Aluminum oxide film, protective

Thermodynamics tells us that aluminum is a reactive metal. Despite this aluminum has excellent corrosion resistance. Because aluminum has strong affinity for oxygen, the surface aluminum oxide film protects the metallic aluminum from corrosion in many environments. Before mentioning the corrosion behavior, it is necessary to know the structure and properties of surface oxide film formed on aluminum. 

There is no stable entity Al2+(aq) to compare with Fe2+(aq) consequently, the mechanism that causes rust to be nonprotective because of migration of Fe2+(aq) through the water before precipitation as FeO(OH) does not apply to aluminum, on which Al(OH)3 or AIO(OH) forms, at once, on the anodic site. Conversely, removal of the protective aluminum oxide film cannot occur by the reductive dissolution mechanism described for iron. 

The aluminum oxide film generally is stable in the pH range of 4 to 9, but is readily dissolved in strong acids and alkalis, with some notable exceptions as mentioned in the section on Natural Protective Films. The rate of corrosion cannot be predicted solely by the pH, but depends on the specific ions present, their concentration, and the temperature. For example, the dissolution rate of aluminum in sulfuric acid becomes appreciable between 50-100 % concentration, with the maximum rate occurring at 70-90 % concentration. Furthermore, the corrosion rate at 50 C can be as much as four times more rapid than at 25°C... 

The chemical composition of alloy 214 will he foimd in Table 15.21. This is a nickel-hased alloy with excellent resistance to 2200 (1204°C). The excellent oxidation resistance is the result of the tenadous aluminum oxide film that protects the metal during prolonged exposiue. 

As would be expected, constant exposme to moisture with a limited supply of oxygen to the aluminum siuface leads to the rapid corrosion of any aluminum apparatus or equipment component. This is due to the highly reactive nature of aluminum that leads to formation of oxides or hydroxides. In the presence of oxygen, a protective aluminum oxide film which is substantially unreactive, develops on any aluminum surface. If the film is removed by mechanical or chemical means and the aluminum exposed to water, a rapid reaction sets in and the fresh aluminum surface is converted to the hydroxide and subsequently to the oxide. 

In near-neutral and deaerated solutions, the oxide film on anodized aluminum is stable and protective in distilled water and chloride solutions, as well as in solutions of inhibitive anions. Thus the inhibition of aluminum corrosion by anions differs from that of iron or zinc in that the presence of dissolved oxygen in the solution is not necessary to stabilize the oxide film. In corrosion inhibition by chromate ions, their interaction with the oxide film on aluminum has been shown to result in the formation of an outer layer of the film that is more protective due to its high electronic resistance and low dissolution rate. Chromate ions were also found to prevent the uptake and penetration of chloride ions into the aluminum oxide film. 

A1 is a very reactive metal with a high affinity for oxygen. This is indicated from its position on the electromotive force series. The metal is nevertheless highly resistant to most atmospheres and chemicals. This resistance is due to the inert and protective character of the aluminum oxide film, which forms on the metal surface and which rapidly reforms if damaged. In most enviroiunents, therefore, the rate of corrosion of A1 decreases rapidly with time. 

CORROSION can combine with mechanical processes (wear, flow velocity, and/or fatigue) to produce severe attack, usually of a localized nature. Because removal of the protective aluminum oxide film by mechanical processes exposes frcsh metal to attack, corrosion occurs at an accelerated rate. 

In dry air at room temperature this reaction is self-limiting, producing a highly impervious film of oxide ca 5 nm in thickness. The film provides both stabihty at ambient temperature and resistance to corrosion by seawater and other aqueous and chemical solutions. Thicker oxide films are formed at elevated temperatures and other conditions of exposure. Molten aluminum is also protected by an oxide film and oxidation of the Hquid proceeds very slowly in the absence of agitation. 

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... 

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... 

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. 

Zirconium is a highly active metal which, like aluminum, seems quite passive because of its stable, cohesive, protective oxide film which is always present in air or water. Massive zirconium does not bum in air, but oxidizes rapidly above 600°C in air. Clean zirconium plate ignites spontaneously in oxygen of ca 2 MPa (300 psi) the autoignition pressure drops as the metal thickness decreases. Zirconium powder ignites quite easily. Powder (<44 fim or—325 mesh) prepared in an inert atmosphere by the hydride—dehydride process ignites spontaneously upon contact with air unless its surface has been conditioned, ie, preoxidized by slow addition of air to the inert atmosphere. Heated zirconium is readily oxidized by carbon dioxide, sulfur dioxide, or water vapor. 

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). 

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). 

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 ... 

The protection current requirement for aluminum ships is considerably less because of the dense adherent oxide films. The necessary protection current requirement is being clarified in current investigations [24] but good results have been obtained by assuming a figure of 10% of that for steel. With aluminum there is only a very narrow permissible potential range [25] (see Section 2.4) so that impressed current protection cannot be used because of the anodic voltage cone and only selected anode materials can be considered. 

Although insulators other than aluminum oxide have been tried, aluminum is still used almost universally because it is easy to evaporate and forms a limiting oxide layer of high uniformity. To be restricted, therefore, to adsorption of molecules on aluminum oxide might seem like a disadvantage of the technique, but aluminum oxide is very important in many technical fields. Many catalysts are supported on alumina in various forms, as are sensors, and in addition the properties of the oxide film on aluminum metal are of the greatest interest in adhesion and protection. 

Eloxal [Electrolytic oxidation of aluminium] An electrolytic process for applying an oxide film to the surface of aluminum in order to protect it from further oxidation or to make it capable of adsorbing a dyestuff. 

Juratka A process for forming a protective oxide film on aluminum. See also Alzac, metal surface treatment. 

---