Films, oxide isolation

Passivity is due to the presence of a thin film which isolates the metal surface from a corrosive aqueous environment. A major impetus for work in neutral solutions came from the research of Nagayama and Cohen [75]. They considered that in the passive region, iron is covered by a thin film of cubic oxide of y Fe203/Fc304. Other compositions and structures were proposed for the passive film, some involving the inclusion of hydrogen or the presence of water [76, 77]. In fact, the composition of the passive film on iron depends on the type of electrochemical parameters during the formation of the film and the nature of solution in which it is formed. In passivity study, it is reported that the passive film on iron is composed of Fe(OH)2, y [78, 79]. 

In general, anodic passivity of metals, regardless of type of corrosion, is associated with the formation of a thin oxide film, which isolates the metal surface from the corrosive environment. Films with semiconducting properties, such as Fe, Ni, Cu oxides, provide inferior protection compared to metals as Al, which has an insulating oxide layer. ... 

Another problem in the construction of tlrese devices, is that materials which do not play a direct part in the operation of the microchip must be introduced to ensure electrical contact between the elecuonic components, and to reduce the possibility of chemical interactions between the device components. The introduction of such materials usually requires an annealing phase in the construction of die device at a temperature as high as 600 K. As a result it is also most probable, especially in the case of the aluminium-silicon interface, that thin films of oxide exist between the various deposited films. Such a layer will act as a banier to inter-diffusion between the layers, and the transport of atoms from one layer to the next will be less than would be indicated by the chemical potential driving force. At pinholes in the AI2O3 layer, aluminium metal can reduce SiOa at isolated spots, and form the pits into the silicon which were observed in early devices. The introduction of a tlrin layer of platinum silicide between the silicon and aluminium layers reduces the pit formation. However, aluminium has a strong affinity for platinum, and so a layer of clrromium is placed between the silicide and aluminium to reduce the invasive interaction of aluminium. 

The analyzer contains three sensitive elements (thin films of zinc oxide) two elements are used in the measurements, whereas the third one is isolated hrom the atmosphere and serves to compensate for time and temperature drifts and to exclude the influence from molecular components of the environment. 

Due to the presence of interactions, the apparent redox potential of a redox couple inside a polyelectrolyte film can differ from that of the isolated redox couple in solution (i.e. the standard formal redox potential) [121]. In other words, the free energy required to oxidize a mole of redox sites in the film differs from that needed in solution. One particular case is when these interations have an origin in the presence of immobile electrostatically charged groups in the polymer phase. Under such conditions, there is a potential difference between this phase and the solution (reference electrode in the electrolyte), knovm as the Donnan or membrane potential that contributes to the apparent potential of the redox couple. The presence of the Donnan potential in redox polyelectrolyte systems was demonstrated for the first time by Anson [24, 122]. Considering only this contribution to peak position, we can vwite ... 

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