High-temperature corrosion atmospheres

These initial results with hot-pressed samples indicate that phase transformations in high-temperature corrosive atmospheres are potentially complex. A better understanding of the phase chemistry under these conditions may be important in determining the performance of alumina-based coating systems, and of monolithic alumina or alumina matrix composites. 

Haynes alloy 556 exhibits useful resistance to a wide variety of high-temperature corrosive atmospheres as well as molten salts. The presence of approximately 18% cobalt results in more resistance to sulfidation than... 

Intermetallics have been developed recently, owing to their specific properties [214], Aluminides are of particular interest because they have high melting points, low densities and considerable resistance to high temperature corrosive atmospheres [215]. Their interesting properties made them excellent candidates for aeronautical applications [216-218]. 

Hancock, ., Corrosion of AUOys Of High. Temperatures in Atmospheres Consisting of, . .Fuel Co.tntmsiion, Products and Associated Impurities, H,M,S.O, (1968). 

The purpose of this review paper is to survey the principles of high temperature oxidation or high temperature corrosion. A typical situation is that of a metal exposed to a hot gas which can act as an oxidant. In many cases the oxidation product forms a layer which separates the reactants, the metal and the gas atmosphere. Under special conditions, the kinetics are diffusion controlled, i. e,, the rate of the reaction (the rate of oxide thickness growth) depends on the diffusion of species, ions and electrons, through the layer (sometimes called a tarnish layer). Actually when a metal or alloy is exposed to a corrosive gas, the reaction kinetics may be controlled by one or more of the following steps ... 

M.A. Uusitalo, P.M.J. Uuoristo, T.A. Mantula, High temperature corrosion of coatings and boiler steels in reducing chlorine-containing atmosphere. Surf. Coat. Tech. 161 (2002) 275-285. 

Corrosion can also be classified based on environments. For example, we can mention the following corrosion phenomena - atmospheric corrosion, fresh water corrosion, sea water corrosion, soil corrosion, high temperature corrosion, and gaseous corrosion. 

TESTCORR (1996), First Draft Code of Practiee for DISCONTINUOUS CORROSION TESTING IN HIGH TEMPERATURE GASEOUS ATMOSPHERES, Leatherhead ERA Technology. 

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). 

Corrosion is a phenomenon that occurs spontaneously in the majority of metals and alloys as a result of their interaction with their environment, which makes them tend toward a situation of stable balance. According to the definition given by the European Corrosion Federation (1974), corrosion is the attack on a metal caused by its reaction to the environment, with the consequent degradation of its properties (Uhlig, 1985 Andrade, 1991). This environment could either be an electrolyte, which is when the process is called electrochemical corrosion, or another high-temperature atmosphere, whenitis called, oxidation, dry corrosion or high-temperature corrosion. 

Fig. 16. High-temperature corrosion (wt.loss) in a Hi/HjO/HCl atmosphere at 980°C (Bendick et al. 1990).

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