Aluminum alloys corrosion protective oxide film

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... 

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). 

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... 

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. 

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... 

Oxygen is the common cathodic reduction species found in water, which is responsible for continued corrosive attack on some engineering materials, such as low carbon steel. However, passive engineering alloys utilize the oxygen to form thin, tenacious, and adherent protective oxide films. Some common alloys with protective films are stainless steels, nickel alloys, copper-base alloys and aluminum alloys. The oxygen concentration at ambient temperatures and atmospheric pressure is approximately 6-8 mg/L. An increase in temperature decreases oxygen solubility, whereas an increase in pressure increases oxygen solubility. 

It is recognized that elements in solid solution are less detrimental to the corrosion of aluminum, but that the existence of secondary phases in the mass are harmful, because a discontinuous and non-protective oxide film is often formed at the matrix-particle interface. The harmful extent of the secondary phases depends on the kinds and amounts of the particles. It is important elec-trochemically to know the potential of the microstructural particle phases. The potential difference between aluminum matrix and secondary phase is of primary importance in the corrosion behavior of aluminum and its alloys. The potentials of solid solu-... 

When strength-to-weight ratio is an important consideration, magnesium alloys compete with aluminum alloys. Magnesium has a density of 1.74 g/cm, which is 36% less than that of aluminum. However, aluminum is less expensive and has a greater corrosion resistance. The oxide film formed on magnesium provides only limited protection, unlike the adherent protective oxide film on aluminum. The primary application of magnesium alloys are for die-cast products. 

The resistance of aluminum alloys to flow induced corrosion depends on the stability of the protective oxide films on the surface. Dissolution of these films leads to accelerated corrosion. The protective films of bayerite and boehmite could be eroded by shear forces resulting from flow beyond a critical velocity. Aluminum alloys of series 5xxx are not adversely affected by velocities up to 3 m/s in the absence of abrasives in water. The removal of a film adjacent to a film surface sets up local corrosion cell which accelerates the corrosion process. AUoys of 5xxx series (such as 5454) show a good resistance to corrosion at velocities up to 3ms at temperatures up to 140°C. The corrosion rate increases with increased velocities in the presence of abrasive particles, which need to be controlled. The water chemistry, water velocity and pH needs to be controlled to minimize the effect of flow on localized corrosion. Maintaining pH below 9 would not allow aluminum to dissolve as AlO. The preventive measures include the minimizing of turbulent flow or changing water chemistry. 

Copper has the highest electrical conductivity of all metals and is used in highly purified form when electrical conductivity is important. Metals that have high electrical conductivity also have high thermal conductivity since the same characteristic is responsible for both. Many alloys of copper and zinc are used. These are called brasses. Alloys of copper and tin are called bronzes. Like aluminum, copper is relatively corrosion resistant due to a protective oxide film that has an esthetic greenish color. 

Its use for handling chlorinated solvents requires careful consideration. Under most conditions, particularly at room temperatures, aluminum alloys resist halogenated organic compounds, but under some conditions they may react rapidly or violent with some of these chemicals. If water is present, these chemicals may hydrolyze to yield mineral acids that destroy the protective oxide film on the aluminum surface. Such corrosion by mineral acids may in turn promote reaction with the chemicals themselves, because the aluminum halides formed by this corrosion are catalysts for some such reactions. To ensure safety, service conditions should be ascertained before aluminum alloys are used with these chemicals. 

A marked increase in aluminum Ep values towards anodic potentials was observed in the presence of marcescens, even in the sterile medium, suggesting the metal surface may experience some protective action by these bacteria. Local acidification enhanced by adhesion processes taking place at the metal/mycelia interface accounts for some of the specific effects of resinae in the corrosion process of aluminum alloys in fuel/water systems (Salvarezza et al., 1979). The acidification also has been reported in the literature as a differential effect between two Pseudomonas spp. in relation to aluminium corrosion. Acidity can prevent repasivation and may hinder the formation of a protective oxide film. Therefore, under acidic conditions, pitting of the metal by chloride anions occurs at more cathodic potentials than in neutral solution (Salvarezza et al., 1983). 

