Corrosion of aluminum and its

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. 

As with other active-passive-type metals and alloys, the pitting corrosion of aluminum and its alloys results from the local penetration of a passive oxide film in the presence of environments containing specific anions, particularly chloride ions. The oxide film is y-Al203 with a partially crystalline to amorphous structure (Ref 13, 59). The film forms rapidly on exposure to air and, therefore, is always present on initial contact with an aqueous environment. Continued contact with water causes the film to become partially hydrated with an increase in thickness, and it may become partially colloidal in character. It is uncertain as to whether the initial air-formed film essentially remains and the hydrated part of the film is a consequence of precipitated hydroxide or that the initial film is also altered. Since the oxide film has a high ohmic resistance, the rate of reduction of dissolved oxygen or hydrogen ions on the passive film is very small (Ref 60). 

L. Tomcsanyi, K. Varga, I. Bartik, G. Horanyi, and E. Maleczki, Electrochemical Study of the Pitting Corrosion of Aluminum and Its Alloys II, Study of the Interaction of Chloride Ions with a Passive Film on Aluminum and Initiation of Corrosion, Electrochim. Acta, Vol 34, 1989, p 855-859... 

Some of the voluminous literature on the oxidation and corrosion of aluminum and its alloys has a direct bearing on DMO. Pure aluminum is normally covered by an amorphous native oxide film which is partially converted to 7-alumina at the interface between the parent metal and the amorphous oxide when heated to 450°C in dry air [7-9]. This unusual behavior is explained by growth of the amorphous phase, through outward cation migration, while thickening of the 7-alumina is by epitaxial growth on the parent metal, controlled by inward oxygen diffusion. Termination... 

Guy D. Davis is a staff scientist and group leader of the Surface Sciences Department at Martin Marietta Laboratories. His current research interests include surface-sensitive measurements and their analyses, corrosion of aluminum and its alloys, effects of acid deposition on painted steel, formation and durability of adhesive bonds, and industrial failure analysis. Dr. Davis was the 1986 Distinguished Young Scientist of the Maryland Academy of Sciences. 

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. 

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. 

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. 

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. 

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... 

Although the degree of atmospheric corrosion of copper and its alloys depends upon the corrosive agents present, the corrosion rate has been found to generally decrease with time. The copper and its alloys such as silicon bronze, tin bronze usually corrode at moderate rates, while brass, aluminum bronze, nickel silver, and copper-nickel corrode at a slower rate.51 The most commonly used copper alloys are Cl 1000, C22000, C38500 and C75200. 

A similar reaction occurs during pitting corrosion of iron and its alloys. Partial hydrolysis, leading to the formation of Al(OH) and Al(OH) may also occur, but all such reactions lead to the formation of acid, making the solution inside the pit much more aggressive than outside. Measurement of the pH inside a pit is not an easy matter, but estimates based on various calculations and on measurements in model pits lead to values as low as 1-2 for chromium-containing ferrous alloys and about 3.5 for aluminum-based alloys, depending on experimental conditions. 

A. J. Epstein, Corrosion Protection of Aluminum and its Alloys Using Electroactive Polymers, Storming Media, Washington DC, 2000. 

Furthermore impurities in the helium coolant, mainly the air constituents, can cause corrosion effects on the outside reformer tube walls which eventually change its properties. Measurements of impurity contents in Dragon and AVR revealed a large scattering of the data. Experimental results obtained within the Dragon project indicate a strong corrosion of aluminum and titanium, i.e., the formation of Cr-, Mn-, Si-, and Ti-oxide layers, and an increased corrosion rate in moist helium compared with a dry atmosphere [26]. 

Magnesium and zinc are the predominantly used galvanic anodes for the cathodic protection of pipelines [13—16]. The corrosion potential difference of magnesium with respect to steel is 1 V, which Umits the length of the pipeline that can be protected by one anode. Economic considerations have led to the use of aluminum and its alloys as anodes. However, aluminum passivates easily, decreasing current output. To avoid passivation, aluminum is alloyed with tin, indium, mercury, or gallium. The electrochemical properties of these alloys, such as theoretical and actual output, consumption rate, efficiency, and open circuit (corrosion) potential, are given in Table 15.1. 

The corrosion resistance of aluminum and its alloys has been well documented [129-132]. The Pourbaix diagram, accepting its limitations, shows that aluminum remains passive in the pH range 4-9. Outside this range, active dissolution occurs according to the following reactions ... 

Corrosion Protection of Iron and Steel Corrosion Protection of Aluminum and Its Alloys Corrosion Protection of Other Metals... 

There have been a few new reports on the use of PANI for corrosion protection of aluminum and its alloys [228-230]. PANI films electrodeposited at pure aluminum from a tosylic acid containing solution... 

As a result of its passive surface layer, aluminum alloys have a good corrosion resistance and a rather constant surface appearance. There are however some reasons, ex-emphfied in Table 37.4, for surface treatment of aluminum and its alloys. 

Corrosion problems will arise in the use of recycled aluminum. For instance, most recycled aluminum is contaminated with impurity elements such as iron, silicon and copper. The contaminated aluminum usually has low corrosion resistance, because of its poor oxide film layer. As the use of recycled aluminum becomes more widespread, corrosion problems will increase in importance in many industrial fields. These considerations lead to the conclusion that corrosion engineering of aluminum and its alloys will be one of the most important subjects to be studied in the next century. 

The metallurgical characteristics of the aluminum oxide layer also depend on its physical metallurgy, such as defects and metallurgical structure included in the oxide layer. For instance, when intermetallic compound particles as secondary phases are exposed on the surface, a discontinuous oxide film with various defects is often produced at the metal-particle interface. This discontinuous oxide film is weakly or non-protective chemically and physically. Because corrosion is a chemical and electrochemical reaction on the surface, corrosion behavior is readily influenced by surface morphology. The aluminum surface is usually adsorbed or contaminated by water, gases and many kinds of micron-sized substances. Microscopic heterogeneous structures such as vacancies, steps, kinks, and dislocations, and macroscopic heterogeneous structures such as scratches, pits and other superficial blemishes influence the corrosion behavior of aluminum and its alloys to different extents. 

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