The effect becomes catastrophic when MoO 3 forms a molten phase at temperatures

The effect becomes catastrophic when moo 3 forms a

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Rate of volatisation equals rate of corrosion. The effect becomes catastrophic when MoO 3 forms a molten phase at temperatures above 813 ˚C. Stable non-volatile oxides would be expected to remain at metal surface. However, the rate of continued oxidation depends upon several factors: (a)The rate of diffusion of reactants through the oxide film.(b) The rate of supply of oxygen to the outer surface of the oxide.(c)Molar volume ratio of oxide to metal. As the oxidation progresses the stresses are developed in the oxide film. The blisters are formed, the film cracks. The film peels off and the corrosion of the new surface starts. This is one type of breakaway corrosion. 13. CORROSION IN MARINE CONCRETE Corrosion of the reinforcement in marine environment can take different forms, ranging from widespread general corrosion to the localised attack. General corrosion leads to early cracking and spalling of the concrete, often with comparatively little reduction in section of the reinforcing bars whereas localised corrosion may result in severe wastage of the bars before there is any significant damage to the concrete cover. A consideration on the mechanism of corrosion of steel in concrete indicates that four conditions must be fulfilled for intense localised corrosion to occur: (i) Local breakdown of passivity. (ii) A low concrete resistivity and often contact with a conducting environment such as wet soil or water. (iii) Migration of chloride ions to the anodic sites and consequent reduction in p H. (iv) Sufficient overall oxygen transport to the steel to support a high localised corrosion rate. Localised corrosion will be most intense when it is driven by an extensive cathode, i.e., when there is a very large cathode/anode area ratio. Such a large macro-cell can occur only when the resistivity of the environment surrounding the concrete is low. Localised corrosion is also favoured by the presence of chloride ions, as ferrous ions hydrolyse. Fe⁺⁺ + OH Fe(OH)⁺ Fe⁺⁺ + 2OH Fe(OH) 2 Electro-neutrality is maintained partly by migration of chloride ions to the anodic area, which therefore becomes progressively more acidic. The cathodic area retains a high alkalinity, which further localises the corrosion. The remaining requirement for intense localised corrosion is that sufficient oxygen should reach the passive steel cathode. This can occur in intermittent wetting and partial immersion conditions, although it has been found that complete immersion of the concrete in water or water logged soil may limit the flux of oxygen to the passive steel to such an extent that it can no longer act as a significant cathode.
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~ Page 10 of 73 ~ FACTORS WHICH AFFECT THE CORROSION OF METALLIC COMPONENTS (I) STRUCTURAL DESIGN The following factors should be observed during the design stage of a component or assembly to reduce corrosion to a minimum.
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