Nitrogen high-temperature corrosion

In industrial applications the environments usually contain more than one reactant. For example high temperature oxidation occurs in air by the combined attack of oxygen, nitrogen and quite frequently water vapour. However, most of the studies concerning the oxidation resistance are performed in dry oxygen or dry air. The oxidation behaviour of the intermetallic phases of theTi-Al system has recently received considerable attention. The influence of water vapour on the oxidation of titanium aluminides has not been studied intensively. There are only a few studies of the high temperature corrosion of titanium and its alloys. 

Vanadium in a fuel forms various metal compounds with low melting points, and causes molten-salt corrosion of steel called vanadium attack. Another example of high temperature corrosion is sulfidation. Carbon monoxide, carbon dioxide, and hydrocarbons form metal carbides at high temperatures and this is called carburization. Nitriding involves chemical reaction of nitrogen with metal. 

Table 5-4 summarizes the types of high-temperature corrosion which occur in various industrial processes (Lai et al., 1985). As can be inferred from this table, many high-temperature processes involve atmospheres which contain oxidants in addition to O2, H2O, or CO2 the most prevalent of these are sulfur, carbon, nitrogen, and chlorine. The corrosive effects of these additional oxidants depend on the oxygen potential in the atmosphere, which is measured by the equilibrium Pq. In relatively high- multi-oxidant atmospheres, such as those resulting from the combustion of hydrocarbon fuels with excess air, the corrosive effects tend to be minimal or even beneficial. An example of a beneficial effect is that of traces of sulfur in the combustion environment of a reheat furnace which acts to slow the oxidation rate of steel as a result of a surface-poisoning effect (Lee, 1997). 

The original route from p-xylene was oxidation in the presence of nitric acid. But the use of nitric acid is always problematical. There are corrosion and potential explosion problems, problems of nitrogen contamination of the product, and problems due to the requirement to run the reactions at high temperatures. Just a lot of problems that all led to the development of the liquid air phase oxidation of p-xylene. Ironically the nitrogen contamination problem was the reason that the intermediate DMT route to polyester was developed, since that was easy to purify by distillation. Subsequently, DMT has secured a firm place in the processing scheme. 

Oxygen is soluble in water to the extent of 9.4 ppm from air at 100 kPa and 20 °C, and 02 is the oxidant responsible for most metallic corrosion. Consequently, deaeration of water by purging with nitrogen or vacuum degassing may be desirable in some circumstances this should not be undertaken without circumspection, since deoxygenation may cause activation of otherwise passive metals or cause cathodic areas to become anodic (Chapter 16). At high temperatures, aqueous oxygen is consumed quite rapidly by hydrazine or sodium sulfite (Section 16.7). 

K. Tantalum. (Tantalum mp 2,996°C), is an extremely chemically resistant metal which is hard but ductile. Because of its high melting point and good corrosion resistance, tantalum is frequently used as a container for high-temperature melts. It is attacked by HF and other fluorides, as well as sulfur trioxide and nitrogen oxides. 

Zirconium metal (mp 1855°C 15°C), like titanium, is hard and corrosion resistant, resembling stainless steel in appearance. It is made by the Kroll process (Section 17-A-l). Hafnium metal (mp 2222°C 30°C) is similar. Like titanium, these metals are fairly resistant to acids, and they are best dissolved in HF where the formation of anionic fluoro complexes is important in the stabilization of the solutions. Zirconium will burn in air at high temperatures, reacting more rapidly with nitrogen than with oxygen, to give a mixture of nitride, oxide, and oxide nitride (Zr2ON2). 

At high temperatures the metal will react slowly with certain gases. With carbon monoxide it produces a surface film of carbide, with nitrogen it produces a nitride film, and with hydrogen sulphide it reacts to form molybdenum disulphide. All of these films presumably interfere with the flow of gas to the metal surface, and in each case only a thin film of the product arises. Molybdenum is also very resistant to corrosive attack by mineral acids except for those such as nitric acid or chromic... 

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