Oxide film, defects

On the other hand, pit initiation which is the necessary precursor to propagation, is less well understood but is probably far more dependent on metallurgical structure. A detailed discussion of pit initiation is beyond the scope of this section. The two most widely accepted models are, however, as follows. Heine, etal. suggest that pit initiation on aluminium alloys occurs when chloride ions penetrate the passive oxide film by diffusion via lattice defects. McBee and Kruger indicate that this mechanism may also be applicable to pit initiation on iron. On the other hand, Evans has suggested that a pit initiates at a point on the surface where the rate of metal dissolution is momentarily high, with the result that more aggressive anions... 

For pure A1 in inorganic electrolytes which form barrier oxide films, such as boric acid-borax solution, surface defects ( flows ) have a dominant role.308 It has been shown312 that in this case only, EL vanishes for electropolished samples. 

Most metals acquire an oxide film or scale on their surfaces on exposure to oxygen or air, especially at elevated temperatures.5-7 The kinetics and mechanism of formation of such films provide examples of applications of the concepts of the defect solid state outlined above, and indeed of solid-state kinetics in general.8... 

Even single metals, however, are subject to aqueous corrosion by essentially the same electrochemical process as for bimetallic corrosion. The metal surface is virtually never completely uniform even if there is no preexisting oxide film, there will be lattice defects (Chapter 5), local concentrations of impurities, and, often, stress-induced imperfections or cracks, any of which could create a local region of abnormally high (or low) free energy that could serve as an anodic (or cathodic) spot. This electrochemical differentiation of the surface means that local galvanic corrosion cells will develop when the metal is immersed in water, especially aerated water. 

If desired, plasma oxide films can be doped much as the plasma nitride film we discussed earlier. In fact, doping with boron and phosphorus has been carried out as an alternative to the standard atmospheric-pressure thermal CVD process for BPSG.11 12 The latter process has the drawbacks of high defect density and poor thickness uniformity, so it was hoped that plasma BPSG would be an improvement. However, there are differences in the films in terms of H2 and N2 content, and their effect on reflow temperature, intrinsic stress and passivation effectiveness had to be examined. 

Any electrons which reach the oxide—oxygen interface will be quickly utilized in the formation of 0" ions, and similarly, any positive holes which manage to reach the metal—oxide interface will be quickly annihilated by electrons from the parent metal. The asymmetrical surface reactions thus lead us to expect widely different electron concentrations in the oxide at the two interfaces of the oxide. This difference in electron concentration is equivalent to a difference in the chemical potential for the electronic species at the two interfaces. As in the case of the ionic defect species, such differences in concentration (and chemical potential) can be expected to produce particle currents of the defect species in question. If the primary electronic defects are excess electrons, then we can expect an electron particle current from metal to oxygen if the primary electron defects are the positive holes, then we can expect a positive-hole particle current from oxygen to metal. Of course, an intermediate situation is also possible in which electrons flow from the metal towards the oxygen while simultaneously a positive-hole current flows from the oxygen towards the metal, with recombination [8] (partial or total) occurring within the oxide film. 

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