High-temperature corrosion oxidation reaction

The reaction of metals with gas mixtures such as CO/CO2 and SO2/O2 can lead to products in which the reaction of the oxygen potential in the gas mixture to form tire metal oxides is accompanied by the formation of carbon solutions or carbides in tire hrst case, and sulphide or sulphates in the second mixture. Since the most importairt aspects of this subject relate to tire performairce of materials in high temperature service, tire reactions are refeiTed to as hot corrosion reactions. These reactions frequendy result in the formation of a liquid as an intermediate phase, but are included here because dre solid products are usually rate-determining in dre coiTosion reactions. 

If local stresses exceed the forces of cohesion between atoms or lattice molecules, the crystal cracks. Micro- and macrocracks have a pronounced influence on the course of chemical reactions. We mention three different examples of technical importance for illustration. 1) The spallation of metal oxide layers during the high temperature corrosion of metals, 2) hydrogen embrittlement of steel, and 3) transformation hardening of ceramic materials based on energy consuming phase transformations in the dilated zone of an advancing crack tip. 

The proper choice of anode material, cathode material and reactor material are of particular importance in conducting WEO treatments. The reaction takes place high temperature at oxidizing conditions in the presence of organics and salts, which make the reaction environment extremely corrosive. 

Most metals occur naturally in their oxide or sulfide forms. The process of metal refining converts these ores into pure metals. Thermodynamically, a metal will return spontaneously to its original oxide form. Metal oxidation can occur at high temperatures, by direct reaction with O2, or at a moderate temperature by reaction with water, O2, and/or H+. The latter oxidation, commonly referred to as wet corrosion, has as its basis the combination of electrochemical cathodic reduction and anodic metal oxidation reactions into a corrosion cell. Thus, many corrosion processes are... 

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 ... 

In designing alloys for use at elevated temperatures, the alloys must not only be as resistant as possible to the effects produced by reaction with oxygen, but resistance to attack by other oxidants in the environment is also necessary. In addihon, the environment is not always only a gas since, in practice, the deposihon of ash on the alloys is not uncommon. It is, therefore, more realishc in these cases to speak of the high-temperature corrosion resistance of materials rather than their oxidation resistance. 

High-temperature corrosion in metals has been studied in order to understand reaction rates, rate-limiting steps, reaction mechanisms, metal oxide crystal structure, oxide composition, and oxide microstructure and morphology. 

F ure 1.5 High-temperature corrosion reaction between the metal and oxygen in the presence of an oxide layer having both ionic and electronic conductivity. 

Although there are no aqueous electrolytes, high temperature corrosion is an electrochemical process, involving anodic and cathodic partial reactions. The metal oxides generated at the corroding surface or molten salts present at the surface form the electrolyte. 

Oxidation, the chemical reaction of a metal with oxygen, is the most studied reaction in high-temperature corrosion. It shows two limiting behaviors that can be described by simple models (Figure 9.4) ... 

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. 

The simplest form of a solid corrosion product on a metal surface is a continuous homogeneous surface scale consisting of one phase. Such a situation is encountered, e.g., in the oxidation of pure nickel where only nickel oxide is formed. Therefore, in many textbook examples, this type of reaction has been used to illustrate the principles of high-temperature corrosion. In the present chapter, the initial stages of the oxidation process are not addressed as this has been part of another chapter in this book. 

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