, giving values around 1,100 for the iron carbide in Ni-Hard and 1,700 for the chromium carbide in HC 250. (These numbers have the same mathematical basis as the more common Brinell hardness numbers.) The microscopically revealed differences in carbide hardness accounts for the superior erosion resistance of these cast irons versus the hardened steels. 

For intermediate temperatures from 400-1000°C (Fig. 11), the volatilization of carbon atoms by energetic plasma ions becomes important. As seen in the upper curve of Fig. 11, helium does not have a chemical erosion component of its sputter yield. In currently operating machines the two major contributors to chemical erosion are the ions of hydrogen and oxygen. The typical chemical species which evolve from the surface, as measured by residual gas analysis [37] and optical emission [38], are hydrocarbons, carbon monoxide, and carbon dioxide. 

Chemical erosion can be suppressed by doping with substitutional elements such as boron. This is demonstrated in Fig. 14 [47] which shows data for undoped pyrolitic graphite and several grades of boron doped graphite. The mechanism responsible for this suppression may include the reduced chemical activity of the boronized material, as demonstrated by the increased oxidation resistance of B doped carbons [48] or the suppressed diffusion caused by the interstitial trapping at boron sites. 

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