Oxidation behavior high temperature

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Figure 17.2 shows a a-8 curve for LiF that illustrates an abrupt elastic-plastic transition. Plastic deformation begins at the upper yield point and there is a decrease in stress. At the lower yield point deformation continues at lower stress levels. This type of behavior is similar to that of some low-carbon steels as well as aluminum oxide and magnesium oxide at high temperatures. 

The first CMCs to be developed consisted of three major components a ceramic matrix, fibers embedded in the matrix, and a tailored interface between the fiber and the matrix. Although these materials show damage tolerance and non-brittle behavior, the non-oxide materials that compose the CMCs are prone to oxidation, especially when matrix cracks are present. Lately, the development of an all-oxide CMC has captured the researchers attention. In these oxide/oxide composites, fracture toughness is achieved through crack deflection inside the matrix. A controlled level of matrix porosity will provide suitable conditions for crack deflection while inherently impeding oxidation during high temperature service. ... 

Furthermore, mass gain is another parameter that can be employed in characterizing oxide kinetic behavior. Using the definition of density and eq. (10.19) yields the mass gain (M) and the rate of mass gain (dM/dt) during surface oxidation at high temperatures... 

The behavior of dissociating oxides at high temperatures is briefly reviewed. At low pressures the composition of the oxide will correspond to an equilibrium oxygen pressure quite different from the ambient oxygen pressure. Evaporation rate studies on cuprous oxide in various atmospheres show that this oxide dissociates according to CuiO (solid) — 2Cu (gas) + iOi (gas). 

High temperature fatigue and fretting fatigue behavior has also been improved by implantation (113,114). This has been achieved by using species that inhibit oxidation or harden the surface. It is generally accepted that fretting behavior is closely coimected to oxidation resistance, perhaps due to third party effects of oxidation products. Oxidation resistance alone has also been improved by ion implantation (118—120). 

A good summary of the behavior of steels in high temperature steam is available (45). Calculated scale thickness for 10 years of exposure of ferritic steels in 593°C and 13.8 MPa (2000 psi) superheated steam is about 0.64 mm for 5 Cr—0.5 Mo steels, and 1 mm for 2.25 Cr—1 Mo steels. Steam pressure does not seem to have much influence. The steels form duplex layer scales of a uniform thickness. Scales on austenitic steels in the same test also form two layers but were irregular. Generally, the higher the alloy content, the thinner the oxide scale. Excessively thick oxide scale can exfoHate and be prone to under-the-scale concentration of corrodents and corrosion. ExfoHated scale can cause soHd particle erosion of the downstream equipment and clogging. Thick scale on boiler tubes impairs heat transfer and causes an increase in metal temperature. 

At high temperature, siUcon carbide exhibits either active or passive oxidation behavior depending on the ambient oxygen potential (65,66). When the partial pressure of oxygen is high, passive oxidation occurs and a protective layer of Si02 is formed on the surface. 

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