HIgh Purlty Water. Suitability of the more corrosion-resistant aluminum alloys for use with high-purity water at room temperature is well established by both laboratory testing and service experience (Ref 21). The slight reaction with the water that occurs initially ceases almost completely within a few days after development of a protective oxide film of equilibrium thickness. After this conditioning period, the amount of metal dissolved by the water becomes negligible. 

Crevice Corrosion. Crevice corrosion is intense locali2ed corrosion that occurs within a crevice or any area that is shielded from the bulk environment. Solutions within a crevice are similar to solutions within a pit in that they are highly concentrated and acidic. Because the mechanisms of corrosion in the two processes are virtually identical, conditions that promote pitting also promote crevice corrosion. Alloys that depend on oxide films for protection (eg, stainless steel and aluminum) are highly susceptible to crevice attack because the films are destroyed by high chloride ion concentrations and low pH. This is also tme of protective films induced by anodic inhibitors. 

The contact ends of printed circuit boards are copper. Alloys of nickel and iron are used as substrates in hermetic connectors in which glass (qv) is the dielectric material. Terminals are fabricated from brass or copper from nickel, for high temperature appHcations from aluminum, when aluminum conductors are used and from steel when high strength is required. Because steel has poor corrosion resistance, it is always plated using a protective metal, such as tin (see Tin and tin alloys). Other substrates can be unplated when high contact normal forces, usually more than 5 N, are available to mechanically dismpt insulating oxide films on the surfaces and thereby assure metaUic contact (see Corrosion and corrosion control). 

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 ... 

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+). 

Anodized films are most often applied to protect aluminum alloys. However there are processes for other metals such as titanium, zinc, and magnesium. Anodized titanium is used in dental implants and sometimes in art and costume jewelry because it generates various colors without dyes. Each color depends on a specific thickness of the oxide [9]. To ensure the preparation of a consistent oxide layer, one must control conditions such as electrolytic concentrations, acidity, and current. Also a sealing process is often needed to achieve corrosion resistance because thick coatings (oxide layers) are generally porous. 

Anodizing is an electrolytic passivation process that increases the thickness of natural oxide layers on the surface of metals [13]. It basically forms an anodic oxide finish on a metal s surface to increase corrosion resistance. For the anodizing process, the metal to be treated serves as the anode (positive electrode, where electrons are lost) of an electrical circuit. Anodized films are most often applied to protect aluminum alloys. An aluminum alloy is seen on the front bicycle wheel in Fig. 2 [14]. For these alloys, aluminum is the predominant metal. It typically forms an alloy with the following elements copper, magnesium, manganese, silicon, tin, and zinc [15]. Two main classifications for these alloys are casting alloys and wrought alloys, both of which can be either heat treatable or non-heat treatable. 

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. 

Except for aluminum alloys that contain copper as a major alloying ingredient, these alloys have a high resistance to weathering in most atmospheres. When exposed to air, the surface of the aluminum becomes covered with an amorphous oxide film that provides protection against atmospheric corrosion, particularly that caused by SO2. 

Halogenated hydrocarbons may decompose by hydrolysis if water is present or by other processes to yield mineral acids such as hydrochloric acid. These acids corrode aluminum alloys because they destix the protective surface oxide film naturally present that provides inherent resistance to corrodon. Corrosion of aluminum alloys by these acids may also promote reactions of the hydrocarbons themselves because aluminum halides formed by corrosion are catalysts for some of these reactions (e.g. AlClj for a Friedel-Crafts reaction). In some instances, aluminum alkyls may be produced. Because of the rapid rate of evolution of heat, corrosion of aluminum and reaction of a halogenated hydrocarbon, once initiated, may tend to become autocacalytic. 

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