Understanding Oxidation of Sigraflex

Much confusion has been generated by recent reports of graphite materials oxidizing at elevated temperatures, and potentially "disappearing" from the gasket joint after sufficient exposure time. Graphite, being a form of carbon, can eventually oxidize if subjected to a high temperature in an oxidizing medium, for a long enough period of time; however, several factors affect the rate of oxidation, particularly in a clamped joint. A through understanding of the effects of the various parameters of possible oxidation is necessary in interpreting these results.                                         

In general, Sigraflex flexible graphite products are rated at the following temperatures:
Air 500C or 932F
Steam  650C or 1200F
Inert or reducing atmospheres, or vacuum 3000C or 5432F
Any oxidation, determined by weight loss, is dependent on the exposed surface area of the sample as compared to the overall weight of material. In other words, only the edges of the gasket are subject to oxidation and for a given size of gasket, thicker samples would exhibit higher oxidation because of the increased edge area. Likewise, of two gaskets with the same inside diameter, one with a larger outside diameter would exhibit more oxidation. Therefore, listing oxidation results as a percentage of initial weight is deceiving without knowing the dimensions and conditions. The gasket surfaces in contact with the flange surfaces will not tend to oxidize, and oxidation within the gasket body will only be accessible via permeation of the oxygen into the pores of the material, hence the onset of oxidation will not lead to a reduction in gasket thickness. The significance of this is that oxidation tests and results are highly dependent on the sample configuration, and the method of sample mounting (i.e. clamped or free state).
bulletThe fluid contained may be an oxidizing environment. In this case, the gasket is most susceptible to oxidation.
bulletThe fluid may be in an inert or reducing environment, but the external edge of the joint may be exposed to air, leaving this portion in an oxidation environment. Due to permeability present in all materials, the contained pressure will tend to fill the pores of the material with the internal fluid, hence reducing/minimizing any oxidizing tendencies.
bulletThe joint may be wrapped or insulated, creating an environment for the gasket different from the external environment.
bulletThe graphite may be used in a packing, in which case the oxidation potential is greatly reduced by the containment of the seal, the final density of the packing, the relative large mass in relationship to the exposed area, and the barrier of any leakage of fluid past or through the packing.

 

Oxidation is highly dependent on "temperature", and "time at temperature". Temperature is generally dependent on the fluid media inside the joint. Heat transfer to the gasket will be dependent on the fluid flow in the pipe or vessel, as well as the proximity of the gasket inside edge to the inner bore of the flange. Heat transfer of the flange and gasket material will create a rapid decrease in temperature as you approach the outer diameter of the joint. One of the features of flexible graphite is its ability to conduct heat, particularly in this parallel direction. Again, wrapped joints will affect this. The overall result is that the fluid could be at or above the oxidation temperature of graphite, while the remaining gasket is not.

 

Joint tightness will also have some affect an oxidation rate, as the the density of the graphite will increase and the permeability will decrease with increasing clamping force. Another factor is the purity of graphite or, more specifically, the ash content. Certain elements within the ash compound can actually increase the rate of oxidation. Higher purity graphite products will oxidize less.
If Sigraflex is selected for use in a known oxidizing atmosphere, the material of choice may be an inhibited grade, Sigraflex BP or SP. The phosphate inhibitor is added to the material to reduce oxidation. These phosphates convert vulnerable areas on the carbon molecule to a glass like structure at the elevated temperatures, creating a physical barrier covering the " active sights" on the graphite crystal lattice thereby protecting the molecules from attach. As a result the effect of oxygen is eliminated or minimized. Inhibitors will raise the threshold of oxidation by about 100 F, and can reduce oxidation time by a factor of ten or more.

For the ultimate in oxidation resistant graphite performance SGL offers its next generation APX2.

This information is based on SGL's current knowledge and is intended to provide general notes on our products and their uses. It should not therefore be construed as guaranteeing specific properties of the products described or their suitability for a particular application. Any existing industrial property rights must be observed. The quality of our products is guaranteed under SGL General Conditions of sale.

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