Passive oxidation diffusion control

Alex is an -> aluminum powder formed by explosion of electrically heated aluminum wires in inert atmospheres with particle sizes between 50 and 200 nm. Due to a passivation layer of a thickness between 2 to 4 nm a substantial amount of the particles is already converted to alumina the formation of which should be avoided by in-situ coating. In addition to the diffusion controlled oxidation at lower temperatures, a partial oxidation of the particles can occur by a fast chemically controlled reaction. Alex can increase the burning rate of solid composite rocket propellants up to a factor of 2. An increase of detonation velocity is not confirmed but Alex might improve -> air blast or fragment velocities of some high explosives. 

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. 

The parabolic rate law is applicable when the controlling step is by diffusion of the oxide-forming metal through a barrier such as an oxide or passive film. The oxidation rate is inversely prop)ortional to time and approaches a slow constant rate after finite time. This is the most common type of high-temperature oxidation found on engineering materials. 

Electropolishing techniques utilise anodic potentials and currents to aid dissolution and passivation and thus to promote the polishing process in solutions akin to those used in chemical polishing. The solutions have the same basic constitution with three mechanistic requirements—oxidant (A), contaminater (B) and diffusion layer promoter (C) —but, by using anodic currents, less concentrated acid solutions can be used and an additional variable for process flexibility and control is available. 

Figure 11-15 shows the corrosion rate observed for a metallic nickel electrode in aerated aqueous sulfate solutions as a function of pH. In addic solutions, nickel corrodes in the active state at a rate which is controlled by the diffusion of hydrated oi en molecules (oxidants). In solutions more basic than pH 6, however, nickel spontaneously passivates by hydrated oiQ n molecules and corrosion is negligible. As shown in the inserted sub-figures in Fig. 11-15, the maximum current of anodic nickel dissolution in the active state is greater in the range of addic pH however, the Tnaximnm current of anodic nickel dissolution is smaller in the range of basic pH than the current of cathodic reduction of os en molecules (dashed curve) which is controlled by the diffusion of hydrated oiQ gen molecules. Consequently, metallic nickel remains in the active state in addic solutions but is spontaneously passivated by hydrated ojQ n molecules in basic solutions. It... 

Rust of iron (the most abundant corrosion product), and white rust of zinc are examples of nonprotective oxides. Aluminum and magnesium oxides are more protective than iron and zinc oxides. Patina on copper is protective in certain atmospheres. Stainless steels are passivated and protected, especially in chloride-free aqueous environments due to a very thin passive film of Cr2C>3 on the surface of the steel. Most films having low porosities can control the corrosion rate by diffusion of reactants through the him. In certain cases of uniform general corrosion of metals in acids (e.g., aluminum in hydrochloric acid or iron in reducible acids or alkalis), a thin him of oxide is present on the metal surface. These reactions cannot be considered hlm-free although the him is not a rate-determining one.1... 

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... 

One example is the hot corrosion of a preoxidized nickel specimen by a thin Na2S04 melt film in a 0.1 wt. % SO2-O2 gas mixture at 1200 K [29]. By variation of the oxide scale thickness and the purity of the material, different regimes of corrosion were investigated passive state, pseudopassive state, and active state. The passive state of 99.9975% of pure nickel, preoxidized in pure O2 for 2 h at 1200 K is controlled by diffusion of 8207 in the salt melt. The corresponding Nyquist plot of impedance data shows linear behavior in the low-frequency range withaslope of45° (Fig. 16).The semicircle at higher frequencies was attributed to the resistance of the NiO layer itself The active state was established on less pure nickel... 

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