Magnesium oxide, passivation

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... 

Beryllium and magnesium are passivated (equation 11.7) and are kinetically inert to O2 and H2O at ambient temperatures. However, Mg amalgam hberates H2 from water, since no coating of oxide forms on its surface Mg metal reacts with steam or hot water (equation 11.8). 

Metals passivation in the resid FCC unit may also be accomplished through the addition of metal traps. Tin, barium titanate, strontium titanate, magnesium oxide, manganese oxide, and specialized zeolite types and contents have all been used for vanadium trapping. In addition, zeolites coated with alumina and catalyst particles coated with rare earth have also been applied to resist vanadium poisoning. Antimony, bismuth, and specialized active high crystalline aluminas have all been used successfully to counteract the negative effect of nickel in the FCC unit. 

Certain metals/alloys - the alkali metals (lidiium, potassium, sodium) and even some metals/ alloys which undergo slow oxidation or are rendered passive in bulk form but which, in the finely divided state, inflame immediately when exposed to oxygen (e.g. aluminium, magnesium, zirconium). 

Titanium, which was in commercial production in 1950, is thermodynamically a very reactive metal (machining swarf can be ignited in a similar fashion to that of magnesium ribbon) but this is offset by its strong tendency to passivate i.e. to form a highly stable protective oxide film. 

Beryllium does not react with water, even when red hot its protective oxide film survives even at high temperatures. Magnesium reacts with hot water (see Fig. 13.22), and calcium reacts with cold water (Fig. 14.21). The metals reduce hydrogen ions to hydrogen gas, but neither beryllium nor magnesium dissolves in nitric acid, because both become passivated by a film of oxide. 

Some metals, such as aluminium or magnesium, cannot function as redox electrodes because of a coating of passivating oxide. Others, such as calcium or lithium, are simply too reactive, and would dissolve if immersed in solution. 

The effect of passivating films on aluminium and magnesium has been the subject of much research. By incorporating chromate/dichromate mixtures and other substances in the electrolyte, a coherent insoluble oxide film is formed which effectively inhibits further corrosion. Sealed cells with aluminium or magnesium anodes may therefore be successfully stored for several years, even at high temperatures. However, once current has been drawn from the cell, the film is broken down and rapid attack on the metal follows due to reactions such as... 

The metallic elements are all potentially very reactive towards air, water and most elements, but Be and Mg form passivating oxide films. Elemental magnesium is manufactured in large... 

Chemical passivity corresponds to the state where the metal surface is stable or substantially unchanged in a solution with which it has a thermodynamic tendency to react. The surface of a metal or alloy in aqueous or organic solvent is protected from corrosion by a thin film (1—4 nm), compact, and adherent oxide or oxyhydroxide. The metallic surface is characterized by a low corrosion rate and a more noble potential. Aluminum, magnesium, chromium and stainless steels passivate on exposure to natural or certain corrosive media and are used because of their active-passive behavior. Stainless steels are excellent examples and are widely used because of their stable passive films in numerous natural and industrial media.6... 

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