Oxide scales mechanical properties

Once the oxidation of the metal reaches the stage where ionic diffusion is rate controlling, a parabolic rate law is found to hold for a period whose duration depends upon factors such as specimen geometry and scale mechanical properties. 

Saunders S R J, Evans FI E and Stringer J A (eds) Workshop on Mechanical Properties of Protective Oxide Scales. Materials at High Temperatures vo 12 (Teddington)... 

Corrosion Resistance Possibly of greater importance than physical and mechanical properties is the ability of an alloy s chemical composition to resist the corrosive action of various hot environments. The forms of high-temperature corrosion which have received the greatest attention are oxidation and scaling. 

Actually, in many cases strength and mechanical properties become of secondaiy importance in process applications, compared with resistance to the corrosive surroundings. All common heat-resistant alloys form oxides when exposed to hot oxidizing environments. Whether the alloy is resistant depends upon whether the oxide is stable and forms a protective film. Thus, mild steel is seldom used above 480°C (900°F) because of excessive scaling rates. Higher temperatures require chromium (see Fig. 28-25). Thus, type 502 steel, with 4 to 6 percent Cr, is acceptable to 620°C (I,I50°F). A 9 to 12 percent Cr steel will handle 730°C (I,350°F) 14 to 18 percent Cr extends the limit to 800°C (I,500°F) and 27 percent Cr to I,I00°C (2,000°F). 

Sulfur Corrosion Chromium is the most important material in imparting resistance to sulfidation (formation of smfidic scales similar to oxide scales). The austenitic alloys are generally used because of their superior mechanical properties and fabrication qualities, despite the fact that nickel in the alloy tends to lessen resistance to sulfidation somewhat. 

In Chapter 1 we emphasized that the properties of a heterogeneous catalyst surface are determined by its composition and structure on the atomic scale. Hence, from a fundamental point of view, the ultimate goal of catalyst characterization should be to examine the surface atom by atom under the reaction conditions under which the catalyst operates, i.e. in situ. However, a catalyst often consists of small particles of metal, oxide, or sulfide on a support material. Chemical promoters may have been added to the catalyst to optimize its activity and/or selectivity, and structural promoters may have been incorporated to improve the mechanical properties and stabilize the particles against sintering. As a result, a heterogeneous catalyst can be quite complex. Moreover, the state of the catalytic surface generally depends on the conditions under which it is used. 

Metal-metal eutectics have been studied for many years due to their excellent mechanical properties. Recently, oxide-oxide eutectics were identified as materials with potential use in photonic crystals. For example, rodlike micrometer-scaled microstructures of terbium-scandium-aluminum garnet terbium-scandium per-ovskite eutectics have been solidified by the micro-pulling-down method (Pawlak et al., 2006). If the phases are etched away, a pseudohexagonally packed dielectric periodic array of pillars or periodic array of pseudohexagonally packed holes in the dielectric material is left. 

A closely related mechanical property which has been used extensively in glass literature is the microhardness. Micro in microhardness only indicates that the hardness measurements have been made on a micron scale. Microhardness actually measures only the scratch resistance of the material and thus a scale of microhardness is a scale of the scratch resistances - harder material can scratch the surface of the softer material. One of the widely used scales is Mohs scale of hardness calibrated with the hardness of the hardest material, namely diamond, marked with a value of 10 and with the hardness of the softest material, namely talc, marked with a value of 1. On this scale most oxide glasses register microhardnesses between 5 and 7. In scientific investigations two other scales are used, namely Knoop s hardness number (KHN) and... 

An additional aspect of the oxidation of Ti3Al alloys is dissolution of oxygen into the alloy at the scale/alloy interface. The embrittlement associated with this phenomenon can be more damaging to the mechanical properties than the surface recession caused by scale formation in the temperature range where Ti-,A1 will likely be used (< 700°C) [58],This subject will be discussed in a separate section. 

The mechanical behaviour of oxide scales has been investigated by a complex of appropriate techniques (cf. for example [12]), where the attention has been focused on the analysis of the stress development in the scale, as well as the measurement of the scale adherence. In the case of weak scale adherence, spontaneous scale failure is often observed during oxidation or cooling of specimens. For systematic investigations of the fracture-mechanical properties of oxide scales, scale failure is induced by a controlled loading of the scale which is produced by an appropriate deformation of the whole specimen. 

In the following, a brief mechanical analysis is given which is to provide a first qualitative understanding of the relationship between the crack patterns and the fracture-mechanical oxide properties. It will be pointed out that appropriate scale loading generates crack patterns which characterise either the tensile strength or the fracture toughness of the oxide scale. 

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