Aluminum alloys protection

Lead—Calcium—Aluminum Alloys. Lead—calcium alloys can be protected against loss of calcium by addition of aluminum. Aluminum provides a protective oxide skin on molten lead—calcium alloys. Even when scrap is remelted, calcium content is maintained by the presence of 0.02 wt % aluminum. Alloys without aluminum rapidly lose calcium, whereas those that contain 0.03 wt % aluminum exhibit negligible calcium losses, as shown in Figure 8 (10). Even with less than optimum aluminum levels, the rate of oxidation is lower than that of aluminum-free alloys. 

Aluminum and aluminum alloys are employed in many appHcations because of the abiHty to resist corrosion. Corrosion resistance is attributable to the tightly adherent, protective oxide film present on the surface of the products. This film is 5 —10 nm thick when formed in air if dismpted it begins to form immediately in most environments. The weathering characteristics of several common aluminum alloy sheet products used for architectural appHcations are shown in Eigure 30. The loss in strength as a result of atmospheric weathering and corrosion is small, and the rate decreases with time. The amount of... 

The resistance to corrosion of some alloy sheet is improved by cladding the sheet with a thin layer of aluminum or aluminum alloy that is anodic to the base alloy. These anodic layers are typically 5—10% of the sheet thickness. Under corrosive conditions, the cladding provides electrochemical protection to the core at cut edges, abrasions, and fastener holes by corroding preferentially. Aircraft skin sheet is an example of such a clad product. 

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 protection of aluminum ships demands particular attention since electrical contact with steel and copper materials can seldom be avoided and a whole range of aluminum alloys are unsuitable for cathodic protection (see Section 2.4 and Fig. 2-11). Later protective measures must therefore be observed during the construction stage since even good coatings in combination with cathodic protection are frequently not adequate to protect gaps or openings. 

Aluminum appears to be resistant to corrosion from SO2 at ambient air concentrations. Aluminum alloys tend to form a protective surface film... 

Table I shows the chemical composition limits of various aluminum alloys presently used for packaging applications (3). In general, these alloys have good corrosion resistance with most foods. However, almost without exception, processed foods require inside enameled containers to maintain an acceptable shelf life (4, 5). Moreover, when flexible foil packages are used for thermally processed foods, the foil is laminated to plastic materials that protect it from direct contact with the food and also provide heat sealability as well as other physical characteristics (6,7).

There are many corrosion mechanisms which can take place in today s engines. The complexity has increased with the use of aluminum alloys in the head and block. Aluminum used in areas such as the head, where large quantities of heat are liberated to the coolant, is subject to a unique heat rejection corrosion. To protect against the heat rejection corrosion of aluminum, a coolant having special corrosion inhibitor systems must be used. 

Select the engine coolant or antirust with care to ensure adequate corrosion protection. Use only those products conforming to recognized standards such as ASTM D 3306 for engine coolant. Additionally, if your vehicle engine contains major cast aluminum components, be certain the engine coolant meets the heat rejection corrosion limits in ASTM specification D 3306 as determined by ASTM Test Method for Corrosion of Cast Aluminum Alloys in Engine Coolants Under Heat-Transfer Conditions (D 4340). 

Aluminum cladding is an alloying process used to help prevent surface corrosion of other underlying metal components. This process involves hot rolling metal to produce a protective aluminum barrier. Clad aluminum alloys can be found in some heat exchanger tubing applications. 

Metals such as aluminium, steel, and titanium are the primary adherends used for adhesively bonded structure. They are never bonded directly to a polymeric adhesive, however. A protective oxide, either naturally occurring or created on the metal surface either through a chemical etching or anodization technique is provided for corrosion protection. The resultant oxide has a morphology distinct from the bulk and a surface chemistry dependent on the conditions used to form the oxide 39). Studies on various aluminum alloy compositions show that while the oxide composition is invariant with bulk composition, the oxide surface contains chemical species that are characteristic of the base alloy and the anodization bath40 42). 

It should protect metal, especially aluminum, from any deleterious effects when exposed to a flame at a temperature of 2000 F. for at least 30 minutes. Thus, for 24S-T3 aluminum alloy the maximum metal temperature should be 350 F. 

Inhibiting the corrosion of aluminum alloys by adding 1-5% of transition metals is a dramatic case of corrosion protection because of the small amounts of additives that are successful in reducing the corrosion rate by 1-2 orders of magnitude. It turns out that the alloying materials shift the pzc toward the positive side on the potential scale. Thus, in many practical situations, the alloys of the transition metals are in a... 

For low-pressure downstream equipment, 316L and 304 stainless steels are good construction materials. Aluminum alloys and piping are quite resistant to the corrosive attack by urea due to the protective oxide film. These alloys can be used in low-pressure piping, floor gratings, hand rails, etc.88... 

Chromate conversion coatings are used widely on aluminum alloys as a pretreatment for painting, though in some applications, where noncondensing atmospheric exposure is expected, they may be used as the primary means of corrosion protection. Chromate conversion coatings are used on magnesium, cadmium, and zinc, and on galvanized steel to suppress the formation of white rust. 

Aluminum spraying is used to coat less corrosion-resistant alloys. In the case of some composites, corrosion is due to the galvanic action between the aluminum matrix and the reinforcing material. Aluminum thermal spraying has been successfully used for the protection of the discontinous silicon carbide/aluminum composites, and continuous graphite/aluminum. Other protection procedures include sulfuric acid anodizing and iron vapor deposition on aluminum.44... 

Chemically, the film is a hydrated form of aluminum oxide. The corrosion resistance of aluminum depends upon this protective oxide film, which is stable in aqueous media when the pH is between about 4.0 and 8.5. The oxide film is naturally self-renewing and accidental abrasion or other mechanical damage of the surface film is rapidly repaired. The conditions that promote corrosion of aluminum and its alloys, therefore, must be those that continuously abrade the film mechanically or promote conditions that locally degrade the protective oxide film and minimize the availability of oxygen to rebuild it. The acidity or alkalinity of the environment significantly affects the corrosion behavior of aluminum alloys. At lower and higher pH, aluminum is more likely to corrode. 

Metallic Pigments. These pigments are metal flakes of various sizes made up of aluminum, copper, zinc, and/or their alloys. The aluminum flakes develop the silver metallic colors, whereas the copper, zinc, and aluminum alloys produce the gold, copper, brass, and bronze colors. The metal effect is modified, not only by shape and size, but also by perticle size distribution within a given product. Since aluminum, in particular, as a fine dry powder, can form explosive mixtures with air, most of these materials are commercially available in paste or liquid concentrates. In addition, surface treatments of these materials enhance their appearance and performance. Flakes of other metals, such as stainless steel, are used for surface protection purposes such as corrosion resistance and electrical conductivity. 

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