Aluminum alkaline corrosion

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). 

Cooling water was used to cool hot aluminum. Calcium carbonate spotting had occurred at alkaline pH. After reducing pH, the staining problem disappeared, but corrosion increased substantially. 

Pitting is also promoted by low pH. Thus, acidic deposits contribute to attack on stainless steels. Amphoteric alloys such as aluminum are harmed by both acidic and alkaline deposits (Fig. 4.4). Other passive metals (those that form protective corrosion product layers spontaneously) are similarly affected. 

The most harmful deposits are those that are water permeable. Truly water-impermeable material is protective, since without water contacting metal surfaces corrosion cannot occur. Innately acidic or alkaline deposits are troublesome on amphoteric alloys (those attacked at high and low pH—e.g., aluminum and zinc). 

Figure 8.1 Effect of pH on corrosion of 1100-H14 alloy (aluminum) by various chemical solutions. Observe the minimal corrosion in the pH range of 4-9. 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. (Courtesy of Alcoa Laboratories from Aluminum Properties and Physical Metallurgy, ed. John E. Hatch, American Society for Metals, Metals Park, Ohio, 1984, Figure 19, page 295.)...

Figure 8.1 shows that aluminum is corroded hy alkaline substances, albeit at different rates, when water pH exceeds 9. Corrosion by inorganic salts between a pH of 5 and 9 is very slow near room temperature. Aluminum shows no significant corrosion in most natural waters up to about 350°F (180°C). Of course, natural waters vary widely in composition, and exceptions do occur. 

Zinc is attacked at high pH. However, in weakly alkaline solutions near room temperature, corrosion is actually very slight, being less than 1 mil/y (0.0254 mm/y) at a pH of 12. The corrosion rate increases rapidly at higher pH, approaching 70 mil/y (1.8 mm/y) at a pH near 14. Just as in aluminum corrosion, protection is due primarily to a stable oxide film that forms spontaneously on exposure to water. High alkalinity dissolves the oxide film, leading to rapid attack. 

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 ... 

Steel slag is mildly alkaline, with a solution pH value of 8 to 10. However, the pH of leachate from steel slag can exceed 11, a level that can be corrosive to aluminum or galvanized steel pipes placed in direct contact with the slag. 

Corrosion—While corrosion is usually not a concern on hot surfaces, recognize that all systems have shut down periods, with the probability that moisture will find ingress and condense. Many insulalions arc alkaline and have little adverse effect on iron and copper, but aluminum is affected adversely. A major concern is stress-corrosion of stainless steel induced by even trace amounts of soluble chlorides un ASTM Test Method maybe used. 

Active metals such as aluminum, titanium, and high-chromium steels become corrosion resistant under oxidizing conditions because of a very adherent and impervious surface oxide film that, although one molecule thick, develops on the surface of the metal. This film is stable in a neutral medium, but it dissolves in an acid or alkaline environment. In a few cases, such as certain acid concentrations, metals can be kept passive by applying a carefully controlled potential that favors the formation of the passive surface film. The ability to keep the desired potential over the entire structure is very critical in anodic control. If a higher or lower potential is applied, the metal will corrode at a higher rate, possibly higher than if it is not protected at all. 

Aluminum is less reactive than are alkaline or alkaline earth metals, and thus its corrosion in polar aprotic solvents is much less pronounced than that of Li, Mg, Ca, etc. 

Aluminum can be attacked by way of cathodic corrosion in strong alkaline media generated at the cathode when magnesium corrodes sacrificially in static NaCI solutions. This mode of attack destroys compatibility in alloys containing significant iron contamination. 

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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