Passive films field strength

The most obvious difference between pore walls and pore tips is their different geometry. For many porous samples the radius of the pore becomes minimal at the pore tip. This produces a maximum of the electric field strength and a minimum of the SCR width at the tip. This is even true if the radius of curvature is constant, due to the transition from the cylindrically curved pore wall to the spherical pore tip. As a result, electrical breakdown of a passive film or an SCR preferably occurs at the pore tip. The breakdown current promotes dissolution, and the pore grows. Junction breakdown is discussed in Chapter 8, which describes the growth of mesopores. 

The rate of the passive film breakdown at Al-Ta is one-tenth that of Al at the same field strength within the oxide. But that is not what would be expected. The concentration of the alloying element is too small (1 place in 20) to retard the... 

Combining the results of Sects. 3.2.2.7 and 3.2.2.8, it becomes clear that ions and electrons move in passive films dependent on the electrode potential and electric field strength respectively, and the history of the film. Therefore, neither the vertical nor the horizontal axis of Fig. 8 can describe the conductivity of all oxides. Many oxides... 

Figure 10.14 Dependence of the thickness of the passive film on iron on the potential. The field strength in the film is If the potential jumps from F, to Fj the field strengths increase as well...

In the ideal case of a passivation that is free of pin-holes and cracks, a film of water which forms on the surface of the die will only come into contact with the metallisation layer at the windows cut to permit bonding. Fortunately, this is the region where there is least risk of failure due to electrochemical corrosion, firstly because the field strengths are lower than for the fine metal tracks over the rest of the... 

Passivation is generally believed to take place by the rapid formation of surface-adsorbed hydrated complexes of metals, which are sufficiently stable on the alloy surface that further reaction with water enables the formation of a hydroxide phase that, in turn, r idly deprotonates to form an insoluble surface oxide film. Failure in any of these stages would lead to continued active dissolution. The passivation potential is critical to this process, in part because it governs the oxidation state of the metal, which in turn governs its solubility. In addition, the electric field strength has to be sufficient to cause deprotonation of the surface hydroxide phase in order to enable the oxide barrier film to become estabilished. No evidence has been found by surface studies that passivity of austenitic stainless steels is possible by formation of a simple hydroxide film. It is peihaps surprising that the passive film formed on austenitic stainless steels does not always contain each of the alloying elements added to stabilize the austenitic phase, even when such additions appear to improve the chemical stability of the steel. Ni exemplifies this behavior. 

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