Salt accelerated oxidation

From these 500 and 550°C test the first stage in salt accelerated oxidation can be deduced. As evident in Figure 8a this consists of localised corrosion in areas of salt deposition. Figure 10 is a schematic diagram of the three stages of scale growth observed for a two phase, -y -t-a2TiAl alloy. 

Stage 2 also follows logarithmic kinetics, reflecting competition between parabolic oxide growth and short circuit diffusion down preferred channels. Initially, the short circuit paths account for the early observed rapid scale growth. A transition is later observed to parabolic kinetics, which marks the onset of the third stage of scale growth in hot salt accelerated oxidation of -y-TiAl. 

Natural rubber is very resistant to water, acids, and alkalis, but is attacked by oxidants and solvents. Some metallic salts accelerate oxidative degradation [32]. 

The ash content of furnace blacks is normally a few tenths of a percent but in some products may be as high as one percent. The chief sources of ash are the water used to quench the hot black from the reactors during manufacture and for wet pelletizing the black. The hardness of the water, and the amount used determines the ash content of the products. The ash consists principally of the salts and oxides of calcium, magnesium, and sodium and accounts for the basic pH (8—10) commonly found in furnace blacks. In some products potassium, in small amounts, is present in the ash content. Potassium salts are used in most carbon black manufacture to control stmcture and mbber vulcanizate modulus (22). The basic mineral salts and oxides have a slight accelerating effect on the vulcanization reaction in mbber. 

Corrosion is described as hot corrosion and sulfidation processes. Hot corrosion is an accelerated oxidation of alloys caused by the deposition of Na2S04. Oxidation results from the ingestion of salts in the engine and sulfur from the combustion of fuel. Sulfidation corrosion is considered a form of hot corrosion in which the residue that contains alkaline sulfates. Corrosion causes deterioration of blade materials and reduces component life. 

Not all sulphates are as readily reduced as sodium sulphate, for instance, calcium sulphate does not usually lead to sulphide penetration, although the presence of other substances with calcium sulphate may lead to accelerated oxidation for other reasons. The results for laboratory tests on a series of metals and alloys in sodium sulphate -F sodium chloride and calcium sulphate + calcium chloride mixtures are shown in Table 7.12 . In many cases sulphide peneration could be noted with the sodium salts but not with the calcium salts. 

Even small traces of certain corrosion stimulants, notably soluble chlorides and sulphates, can maintain a continuing corrosion process under a paint film because the salts accelerate the initial dissolution of ferrous iron (and other metal ions) but are not immobilised in the hydrated oxide corrosion products. Filiform corrosion is the most spectacular example of this phenomenon, but progressive spread, preceded by blistering, is also observed from scratches or other breaks in a coating, for example during salt spray tests. 

This leads to chain termination in the absence of hydroperoxide. There are experimental examples when the introduction of transition metal salt does not accelerate oxidation but... 

Compounds of transition metals (Mn, Cu, Fe, Co, Ce) are well known as catalysts for the oxidation of hydrocarbons and aldehydes (see Chapter 10). They accelerate oxidation by destroying hydroperoxides and initiating the formation of free radicals. Salts and complexes containing transition metals in a lower-valence state react rapidly with peroxyl radicals and so when these compounds are added to a hydrocarbon prior to its oxidation an induction period arises [48]. Chain termination occurs stoichiometrically (f 1) and stops when the metal passes to a higher-valence state due to oxidation. On the addition of an initiator or hydroperoxide, the induction period disappears. 

Earp and Hill (99) find that the addition of salts to graphite usually accelerates oxidation markedly the notable exceptions being most of the borates and phosphates. 

T he rate of metal salt-catalyzed autoxidation of hydrocarbons reaches - a maximum at a certain catalyst concentration (1, 7, 13), and any further increases in this concentration do not accelerate the rate. However, when bromide ion is added to the solution of hydrocarbon and fatty acid with metal salts, the oxidation rate increases over the maximum value of k32(RH)2/2k6 as a function of metal concentration. 

Addition of cerium salts accelerates the electrolytic oxidation of anthracene, naphthalene, and phenanthrene, which yield the corresponding quinones. The hydrocarbons may be in solution or in the form of a finely divided suspension. Anthracene, for example, is oxidised to Anthraqtjinone in 20 per cent, sulphuric acid anode current density is about 5 amps, per dm.2, and by the addition of 2 per cent, of cerium sulphate the current efficiency 8 is stated to reach nearly 100 per cent. 

ETFE resins have a good thermal stability however, for high-temperature applications thermal stabilizers are often added67 A wide variety of compounds, mostly metal salts, such as copper oxides and halides, aluminum oxide, and calcium salts, will act as sacrificial sites for oxidation. Addition of certain salts can alter the decomposition from oligomer formation to dehydrofluorination. Iron and other transition metal salts accelerate the dehydrofluorination. Hydrofluoric acid itself destabilizes ETFE at elevated temperatures and the degradation becomes self-accelerating. For that reason, extrusion temperatures higher than 380°C (716°F) should be avoided.68... 

Hence, in conclusion, studies on the oxidation of PC solutions clearly differentiate between LiC104 solutions and other salt solutions. It is clear that C104 oxidation occurs in parallel to solvent oxidation, and the latter is accelerated by anion oxidation. Obvious products obtained from PC oxidation are C02 and polymeric species, probably derivatives of poly(propylene oxide). In the case of BF4 or PF6 salts, the oxidation products also include fluorinated derivatives of... 

Selenious acid oxidizes pyrrole to a blue dyestuff of unknown composition ( pyrrole blue ). Iron salts accelerate the reaction when it is carried out in phosphoric acid solution. Selenic, tellurous and telluric acids do not react under the conditions given below the test therefore provides a method of distinguishing selenites and selenates. 

Ferrous salts accelerate the oxidation of sulphurous acid to sulphuric acid in the presence of oxygen.3... 

The presence of small amounts of salt, typically 37 xg/cm2, result in accelerated oxidation rates for a 7 + a2 TiAl alloy over the temperature range 500-700 °C. 

In addition to attack by reactive gases, alloys used in practical environments, particularly those involving the combustion products of fossil fuels, undergo an aggressive mode of attack associated with the formation of a salt deposit, usually a sulphate, on the metal or oxide surface. This deposit-induced accelerated oxidation is called hot corrosion. The severity of this type of attack, which can be catastrophic, has been shown to be sensitive to a number of variables including deposit composition, and amount, gas composition, temperature and temperature cycling, erosion, alloy composition, and alloy microstructure. A number of comprehensive reviews on hot-corrosion have been prepared. The purpose of this chapter is to introduce the... 

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