Introduction and basic concepts of the high temperature oxidation oxidation

During their operation materials are exposed to different destructive phenomenon, which significantly reduce their component lifetime and influence the selection of materials used for particular applications. Among the destructive processes influencing materials performance, oxidation is often of crucial importance due the fact that it takes place in most atmospheres and at all temperatures. It occurs in air and, oxygen, as well as gases with relatively low oxidising potentials as sulphur dioxide, carbon dioxide wet gas mixtures and pure steam. There are techniques to reduce the impact of oxidation on material performance but this process is still highly problematic. High temperature oxidation is described as a reaction between a material operating at high temperature and its surrounding atmosphere, which result in an oxide forming on the surface of the material [57, 58, 59]:

(2-40)

Equation (2-40) shows basic oxidation of metal in oxygen resulting in formation of metal oxide on the surface [57] The basic model of high temperature oxidation divides the process into two basic stages: initial oxidation, which describes formation of the first, most stable oxides on the surface of the bare material, and thick layer oxidation [57, 58, 59, 60]. Study of the initial stage of the high temperature oxidation is of crucial importance; however, it is a difficult to predict the behaviour of the material during this period and to estimate the interaction between the gas/metal.

In general, the initial stage of oxidation involves adsorption of the oxygen and leads to the formation of an oxide; those scale a few nanometres thick. The initial period of oxidation starts with the adsorption of oxygen on the clean surface, which is followed by formation of a thin chemisorbed layer. The next step is development of additional oxide layers. This stage is controlled by island

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growth and exchange [57, 61], which drives formation of the thicker oxides that spread along the surface. Finally, in order for the oxide growth to continue ions and electrons have to be transported across the layers formed; the kinetics of this process are depended on different possible transport mechanisms. In literature, the first two stages of initial oxidation are believed to follow linear rate dependencies the third stage exhibits a logarithmic kind of the rate dependency [57, 61].

After the initial oxidation stages, which for the high temperature processes are relatively short, thicker oxides start to grow, and for the reaction to occur either metal ions from the substrate or oxygen ions from the oxidising atmosphere have to be transported [46, 57. 58, 59, 60]. The migration of the ions involved is controlled by diffusion, which is enhanced by defects in the oxide(s) formed. The thicker oxides oxidation is described by Wagner’s theory of oxidation that is often used in high temperature oxidation studies [57, 58]. For the theory to be applied following assumption have to be made [57, 60]:

1. Oxide scales are adherent and compact;

2. Migration of ions, electron and electron holes determines the process rate;

3. Thermodynamic equilibrium is established on both metal/oxide and oxide/gas interfaces;

4. Oxides are stoichiometric in nature (but small deviation from stoichiometry are possible); and

5. Oxygen solubility in the metal is neglected.

Because of the equilibrium at both interfaces (gas/oxide and scale/oxide) the activity gradients of metal and oxygen are established across the scale. In consequence, the metal and oxygen ions are able to counter - diffuse throughout the scales. Due to fact that ions are charged thus migration drives the formation of an electric field across the oxides (which moves from the metal to the atmosphere); thus the net migration of ions, electrons and electron holes has to be balanced in order to maintain the electro-neutrality condition.

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Figure 2-17 Schematic oxide scale formation following the wagner’s theory of oxidation [57, 60]

Wagner’s theory of oxidation is frequently quoted in the high temperature oxidation; however there are only a few oxides which strictly follow the Wagnerian model. This is a result of the basic assumptions; most of the oxides formed on metal are neither adherent nor compact in nature. Moreover, most oxides are not stoichiometric, and finally total equilibrium at each interface is rarely established.

am’ M = M⁺⁺ + 2e M⁺⁺ +2e +1/2O2 = MO pO2’’

O 2-e

-Metal Oxide Oxygen

M⁺⁺

am’’ pO2’

41 2.4.2 Oxidation rates

Oxidation is controlled by diverse processes, which can change with time and exposure conditions. The literature review shows that high temperature oxidation can exhibit different rate law dependences:

 Linear for the initial phase of oxidation;

 Parabolic in the thicker oxides regime; and

 Logarithmic or inverse logarithmic in thin scales and at low temperatures.

Oxidation exhibits linear rate law dependence when the oxide scales are thin and the reactions on scale/gas interface are the rate-determining [60].

Under such conditions, the rate may be written as:

(2-41)

where is the oxide thickness [μm], is linear rate constant [um/s], t is time [s]

In many cases oxidation of materials at high temperature is reported to exhibits a parabolic rate law dependence [57]. Since the oxides formed on the alloy surface are thicker, the diffusion of cations and anions throughout the scales become the rate determining processes. The cation and anions diffusion across the oxide scale shows parabolic rate dependence [62], therefore the oxidation process at this stage is believed to exhibit the parabolic rate dependence [59]. The oxidation rate is inversely proportional to the scale thickness and may be written as [57]:

(2-42)

where is the oxide thickness [μm], is parabolic rate constant [μm²/s], C is constant [μm²], t is time [s].

Oxidation follows the logarithmic rate law under specific conditions: firstly for thin oxide scales and secondly at low temperatures [60]. Under such conditions the reaction rate is rapid at the beginning at the process, however

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after some period starts to decrease with time, following the direct or inverse logarithmic rate law.