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High-Temperature Corrosion: Part Two :: Total Materia Article
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High-Temperature Corrosion: Part Two
Vapor species that form in any given high-temperature corrosion situation often have a strong influence on te rate of attack, te rate generally being accelerated when volatile corrosion products form.
Although most metals and alloys form a layer of oxide in contact with air, te differences in oxide growth rate can be significant.
Vapor species that form in any given high-temperature corrosion situation often have a strong influence on te rate of attack, te rate generally being accelerated when volatile corrosion products form. Gulbransen and Jansson have shown that metal and volatile oxide species are important in te kinetics of high-temperature oxidation of carbon, silicon, molybdenum, and chromium.
Six types of oxidation phenomena were identified:
1. At low temperature, diffusion of oxygen and metal species through a compact oxide film
2. At moderate and high temperatures, a combination of oxide film formation and oxide volatility
3. At moderate and high temperatures, te formation of volatile metal and oxide species at te metal-oxide interface and transport through te oxide lattice and mechanically formed cracks in te oxide layer
4. At moderate and high temperatures, te direct formation of volatile oxide gases
5. At high temperature, te gaseous diffusion of oxygen through a barrier layer of volatilized oxides
6. At high temperature, spalling of metal and oxide particles.
Although most metals and alloys form a layer of oxide in contact with air, te differences in oxide growth rate can be significant. For engineering purposes te differences are vital since they determine te component life-time. A linear growth rate implies that te rate of oxidation remains constant during exposure. Linear oxidation rates can be observed during te initial exposure, before a continuous oxide film has formed. A similar behaviour is also seen when a bare metal surface is revealed due to severe oxide spallation.
According to te linear rate equation described in Equation 6, te exposure time does not affect te oxidation rate and it is reactions at te surface or phase boundary that limit te oxidation rate.
Where x represents te oxide thickness, K1 te linear rate constant, t denotes time and C is a constant.
During exposure of metals and alloys suitable for use at high temperatures, te transport of reactants across te oxide scale is usually te rate-determining step and te oxidation rate decreases as te oxide thickens. In a model developed by Wagner te diffusion of ions and electrons are limiting te oxidation rate, leading to a parabolic growth rate as shown in Equation 7.
Where x represents te oxide thickness, KP te parabolic rate constant, t denotes time and C is a constant.
Plots showing oxide growth rates are commonly used when assessing alloy performance during exposure in high temperature atmospheres. With a double logarithmic plot te constants of te rate equation can be easily determined. In reality this often results in a growth rate that deviates from te existing models. Te obvious reason is that these are idealized models that cannot perfectly describe oxide growth on engineering alloys when several factors are simultaneously influencing te oxidation process.
Te presence of fast diffusion paths at localized positions (grain boundaries, lattice defects etc.) both within te oxide and in te metal virtually means that te scale growth rate varies over te surface. Localized spallation may also occur which governs a rapid oxide growth at te position where te oxide film has ‘spalled off’. Despite discrepancies from te described growth models, a diffusion controlled oxidation is desired and commonly observed for engineering alloys. Severe cracking or spallation of te oxide film is also an important feature which results in a rapid increase in corrosion rate.
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High Temperature Corrosion: Part One
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