Rate of film formation

Various finishes can be achieved — gloss, satin (or egg-shell) or matt. This is accomplished by the addition of particles of size 1-5 pm of, for example, silica, china clay or the white pigment, TiCK The degree of mattness depends on various factors, such as particle size, surface treatment of the particles, rate of film formation, and the polymer composition, e.g., urethane/acrylate compared with epoxy/acrylate. The former requires smaller particles larger particles cannot be used as they create a rough surface. 

In the stationary state of anodic dissolution of metals in the passive and transpassive states, the anodic transfer of metallic ions metal ion dissolution) takes place across the film/solution interface, but the anodic transfer of o Q en ions across the Qm/solution interface is in the equilibrium state. In other words, the rate of film formation (the anodic transfer oS metal ions across the metal lm interface combined with anodic transfer of osygen ions across the film/solution interface) equals the rate of film dissolution (the anodic transfer of metal ions across the film/solution interface combined with cathodic transfer of oitygen ions across the film/solution interface). 

An alternative explanation is suggested by some later work of Fleischauer and Bauer °°. They found that the best performance of transfer films of molybdenum disulphide was obtained when molybdic oxide was present in the lowest layers of the film adjacent to the steel surface. Oxidation to molybdic oxide is increased in the presence of moisture, so that reduction of moisture content due to frictional heating may reduce the amount of molybdic oxide present, and thus have a direct adverse effect on transfer film life as well as reducing the rate of film formation. Fleischauer and Bauer also found indications that transfer film life was improved if a slight excess of sulphur was present at the interface between the film and the substrate. The presence of molybdic oxide or excess sulphur are undesirable in the bulk of the lubricant and especially on the sliding surface and they suggested that for optimum... 

Because powder formation can be characterized as the rapid formation of polymeric species in a localized gas phase, the quantity of particles or powders mixed in a coherent film that forms at a substrate surface should also be related to the rate of film formation. Thompson and Smolinsky [8] found a direct correlation between the particle density on the surface of a plasma-polymerized film and the growth rate of the film as depicted in Figure 8.14. 

Assume a defect-free single crystal and a mechanism of oxide film growth by vacancy migration. Thus, the rate of film formation for a single crystal, related to Faraday s law, can be approximated as [13]... 

The kinetics of passivation is normally characterized through Faraday s law for determining the rate of film formation in terms of growth of film thickness according to eq. (6.1). As a cmde approximation, the rate of film formation dxjdt) is related to vacancy diffusion and it is assumed to obey the Arrhenius equation (6.2). In fact, dx/dt increases provided that there exists a net anodic current density i and an overpotential r/, at a distance x from the electrode surface. 

Zinc ions inhibit corrosion by a cathodic polarization mechanism based on the precipitation of a zinc hydroxide film at cathodic sites on the metal surface. Zinc in combination with phosphates will lead to a protective film containing zinc phosphate. Film formation is usually rapid due to the low solubility of the zinc compounds at an alkaline pH. The low solubility of zinc in alkaline solutions requires the incorporation of dispersants. The rate of film formation with cathodic inorganic inhibitors should be carefully controlled, as dangerous fouling may occur. Protective films caused by cathodic inhibition are macroscopic and often easily visible, whereas anodic inhibitors generally from very thin, hardly detectable passive films. 

When there is a passive film on the surface, the situation is somewhat different, as is shown in Fig. 2.2(c). The oxidation, as well as the reduction reaction, occurs at the metal-oxide interface, and the dissolution, which balances the rate of film formation and film breakdown, occurs at the film-solution interface. Since passive films are not very conductive, a large potential drop can occur across a very thin film. The transport of ions across the film is governed by high field conduction (Schmuki, 2002) ... 

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