Aluminum alloy, alkaline corrosion

FIG. 24-60 Effect of corrosion on 1100-H14 aluminum alloy by various chemical solutions. Observe the minimal corrosion in the pH range from 4.0 to 9.0. The low corrosion rates in acetic acid, nitric acid, and ammonium hydroxide demonstrate that the nature of the individual ions in solution is more important than the degree of acidity or alkalinity. (With permission from ASM International www. asmintemaUonal.org. Courtesy of Com-husUon Energy Systems. Ltd. www.eondexenergy.com.)... 

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 first step of SAIE in the case of corrosion protection of aluminum alloys is the preparation of oxides. The top layer of an aluminum alloy is generally covered with hydrated mixed oxides. Either alkaline cleaning or a combination of alkaline cleaning and deoxidization removes major organic contaminants and this potentially unstable oxide layer. A thin layer of plasma polymer is deposited on the stabilized oxide layer thus created. 

Acidity and alkalinity affect the corrosion rate. Generally, alkaline conditions favor lower corrosion rates however, some metals, such as aluminum and zinc, are amphoteric and show increased corrosion at pH values above 9. For iron alloys, the corrosion rate is relatively steady between a pH of 4 and 10 at ambient temperature and where oxygen reduction is the primary cathodic corrosion reaction however, the corrosion rate increases rapidly below pH 4 [4]. Iron passivates and the corrosion rate decreases rapidly above a pH of 10, except at very high pH levels where the corrosion rate again can increase. 

Transition metal alloys, notably Raney nickel, have also been investigated extensively as catalysts because of their interesting electronic and chemical properties [94]. Raney nickel is a solid catalyst, composed of fine grains of a nickel-aluminum alloy, and has been used in many industrial processes. Its application in the fuel cell field has been focused on alkaline fuel cells (AFC) rather than PEM fuel cells, due to potential corrosion in PEM operation media. Raney nickel s unique catalytic activity for the HOR as a non-noble catalyst makes it worth inclusion in this chapter. 

Alloys of the 5XXX series have the same high resistance to general corrosion as the other non-heat-treatable alloys in most environments. In addition, they exhibit a better resistance in slightly alkaline solutions than that of any other aluminum alloy. These alloys are widely used because of their high as-welded strength when welded with a compatible 5XXX series filler wire. 

ALKALINE SOLUTIONS. Alkaline solutions generally have some action on aluminum alloys. The pH of these solutions alone is not a reliable indicator of the performance of aluminum alloys. Usually, weak bases such as ammonium hydroxide, hex-amine, alkanolamines and their aqueous solutions can be handled in aluminum because a protective film forms on aluminum after an initial period of reaction. Solutions made alkaline by hydrolysis of basic salts such as sodium carbonate form protective films on Al-Mg alloys containing 3.5% or more magnesium. Strong bases such as sodium hydroxide and potassium hydroxide dissolved in water are very corrosive and should not be handled in aluminum. See also Ref (4) pp. 35, 37. 

GLUE. Originally an impure form of gelatin. In more modem times, glue is one of many types of adhesives used for bonding. In laboratory tests, most adhesives were found to be either innocuous or protective to aluminum alloys. However, exceptions were found and included the alkaline water base latex adhesives, acetic anhydride adhesives, and adhesives that have been made electrically conductive by the addition of copper, silver, or carbon. Such adhesives should be used with caution and with the knowledge that corrosion could develop. Adhesives are used with aluminum alloys in many applications. SeeaIsoRef (I)p. 133. (3) pp. 124, 199, 231. 233. (4) pp. 107, 115, R. L. Patrick, Editor. Treatise of Adhesion and Adhesives," Vol. Ill, Marcel Dekker, New York, 1973. 

SOAP. Salt of fatty acids. In laboratory tests, the action of soaps on aluminum alloys is variable. Many soaps cause less than 1 mpy attack while others, usually those more alkaline, are corrosive. Aluminum alloy screw conveyors, compactors, packaging equipment, and tote bins have been used in the production of soap. Bar soap has been wrapped in aluminum foil laminates. See also Ref (1) p. 142. (2)p, 647, (3)pp. 117, 239. 245 (7) p. 160,... 

SODIUM DISnJCATE. Na.SijOs. Sodium disiii-cate has been used as an inhibitor of corrosion of aluminum alloys in alkaline solutions. See also Ref (7 p. 165. 

This corrosion of the anode, in an alkaline electrolyte, is one of the factors that reduce the cou-lombic efficiency and the energy density of the Al-air cell. Several methods have been developed to decrease this corrosion. Aluminum alloys have been used to decrease the corrosion, as have corrosion inhibitors that are added to the caustic electrolyte. Non-caustic electrolyte, such as salt water (saline solution), are also used to decrease the corrosion of the aluminum anode. The alkaline electrolyte system has the advantage of higher conductivity over the salt water system, which results in a high discharge rate for the battery. 

Magnesium and its alloys are definitely anodic to the A1 alloys and, thus, contact with aluminum increases the corrosion rate of magnesium. For example, in sodium chloride solutions (3-6%), the potential of Mg alloys is -1.67 V/SHE while that of Al-12%Si and pure aluminum are -0.83 to -0.85, respectively. However, such contact is also likely to be harmful to aluminum, since magnesium may send sufficient current to the aluminum to cause cathodic corrosion in alkaline medium. Aluminum oxide is amphoteric and so it is soluble in acid as well as in alkaline solutions. The standard reduction potentials of these two half-reduction reactions are (-1.66 V/SHE) and (-2.35 V/SHE), respectively. Alkaline reaction of the possible existence of aluminum phase in sacrificial Mg anodes is ... 

Nickel—Copper. In the soHd state, nickel and copper form a continuous soHd solution. The nickel-rich, nickel—copper alloys are characterized by a good compromise of strength and ductihty and are resistant to corrosion and stress corrosion ia many environments, ia particular water and seawater, nonoxidizing acids, neutral and alkaline salts, and alkaUes. These alloys are weldable and are characterized by elevated and high temperature mechanical properties for certain appHcations. The copper content ia these alloys also easure improved thermal coaductivity for heat exchange. MONEL alloy 400 is a typical nickel-rich, nickel—copper alloy ia which the nickel content is ca 66 wt %. MONEL alloy K-500 is essentially alloy 400 with small additions of aluminum and titanium. Aging of alloy K-500 results in very fine y -precipitates and increased strength (see also Copper alloys). 

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