Surface oxide film, aluminum-based alloys

When considering zinc-aluminum alloys, the surface oxide film normally present is likely to reduce any corrosion current. The risk of bimetallic corrosion is small in atmospheric exposure trials by Noranda have been in progress since 1984 on ZA alloys coupled to other common metals. No visual effects were noted at the 5-year examination (Barmhurst and Belisle, 1992). A zinc-25% aluminum-0.05% magnesium alloy coupled to other materials and exposed on the Noranda Research Center roof showed pitting attack on the zinc-based material (but only up to 0.38 mm deep in 10 years) when joined to copper, brass, or steel, but less when joined to stainless steel or lead and least when joined to aluminum. 

Zinc however, does not produce a surface oxide film that is as effective a barrier as the oxide film on aluminum. The original barriers of zinc and zinc alloy coatings result from electrochemical properties based on the structure of the coating layer. 

An especially insidious type of corrosion is localized corrosion (1—3,5) which occurs at distinct sites on the surface of a metal while the remainder of the metal is either not attacked or attacked much more slowly. Localized corrosion is usually seen on metals that are passivated, ie, protected from corrosion by oxide films, and occurs as a result of the breakdown of the oxide film. Generally the oxide film breakdown requires the presence of an aggressive anion, the most common of which is chloride. Localized corrosion can cause considerable damage to a metal stmcture without the metal exhibiting any appreciable loss in weight. Localized corrosion occurs on a number of technologically important materials such as stainless steels, nickel-base alloys, aluminum, titanium, and copper (see Aluminumand ALUMINUM ALLOYS Nickel AND nickel alloys Steel and Titaniumand titanium alloys). 

The corrosion resistance of aluminum and At alloys is largely due to the protective oxide film, which can attain a thickness of about 10 A within seconds on a freshly exposed aluminum surface [8]. A good corrosion protection system should include protection of the oxide layer and, in addition, should provide a good adhesive base for subsequent paint. The conventional corrosion protection system of aluminum... 

Corrosion. Aluminum is a not a noble metal and is attacked by both alkali and acidic solutions. Because of the presence of a surface A1203 film, the metal is protected against corrosion [Diggle et al.136, Borgmann et al.137]. This oxide film, however, is easily penetrated, for instance, by the presence of chlorine ions which remain in the resist after a chlorine based plasma etch. Also, the presence of Cu in the aluminum weakens the corrosion resistance of the alloy by the presence of an unfavorable electrochemical couple (A1/Cu2+). 

Metals in the passive state (passive metals) have a thin oxide layer on their surface, the passive film, which separates the metal from its environment. Metals in the active state (active metals) are film free. Most metals and alloys that resist well against corrosion are in the passive state stainless steel, nickel-chromium based superalloys, titanium, tantalum, aluminum, etc. Typically, the thickness of passive films formed on these metals is about 1-3 nm. 

Metallized metal, polymer and carbon. Types la, lb, and Ic, are variants of the solid metal fibers and are distinguished therefrom by a metal layer upon the base fiber s periphery. They are fabricated by electrochemical deposition or grafting of a suitable metal, such as nickel, copper, aluminum, and their alloys, as a thin layer upon the fiber s surface. In general, these variants evolved in attempts to improve upon one, or more, properties of the Type 1 fibers. Applications of metal-on-metal. Type la, are typified by the structures described in 1972 by McNab(26) who used refractory, non conducting, base fibers for example, aluminum oxide and boron nitride, upon which were deposited films of noble metals. McNab s objective was to improve upon the strength and flexibility of Type 1 fibers by selecting a base fiber for its mechanical... 

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