Oxidation high-temperature

For alloys 1 and 3 in ultrahigh vacuum (UHV) and 5 x 10 Pa O2 at 973 K, the XPS peak areas for each elemental region were normalized by using their corresponding sensitivity factors. The normalized, relative-peak-area percentages for the sputtered and reacted surfaces of these alloys are shown in Table 8.2 in 

The relative reactivity and extent of reactions of Nb, Ni3Nb, and NbC with O2 were determined by XPS [33]. Specimens following exposure to 5 x 10 Pa of oxygen at 873, 923, and 973 K for 2,700 seconds were analyzed. Niobium, Ni3Nb, and NbC were found to oxidize readily. Representative spectra for Ni3Nb and the NbC film at 973 K are shown in Fig. 8.27 along with the component spectra for Ni3Nb and 

NbC and the various oxides that were formed. The results confirm the fact that both NisNb and NbC react with oxygen at these temperatures to form niobium oxides of ranging stoichiometry, with a greater propensity for NisNb to oxidize to Nb205. 

Additional experiments on NisAl and NisTi confirmed that these precipitates also oxidized readily. The oxidation of these precipitates ahead of the crack tip contributed to the enhancement of crack growth in the y -strengthened alloys [36, 38]. The increase in growth rate in the y -strengthened alloys appears to depend on the volume fractions of these precipitates [38]. 

Corrosion is normally associated with aqueous solutions but oxidation can occur in dry conditions. Carbon and low alloy steels will oxidise rapidly at high temperatures and their use is limited to temperatures below 500°C. 

Chromium is the most effective alloying element to give resistance to oxidation, forming a tenacious oxide film. Chromium alloys should be specified for equipment subject to temperatures above 500°C in oxidising atmospheres. 

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Figure C2.8.7. Principal oxide growth rate laws for low- and high-temperature oxidation inverse logarithmic, linear, paralinear and parabolic.

Since 1960, the Hquid-phase oxidation of ethylene has been the process of choice for the manufacture of acetaldehyde. There is, however, stiU some commercial production by the partial oxidation of ethyl alcohol and hydration of acetylene. The economics of the various processes are strongly dependent on the prices of the feedstocks. Acetaldehyde is also formed as a coproduct in the high temperature oxidation of butane. A more recently developed rhodium catalyzed process produces acetaldehyde from synthesis gas as a coproduct with ethyl alcohol and acetic acid (83—94). 

Metallurgy. The strong affinity for oxygen and sulfur makes the rare-earth metals useflil in metallurgy (qv). Mischmetal acts as a trap for these Group 16 (VIA) elements, which are usually detrimental to the properties of steel (qv) or cast iron (qv). Resistance to high temperature oxidation and thermomechanical properties of several metals and alloys are thus significantly improved by the addition of small amounts of mischmetal or its siUcide (16,17). 

Galvalume has been shown to have two to six times the life of an equivalent thickness of 2inc, including marine atmospheres. Eor high temperature oxidation resistance up to 700°C, Galvalume is equivalent to pure aluminum. 

There is also a two-step process of chromizing foUowed by aluminizing. Above 900°C the chromizing begins to rediffuse and the protective oxide changes to Al O from Cr202. Aluminum oxide is less volatile than chromium oxide and better for high temperature oxidation resistance above 1000°C. 

N. Birks and G. H. Meir, Introduction to High Temperature Oxidation of Metals, E. Arnold, London, 1983. 

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. 

High Temperature Oxidation Resistant Coatings, Materials Advisory Board, National Academy of Science, Washington, D.C., 1970. 

Table 1. High Temperature Oxidation Resistant Beryllides...

Fluorinated Excellent resistance to high temperature, oxidizing acids, and oxidation good resistance to fuels containing up to 30% aromatics... 

P. Kofstad. High Temperature Oxidation of Metals. J. Wiley Sons. New York (1966) TA 462. K57. 

A further indication of the rapid advances that have occurred in the chemistry of the elements during the past 15 years can be gauged from the several completely new sections which have been added to review work in what were previously both nonexistent and unsuspected areas. These include (a) coordination compounds of dihapto-dihydrogen, (b) the fullerenes and their many derivatives, (c) the metcars, and (d) high-temperature oxide superconductors. 

The electrodeposition of Cr in acidic chloroaluminates was investigated in [24]. The authors report that the Cr content in the AlCr deposit can vary from 0 to 94 mol %, depending on the deposition parameters. The deposit consists both of Cr-rich and Al-rich solid solutions as well as intermetallic compounds. An interesting feature of these deposits is their high-temperature oxidation resistance, the layers seeming to withstand temperatures of up to 800 °C, so coatings with such an alloy could have interesting applications. 

In an ionizing solvent, the metal ion initially goes into solution but may then undergo a secondary reaction, combining with other ions present in the environment to form an insoluble molecular species such as rust or aluminum oxide. In high-temperature oxidation, the metal ion becomes part of the lattice of the oxide formed. 

Kofsted, P., High Temperature Oxidation of Metals, John Wiley, New York 0965) Kubaschewski, O. and Hopkins, B. F., Oxidation of Metals and Alloys, Butterworths, London... 

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