High-temperature corrosion thermodynamics, formation

Chapter 10 deals with high temperature corrosion, in which the thermodynamics and kinetics of metal oxidation are included. The Pilling Bedworth Ratio and Wagner s parabolic rate constant theories are defined as related to formation of metal oxide scales, which are classified as protective or nonprotec-tive. 

The outstanding characteristics of the noble metals are their exceptional resistance to corrosive attack by a wide range of liquid and gaseous substances, and their stability at high temperatures under conditions where base metals would be rapidly oxidised. This resistance to chemical and oxidative attack arises principally from the Inherently high thermodynamic stability of the noble metals, but in aqueous media under oxidising or anodic conditions a very thin film of adsorbed oxygen or oxide may be formed which can contribute to their corrosion resistance. An exception to this rule, however, is the passivation of silver and silver alloys in hydrochloric or hydrobromic acids by the formation of relatively thick halide films. 

Nearly all metals are thermodynamically unstable in most environments and the result of this instability is corrosion, such as oxidation or some other reaction with the environment. In both "wet" and "dry" corrosion three general phenomena occur. First, material from the metal can dissolve in the environment. This takes forms such as evaporation and volatile compound formation at high temperatures and material dissolution in aqueous solutions. Material loss by such processes may weaken a structure or cause loss of a protective layer. Second, a reaction layer may form on the surface of the metal. Frequently, these layers reduce the rate of a reaction and thus protect the material (passivate a... 

It is important to realize that corrosion rates may be controlled by any of several thermodynamic or kinetic properties of the alloy-scale-environment system and not just by surface or interface reactions. The three stages of high temperature oxidation of a metal, shown schematically in Fig. 1, serve as an example (7). The first or transient stage includes initial gas adsorption, two-dimensional oxide nucleation, initial three-dimensional oxide formation and finally, formation of the dominant oxide that will control the oxidation rate in Stage II. Various portions of Stage I have been widely studied using surface analytical techniques, but its duration can be very short and it is usually assumed (not always correctly) that Stage I has little impact on ultimate corrosion properties of the material. 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

Elevated temperatures along with high compression and flow rates in the turbine are the main reasons for the formation of a uniform HTHC region. The high temperature has a major effect since it changes the chemical composition, fluidity, and thermodynamic properties of corrosion products. High-temperature gradients between the gas flux and the surface of cooled blades help... 

Alloys generally rely upon an oxidation reaction for the formation of a protective scale that will improve the corrosion resistance to sulfidation, carburization, and the other forms of high-temjjerature attack. The properties of high-temperature oxide films, such as their thermodynamic stability, ionic defect structure, and detailed morphology, therefore play a crucial role in determining the oxidation resistance of a metal/alloy in a specific environment. 

The austenitic stainless steels and most other metals and alloys of practical importance in large-scale homogeneous reactors are thermodynamically unstable in aqueous solutions and depend on protective films for their corrosion resistance. Tortunately, when austenitic stainless steels are oxidized in high-temperature uranyl sulfate solutions, the steel oxidizes uniformly so that no element (or elements) is leached preferentially from the alloy. However, not all the alloying elements contribute to film formation. 

In order to predict the direction of corrosion and mass transfer, it is essential to have data on thermodynamic properties of chemical compositions and steel components as a function of temperature. If the liquid metal is flowing at high velocity, the material is subject to erosion. Formation of the film (consisting of both steel and liquid metal coolant components) on the structural metal surface is another type of corrosion, since this is not protective film. Due to the difference in chemical activity between sodium and lead, technologies of these coolants are quite different, although some methods share a number of common features. 

According to thermodynamic simulation [100], at a carbide water ratio of 1 10, tungsten carbide should be completely oxidized producing H2WO4 or WO3 at lower temperatures and WO2 at higher temperatures. Carbon is oxidized to CO and CO2 in the case of a low WC H2O molar ratio. Formation of free carbon upon hydro-thermal corrosion of WC was predicted to be possible at a high WC water ratio upon hydrothermal corrosion of WC. The stability of WC increases with increasing pressure. 

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