corrosion effects/behavior of Carbon-carbon composites

Corrosion effects of carbon-carbon: One of the key concerns with carbon or graphite is its reactivity with oxidizing species, especially at high-temperatures. High-temperature gas-cooled reactor literature suggests that the typical gas composition in a helium gas-cooled reactor will consist of impurities H2, H2O, CO, CH4, N2, and O2. The main oxidizing species are O2 and H2O, although the amount of O2 present in the gas coolant may be negligible. Even if great care was taken to prevent oxygen potential in the gas, H2O is difficult to remove from the system completely. Therefore, it is likely that the corrosion reaction of carbon-carbon in gas-cooled reactor will take the form:

C(s) + H2O(g) →CO(g) + H2(g)

The primary consequences of the suggested reaction are that solid carbon is gasified and the reaction products can alter coolant chemistry. As the solid carbon is lost, the interfacial bond between the fiber and matrix becomes debonded and a significant reduction in mechanical and thermal properties of the composite becomes unavoidable.

Considering coolant chemistry, HTGR studies 17,18,19,20,21 have concluded that corrosion of high-temperature superalloys is related to the presence of low partial pressures of H2O, CO and CH4 I the He gas. Depending on the gas composition and temperature, superalloys can carburize, decarburize, or oxidize. This will depend primarily on the CO and H2O concentrations, both of which are affected by the carbon-water gas equilibrium reaction. Variations in CO concentration will have a major effect on gas/metal interactions and can represent a critical partial pressure at which oxides and carbides coexist in the superalloy. Below this critical CO level, excess H2O concentration may cause the formation of surface oxides and total decarburization; whereas, low H2O concentration may cause surface carbides and continuous carburization.

Refractory metal alloys retain useful strength and properties at high-temperatures, although there are significant concerns regarding interstitial embrittlement in the presence of C,O,N and H. One of the most significant differences between te refractory metals of group Vb and group VIb is the solubilities of interstitial elements. Molybdenum and tungsten have considerably lower interstitial solubility than the group Vb refractor metals. At higher impurity levels, refractory metals can be expected to form carbides, oxides, and nitrides. Therefore, the change in coolant chemistry due to carbon gasification or the release of free carbon “dust” is of great concern.

Corrosion behavior of carbon-carbon: Carbon-carbon corrosion is governed by structural defects or stress accumulation within the matrix as a result of carbonization shrinkage. The selection of matrix precursor, fiber and processing conditions will therefore have an important effect. The reaction will commence at edge sites or porosity and proceed to regions of the laminar matrix, anisotropic matrix, isotropic matrix, fiber lateral surface, fiber ends, and finally, fiber cores. Graphitic carbon, with its denser, crystalline structure and lower proportion of reactive edge sites, does not begin to corrode until slightly higher temperatures. Gasification rate is increased by increasing operating temperature and reduced by increasing heat treatment temperature. The latter is believed to occur as a result of reducing reactive edge sites, retained impurities, and residual carbonization stresses.

The gasification of carbon-carbon typically begins at 350C and the corrosion rate is found to increase exponentially with temperature. Depending on the temperature regime, gasification attack can be uniform throughout the material or limited to the geometrical surface. At low temperatures, the rate-controlling steps of the reaction are chemical in nature and the reactions are so slow that corrosive gas can penetrate the carbon in depth. Corrosion occurs primarily at the high-energy reactive sites causing rather uniform attack and thus reducing strength without changing the component geometry.

At high-temperatures, the rate-controlling step changes to diffusion controlled as gas diffuses through the boundary layer close to the solid carbon. In this regime, chemical reactivity is so high that all corrosive gas penetrating the boundary layer reacts immediately with the hot carbon surface. Therefore, the rate of gasification is controlled by the transport of gas to and from the reaction front. This type of corrosion will cause geometrical changes of the solid carbon.

At temperatures between to regimes, the corrosion rate is controlled by in-pore diffusion, and the composite pore structure becomes a rate determining factor. The critica temperatures separating the regimes are not well defined or exactly determined, and the reactions are very difficult to model with any degree of confidence. The reaction rate is difficult to express due to changes in the reaction surface over time and the generation of open pores and microcracks during oxidation. Therefore, the majority of corrosion information tends to be empirical and based on experimental observations.

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