Corrosion mechanism diffusion

Film Adhesion. The adhesion of an inorganic thin film to a surface depends on the deformation and fracture modes associated with the failure (4). The strength of the adhesion depends on the mechanical properties of the substrate surface, fracture toughness of the interfacial material, and the appHed stress. Adhesion failure can occur owiag to mechanical stressing, corrosion, or diffusion of interfacial species away from the interface. The failure can be exacerbated by residual stresses in the film, a low fracture toughness of the interfacial material, or the chemical and thermal environment or species in the substrate, such as gases, that can diffuse to the interface. 

The temperature dependence of corrosion rate is given by the temperature dependence of all the parameters mentioned above and participating in the corrosion process. The main roles are played by the temperature dependence of the diffusion coefficient and that of viscosity which determines the convection rate. Solubility and the other characteristics are of lesser significance. As the parameters involved do not have the same temperature coefficienis, the activation energy evaluated directly from the corrosion kinetics is not reliable for interpretation of the corrosion mechanism. 

The small slope of the stage II section of the crack-growth rate versus K curve is attributed to corrosion-related, diffusion-controlled processes in the crack. Steady-state diffusion mechanisms are required to account for the fact that the crack growth rate is essentially constant... 

The diffusion constant D is determined by the concrete quality. At the carbonation front there is a sharp drop in alkalinity from pH 11-13 down to less than pH 8. At that level the passive layer, which we saw in Chapter 2 was created by the alkalinity, is no longer sustained so corrosion proceeds by the general corrosion mechanism as described in the Chapter 2. 

During low temperature oxidation, oxide films grow by high-field conduction (Section 8.1) rather than by solid-state diffusion because the value of the diffusion coefficients is too small. Under these conditions the thickness of the oxide layer does not exceed a few nanometers. In contrast, in high temperature corrosion, volume diffusion and grain-boundary diffusion are the principle transport mechanisms by which oxide layers grow. As a consequence their thickness can reach much larger values. 

The corrosion mechanism entails the simple depletion of a passivating specie in the corrosive. This mechanism applies equally to nonconductive and electrically conductive ceramics. The passivating specie within the crevice is consumed by the corrosion reaction, making the crevice environment more corrosive than the bulk solution. Replacement of the passivating specie by diffusion is prevented by the occluded nature of the crevice therefore, the environment within the crevice remains more corrosive. The crevice grows as corrosion proceeds forming additional corrosion product, which further restricts diffusion. 

The corrosion mechanism for weathering steels is similar to that of unalloyed carbon steels. The rust forms a more dense and compact layer on the weathering steels than on unalloyed carbon steels. The rust layer more effectively screens the steel surface from the corrosive envirorunents of the atmosphere. The corrosion process may be affected in several ways by this rust layer. The cathodic reaction may be affected by the low diffusion rate of oxygen, whereas the anodic reduction may be retarded by limiting the supply of water and corrosion-stimulating ions that can reach the surface of the steel. In addition, the increased electrolyte resistance may also decrease the corrosion rate. 

While acid corrosion in glass fibers is diffusion-controiied and therefore /f-kinetics are expected, the process in aqueous and aikaiine soiutions is considered much more complicated because of the many influencing factors. The reaction kinetics depends on the (local) pH value. It is conventional opinion that, with the switch from a diffusion-controlled corrosion mechanism to an interfacial-controlled mechanism, a rapid shift from ft-to t-kinetics takes place, and the process follows linear t-kinetics except for short exposure times and low temperatures. However, in the literature, dependencies on t are also found, with values for a varying between 0.5 and 1 [819]. The chemical stability of glass fibers under alkaline attack is also significantly influenced by insoluble corrosion or reaction products on the fiber surface. 

High Ni alloys used either as base or welding filler metals are often used to resist carburizing conditions. Ni slows the diffusion of carbon in alloys, which is important because carburization is essentially a corrosion mechanism limited by the rate of carbon diffusion in the alloy. However, carburization of high Ni alloys can be especially rapid and 5deld rates greater than 2.5 mm y 1, if the temperature exceeds 980°C. 

The environment may be either acidic or basic. A technique using the eifects of diffused hydrogen on current flow between electrodes in a vacuum has been used to study corrosion mechanisms and should be valuable for inhibitor studies, because the sensitivity of this method is considerably better than that of the pressure build-up technique. 

This phenomenon is usually observed for experiments carried out on bare metals immersed in corrosive solutions, showing that the corrosion mechanism is under diffusion control [26,27]. 

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