Internal oxidation zone cannot be removed by conventional descaling methods. Kinetics of high temperature oxidation. High temperature oxidation of alloys. Effect of oxide structure on oxidation. Internal oxidation. Protective scale prevents access of oxygen to the metal surface due to non-porous continuous structure of the oxide layer.
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Non-protective scale has loose porous structure providing free access of oxygen to the underlaying metal. The scale is protective adherent and non-porous if the volume of the oxide is not less than the volume of metal, from which the oxide was formed. The scale is non-protective porous if the volume of the oxide is less than the volume of metal, from which the oxide was formed.
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The structure of an oxide scale determines the low, according to which the scale weight increases: Non-protective porous scales are formed in the process, rate of which is independent on the scale thickness due to fast transfer of oxygen to the metal surface. The process rate is controlled by the reaction of oxidation. In this case the process rate is constant:. Protective adherent and non-porous scales are formed in the process, rate of which is controlled by diffusion of oxygen through the oxide scale.
The weight-time dependence obtained from the First Fick's law follows the parabolic law:. Thin protective films formed at lower temperatures are described by asymptotic logarithmic law providing more rapid decrease of rate with time:.
Contents of metals in the oxide scale differ from the alloy composition due to the following factors: The alloy components may have different affinities for oxygen. Some of them oxidize more rapidly than other. In some alloys only one component most reactive oxidizes - the process is called selective oxidation.
Different alloy components metal ions may have different diffusion coefficients in the oxide and alloy, which causes preferential oxidation of the component having higher diffusivity.
Some alloy components may oxidize out of the scale - within the alloy below the metal-scale interface Internal oxidation. Anion: negatively charged ion oxygen. Instead of the the iron corroding, the Zn acts as the sacrificial anode in the cell and protects the iron. Outside links For more quick information on corrosion, visit the Wikipedia site on corrosion.
References Landolt, Dieter. Corrosion and Surface Chemistry of Metals. EPFL Press, Perez, Nestor. Electrochemistry and Corrosion Science.
Kluwer Academic Publishers, Petrucci, Ralph H. Harwood, F G. Herring, and Jeffry D. General Chemistry Principles and Modern Applications. New Jersey: Pearson Education, Inc, Evans, M. Tchorabdji Simnad. The Royal Society. Proceedings of the Royal Society of London. Butler, H.
Corrosion, Design and Materials: General and Pitting. Philosophical Transactions of the Royal Society of London. New Light on Corrosion. Science News , Vol. Problems Observe an example of corrosion and identify the conditions that allowed the attack and why it is a problem.
Explain under what conditions general corrosion would be problematic. What type of corrosion is pitting and why is it often more detrimental to functionality than general corrosion? What is the smallest number of electrochemical reactions needed to describe corrosion? When iron and magnesium are in contact with an electrolyte, which will corrode and which will have reduced susceptibility to corrosion?
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Fasasi A. Datta and J. Gray eds. Pierraggi B. Wagner C. B21 , 25 Solid State Chem. Smeltzer W. Personalised recommendations.