Aluminum protective oxide coating

The ratio of the oxide formed to the metal consumed is called the Piling and Bedworth number. When the number is over 1, the metal rusts. Aluminum and magnesium are the best examples of metals that do not rust because a protective oxide coat forms that is, they have a Piling to Bedworth number of 1. Scratch an aluminum ladder and notice a bright fissure forms and quickly self-coats. The heat release in the sealing aluminum oxide is dissipated to the ladder structure. 

Group 13 React slowly. Aluminum forms an oxide coating that protects the metal the compound AI2O3 can act as a base or an acid. A1 displaces H2(g) from steam. A1 reacts readily if the protective oxide coating is removed, to generate H2(g). 

The chemical resistance of aluminum is fair, but any chemical which destroys the protective oxide coating will lead to rapid corrosion therefore, strong acids, alkalies, and mercury must be kept out of contact with aluminum. 

Chemidize. [Witco] Chemical for pre-treating and forming protective oxide coating on aluminum and aluminum alloy surfaces. 

Some metals, such as aluminum and chromium, form a thin protective oxide coating that prevents further corrosion... 

An oxide coating forms on the surface of aluminum particles, which tends to protect it against chemical action. This oxide layer is also formed on magnalium, but is less protective. (In alloys of less than 30% aluminum, the oxide coating s effectiveness has been significantly reduced.) In terms of safety ... 

Aluminum pacification — 0.5% Boric acid (Strengthens protective oxide coating.)... 

Corrosion involves the oxidation of metals to form mainly oxides and sulfides. Some metals, such as aluminum, form a thin protective oxide coating that inhibits their further corrosion. Corrosion of iron can be prevented by a coating (such as paint), by alloying, and by cathodic protection. 

These alloys are of vital importance in the construction of modern aircraft and rockets. Aluminum, evaporated in a vacuum, forms a highly reflective coating for both visible light and radiant heat. These coatings soon form a thin layer of the protective oxide and do not deteriorate as do silver coatings. They are used to coat telescope mirrors and to make decorative paper, packages, toys. 

Aluminum containers are recommended for many appHcations because of the very hard, corrosion-resistant oxide coating. They are deficient in only one respect once the protective skin has been penetrated, aluminum corrosion accelerates. 

In some cases, the oxide-coating protects the surface from further oxide buildup. One example is that of aluminum where an oxide coating appears almost instantaneously once the pristine surface is exposed to air. Yet, there are many cases where the oxide layer continues to buildup until the metal is totally consumed (One example is that of iron and "rust"). How is this possible Wagner hypothesized that both metal and oxide ions difiosed through the metal oxide layer so as to build up the layer thickness from both sides. The following diagram is one representation of such a mechanism ... 

Aluminum surfaces are readily oxidized by the oxygen in air, and a tight surface coating of aluminum oxide (A12O 3) is formed that protects the inner metal from further oxidation. Hence, aluminum powder can be stored for extended periods with little loss of reactivity due to air oxidation. Metals that form a loose oxide coating on exposure to air - iron, for example - are not provided this surface protection, and extensive decomposition can occur during storage unless appropriate precautions are taken. 

Because corrosion is electrochemical, we can use our knowledge of redox reactions to combat it. The simplest way to prevent corrosion is to protect the surface of the metal from exposure to air and water by painting. A method that achieves greater protection is to galvanize the metal, which involves coating it with an unbroken film of zinc (Fig. 12.16). Zinc lies below iron in the electrochemical series, so if a scratch exposes the metal beneath, the more strongly reducing zinc releases electrons to the iron. As a result, the zinc, not the iron, is oxidized. The zinc itself survives exposure on the unbroken surface because, like aluminum, it is passivated by a protective oxide. 

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