Aluminum and its alloys

Aluminum is attacked by salts of more noble metals. In particular, aluminum and its alloys should not be used in contact with mercury [7439-97-6] or mercury compounds. 

Aluminum, although highly electropositive, does not react with water under ordinary conditions because it is protected by a thin (2—3 nm) impervious oxide film that rapidly forms even at room temperature on nascent aluminum surfaces exposed to oxygen. If the protective film is overcome by amalgamation or scratching, water rapidly attacks to form hydrous aluminum oxide. Because of the tendency to amalgamate, aluminum and its alloys... 

Softer metals such as aluminum and its alloys can be blast cleaned using abrasives that are not as hard as those used on steel. Garnet, walnut shells, corncobs, peach pits, glass or plastic beads, and soHd carbon dioxide have been used successfully. 

Aluminum and Alloys Aluminum and its alloys are made in practically all the forms in which metals are produced, including castings. Thermal conductivity of aluminum is 60 percent of that of pure copper, and unalloyed aluminum is used in many heat-transfer applications. Its high electrical conductivity makes aluminum popular in electrical apphcations. Aluminum is one of the most workable of metals, and it is usually joined by inert-gas-shielded arc-welding techniques. 

The corrosion resistance of aluminum and its alloys tends to be veiy sensitive to trace contamination. Veiy small amounts of metalhc mer-cuiy, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fuUy acceptable. 

Figure 2-11 shows weight loss rate-potential curves for aluminum in neutral saline solution under cathodic protection [36,39]. Aluminum and its alloys are passive in neutral waters but can suffer pitting corrosion in the presence of chloride ions which can be prevented by cathodic protection [10, 40-42]. In alkaline media which arise by cathodic polarization according to Eq. (2-19), the passivating oxide films are soluble ... 

Other passivating materials suffer pitting corrosion by chloride ions [62] in a way similar to stainless steels (e.g., Ti [63] and Cu [64]). The pitting potential for aluminum and its alloys lies between = -0.6 and -0.3 V, depending on the material and concentration of chloride ions [10,40-42]. 

The main criteria in the selection of aluminum and its alloys for chemical plants are corrosion resistance, ease of fabrication and price. High-quality aluminum grades are used for chemical and process plant applications. 

Aluminum and its alloys are excellent for low temperatures as well as for cryogenic applications because their tensile strength and ductility are increased at low temperatures. 

Clean metallic aluminum is extremely reactive. Even exposure to air at ordinary temperatures is sufficient to promote immediate oxidation. This reactivity is self-inhibiting, however, which determines the general corrosion behavior of aluminum and its alloys due to the formation of a thin, inert, adherent oxide film. In view of the great importance of the surface film, it can be thickened by anodizing in a bath of 15% sulfuric acid (H2SO4) solution or by cladding with a thin layer of an aluminum alloy containing 1 % zinc. 

Certain anions, especially chloride, penetrate the protective films, which are naturally present on some metals (e.g. aluminum and its alloys and stainless steels). This process is an initiator for corrosion, especially for localized corrosion. 

The Alclad alloys have been developed to overcome this shortcoming. Alclad consists of a pure aluminum layer metallurgically bonded to a core alloy. The corrosion resistance of aluminum and its alloys tends to be very sensitive to trace contamination. Very small amounts of metallic mercury, heavy-metal ions, or chloride ions can frequently cause rapid failure under conditions which otherwise would be fully acceptable. When alloy steels do not give adequate corrosion protection—particularly from sulfidic attack—steel with an aluminized surface coating can be used. 

Reacts with sodium and potassium permanganates (oxidizing agents) yielding carbon dioxide, water, and chloride ions. Incompatible with other strong oxidizing agents (e.g., ozone), aluminum and its alloys. 

Fig. 12.57. Correlation between pitting potential and pzc. (Reprinted from J. O M. Bockris and J. Kang, The Pro-tectivity of Aluminum and its Alloys with Transition Metals, J. Solid State Elec-trochem. 1 28,1997, with permission from Springer-Verlag.)...

Bockris and J. Kang, The Protectivity of Aluminum and its Alloys with Transition Metals, J. Solid State Electrochem. 

The values of activation energies obtained from treatment of the experimental data for aluminum and its alloys lie in the range from 0.2 eV to 0.8 eV that allows us to conclude that the chemically bound hydrogen is absent in this alloys. 

S. Wernick, R. Pinner, P. G. Sheasby. The Surface Treatment and Finishing of Aluminum and Its Alloys. 5th ed. 1987, Metals Park, OH. ASM International. 

Tests for aluminum and its alloys consist of intergranular corrosion, stress corrosion cracking and corrosion fatigue as detailed below ... 

Fresh water. Aluminum and its alloys are not prone to corrosion on exposure to distilled water up to 180°C. Thus storage tanks, piping and fittings of the alloy can be used for handling distilled water. The composition of natural fresh water is variable. In spite of this restriction, the alloys are not attacked, even at 180°C in natural waters. It should be noted that pitting may occur when a small thickness of the sample is exposed. In this case Alclad 3003 is recommended for use to avoid failure due to pitting. 

The performance of aluminum and its alloys in some environments is given below. 

Aluminum and its alloys are inert to dry gases. Moist S02 gives rise to sulfurous acid, which is corrosive Moist H2S, is not corrosive moist C02 is also not corrosive... 

The various forms of corrosion encountered in the case of aluminum and its alloys are ... 

Corrosion inhibitors such as chromates, silicates, polyphosphates, nitrites, nitrates, borates and mercaptobenzothiazole have been used in corrosion inhibition of aluminum and its alloys.45... 

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. 

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