Oxygen diffusion-convection

Eventually oxygen becomes depleted in the crevice. Replenishment of oxygen by convective transport cannot occur since the crevice is too tight to allow water to move in and out freely. Also, oxygen diffusion... 

It is appropriate to consider first the crevice corrosion of mild steel in oxygenated neutral sodium chloride, and then to consider systems in which the metal is readily passivated. Initially, the whole surface will be in contact with a solution containing oxygen so that attack, with oxygen reduction providing the cathodic process, occurs on both the freely exposed surface and the surface within the crevice (Fig. 1.50). However, whereas the freely exposed surface will be accessible to dissolved oxygen by convection and diffusion, access of oxygen to the solution within the crevice can occur only... 

In the film oxygen diffuses to the flame front, and combustion products and heat are transported to the surrounding atmosphere. The position of the boundary designated by °o is determined by convection. A stagnant atmosphere places the boundary at an infinite distance from the fuel surface. 

Polarization of the galvanic cell. The different phenomena of polarization of the anodic and cathodic reactions (activation, diffusion, convection, etc.), should be well known as a function of the evolution and change of the properties of the interface as a function of time. The polarization behavior of the cathodic and anodic reactions on the two electrodes should be examined (see Figure 6.5). In natural atmospheres, the cathodic reaction controls frequently the attack rate. The diffusion of oxygen is an important parameter to avoid control and polarization of the corrosion by the rate of the cathodic reaction (Figure 6.12).7... 

Humidified air containing molecular oxygen is convectively transported across the cathode plate area to the back side of the gas diffusion layer forming the electrode interface. 

Mass transport-control Mass transport-controlled corrosion implies that the rate of corrosion is dependent on the convective mass-transfer processes at the metal/fluid interface. When steel is exposed to oxygenated water, the initial corrosion rate will be closely related to the convective flux of dissolved oxygen toward the surface, and later by the oxygen diffusion through the iron oxide layer. Corrosion by mass transport will often be streamlined and smooth. 

Exceptions to these rules of thumb abound, however. For example, although diffusion is faster at elevated temperatures, dissolved oxygen concentration may be lower. Convective transport is stimulated by high temperatures, but increased... 

Slime is a network of secreted strands (extracellular polymers) intermixed with bacteria, water, gases, and extraneous matter. Slime layers occlude surfaces—the biological mat tends to form on and stick to surfaces. Surface shielding is further accelerated by the gathering of dirt, silt, sand, and other materials into the layer. Slime layers produce a stagnant zone next to surfaces that retards convective oxygen transport and increases diffusion distances. These properties naturally promote oxygen concentration cell formation. 

Oxygen from the atmosphere, dissolved in the electrolyte solution provides the cathode reactant in the corrosion process. Since the electrolyte solution is in the form of thin films or droplets, diffusion of oxygen from the atmosphere/electrolyte solution interface to the solution/metal interface is rapid. Moreover, convection currents within these thin films of solution may play a part in further decreasing concentration polarisation of this cathodic process . Oxygen may also oxidise soluble corrosion products to less soluble ones which form more or less protective barriers to further corrosion, e.g. the oxidation of ferrous species to the less soluble ferric forms in the rusting of iron and steel. 

The impact of the diffusion coefficient was also analysed here values of isoprene and oxygen were taken [85], It was found that the RTD of one component in a cycling experiment may be narrow, while the other component is not at optimum. By changing the channel diameter, such a sequence may even be reversed. It is advised [85] to operate in the diffusion-dominated branch rather than in the convection-dominated branch. 

For the well-mixed continuous-flow liquid phase shown in Fig. 1, the balance equations for oxygen and substrate must account for the supply of each component both by convective flow and by gas-liquid transfer, as well as by the diffusion rate into the biofilm. 

The uniformflux of oxygen, S, into the fluid along fhe Pt surface and zero flux along the Au surface provide boundary conditions for the convection-diffusion equation. 

---