Passive oxidation mixed control

Mixed Control The interfacial reaction and diffusion processes in the passive oxidation of Si are tightly coupled. Thus, it is better not to consider them in isolation, but instead to consider them together. Since these reactions occur in series, their rates must be equal. In other words. 

XPS study by Buckley and Woods (1984b) showed that freshly fractured chalcopyrite surfaces exposed to air formed a ferric oxyhydroxide overlayer with an iron-deficient region composed of CuSi. Acid-treated surfaces of fractured chalcopyrite showed an increase in the thickness of the CuS2 layer and the presence of elemental sulfur. Hackl et al. (1995) suggested that dissolution of chalcopyrite is passivated by a thin (< 1 pm) copper-rich surface layer that forms as a result of solid-state changes. The passivating surface layer consists of copper polysulhde, CuS , where n > 2. Hackl et al. (1995) described the dissolution kinetics as a mixed diffusion and chemical reaction whose rate is controlled by the rate at which the copper polysulhde is leached. The oxidation of chalcopyrite in the presence of ferric ions under acidic conditions can be expressed as... 

Figure 3. Mixed potential diagram illustrating controls on the kinetics of corrosion at a pitted, oxide-covered metal. The potential range is from -700 to +300 mV/NHE. Arrows (B) corrosion current at the bottom of the pit, controlled by Fe Fe + (acid) and 2H - H2 (M) corrosion current at the mouth of the pit, controlled by the partial currents for Fe -> Fe2+ (passivated) and RX RH (Pit) corrosion current for the short-circuited pit, controlled by Fe Fe + (acid) and RX - RH. The three solid curves are generated using the Tafel equation and exchange current densities and Tafel slopes from reference (9). The dashed curve was measured at 5 mV s in pH 8.4 borate buffer, using methods described in reference (9).

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