Aluminum alloys corrosion processing

Refractories in the Aluminum Industry. Carbon materials are used in the HaH-Heroult primary aluminum cell as anodes, cathodes, and sidewalls because of the need to withstand the corrosive action of the molten fluorides used in the process (see Aluminumand aluminum alloys). 

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).

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. 

Annually, sessions are held on advanced aluminum alloys as part of the conference on Advanced Aerospace MaterialsAProcesses and Exposition." Topics discussed have included rapid-solidilication alloys, very low density alloys, laminates, new alloying elements, such as lithium, and such processes as diffusion bonding and means to improve fracture toughness and reduce fatigue and corrosion. 

Steel sheets are aluminized by a hot-dip process similar to galvanizing. The principal applications for such a product are furnaces and ovens, automobile mufflers, and other equipment requiring heat and corrosion resistance. When a sheet which has been coated with aluminum by a hot-dip process is exposed to a temperature over l,000°F (538aC). the aluminum forms an iron-aluminum alloy which is heat- and corrosion-resistant. 

Aluminum and Aluminum Alloys. Aluminum can be employed in sea water as a resistant material of construction. Experiments at Fort Bel voir, Virginia, and elsewhere, indicate that by proper corrosion-control practices, aluminum can be used for an entire plant which processes sea water. The sea water entering the plant should be free of all metallic ions, especially copper or nickel. It is essential, in such a plant, that no copper-base alloys be used at all and that galvanic couples to most other metals be avoided. 

A number of techniques have been developed to convert corrosion-prone, clean surfaces to less reactive ones. Three common conversion processes are phosphating, anodizing, and chromating. These processes remove the inconsistent, weak surface on metal substrates and replace it with one that is strong, permanent, and reproducible.58 Figure 15.18 shows the effect of various pretreatments on the durability of aluminum alloy-epoxy joints subjected to aging in water at 50°C. 

Aluminum pistons in an engine that bums H2 will be exposed to not only H2 but also H2O at temperatures of 80 to 120°C. Aluminum alloys can be totally immune to H2 embrittlement and H2-induced crack growth if the natural AI2O3 oxide is intact. However, there are processes that can disrupt this film, and it is known that aluminum alloys will absorb H2 when exposed to H2O vapor at 70°C. There will also be periods when the engine is cool and condensed water will be present so that aqueous corrosion could occur, but this is not expected to be any different than with an engine with cast aluminum pistons that bums gasoline. 

An ultrathin layer of plasma polymer of trimethylsilane (TMS) has been utilized in the corrosion protection of aluminum alloys by means of system approach interface engineering (SAIE) [1 ]. SAIE by means of low-temperature plasmas utilizes low-temperature plasma treatment and the deposition of a nanolilm by luminous chemical vapor deposition (LCVD). This approach does not rely on the electrochemical corrosion-protective agents such as six-valence chromium, and hence the process is totally environmentally benign. 

Besides the advantageous features described earlier, DC cathodic plasma polymerization of TMS mixed with argon also provides an opportunity to combine the two processes of TMS deposition and second plasma treatment into a single step. TMS plasma coating thus produced also maintains excellent corrosion protection properties on the aluminum alloy substrates. 

System approach interface engineering (SAIE) by means of low-pressure plasma in corrosion protection of metals was first applied on cold-rolled steel (CRS) [1-3]. Unlike aluminum alloys described in the previous chapters, oxides on the surface of steel and iron are less stable than those on aluminum alloys, i.e., iron oxides formed on the surface do not have corrosion protecting capability of iron underneath and hence are more vulnerable for corrosion. On the other hand, the oxides on the surface of steel can be removed by plasma treatment, particularly of (Ar- -H2) plasma. Hence, SAIE could be performed in totally dry processes. The improvement of corrosion protection by means of environmentally benign process was the motivation for the approach. 

Use Ferroalloys (steel manufacture), nonferrous alloys (improved corrosion resistance and hardness), high-purity salt for various chemical uses, purifying and scavenging agent in metal production, manufacture of aluminum by Toth process. 

The corrosion resistance of stainless steels and nickel-based alloys in aqueous solutions can often be increased by addition of chromium or aluminum. " Chromium protects the base metal from corrosion by forming an oxide layer at the surface. Chromium is also considered to be an important alloying metal for steels in MCFC applications. Chromium containing stainless steel, however, leads to the induced loss of electrolyte. Previous studies done to characterize the corrosion behavior of chromium in MCFC conditions have shown the formation of several lithium chromium oxides by reaction with the electrolyte. This corrosion process also results in increased ohmic loss because of the formation of scales on the steel. Aluminum additions similarly have a positive effect on corrosion resistance. " However, corrosion scales formed in aluminum containing alloys show low conductivity leading to a significant ohmic polarization loss. 

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