Mechanism of internal oxidation involves diffusion of Oxygen inward below the metal surface , nucleation of oxides, growth of the oxide particles due to diffusion of both ions of oxygen and the ions of alloying element.
Internal oxidation takes place under th following conditions:. Internal oxidation is used for surface dispersion hardening of alloys.
Internal oxidation zone cannot be removed by conventional descaling methods. Kinetics of high temperature oxidation. High temperature oxidation of alloys.
Three Examples of High-Temperature Corrosion of Metals by Molten Glasses
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. Non-protective scale has loose porous structure providing free access of oxygen to the underlaying metal.
High Temperature Oxidation and Corrosion of Metals, Volume 1 - 1st Edition
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. 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:.
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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.
Why Study Corrosion?
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. Cation: positively charged ion metal. Addition of higher valency cation eg. Addition of lower valency cation eg.
Internal oxidation takes place under th following conditions: The alloying element has a greater affinity for oxygen than the matrix metal. Diffusivity of oxygen in the matrix should be greater than that of the alloying element.
Penetration of the internal oxidation inward should be faster than the rate of the scale formation. Electrode potentials. Pourbaix diagrams. We have carried out testing on metals, alloys, ceramics, graphite and graphite-based materials, composites and coatings. TWI has considerable expertise in designing and building equipment for high temperature testing in extreme environments.
An example of TWI's expertise is shown in a series of two recent projects.
The first dealt with corrosion-resistant coatings for a biomass plant. As an offshoot from this and other work, TWI has recently begun a group-sponsored project looking at developing new coating technologies for high temperature corrosion mitigation in process plants, each with its own unique highly corrosive environment. Support for SMEs.
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