THE RESISTANCE OF ALUMINIUM TO ATMOSPHERIC CORROSION

The resistance of aluminium and aluminium alloys to atmospheric corrosion (weathering) is nowadays well known. This has been achieved through 

Gioacchino church in Rome. Some of these are subject to regular inspections by aluminium producers [1]. 

Results obtained in outdoor corrosion testing stations since 1935. These are mostly long-term tests (20 years and longer) in various types of atmospheres, especially marine [2, 3] and industrial. 

3-mm-thick aluminium sheets were manufactured in Neuhausen (Switzerland) in 1895. The composition of the metal corresponds to what was commonly achieved when the production of aluminium by molten salt electrolysis started (Table C.5.1). Very high in iron and siUcon, the metal contained 98.3% aluminium. In the last 50 years, when unalloyed aluminium (1000 series) has been used for this type of application, it contains at least 99.5% aluminium. 

The first inspection by Panseri in 1937 [4], as well as the second inspection in 1949 [5], on the occasion of the 50th anniversary of the church s completion, demonstrated the good corrosion resistance of this roofing. The micrographs included in the report show that the pitting depth did not exceed 100 pm. The areas of overlap between sheets did not exhibit preferential or more intense corrosion than the surfaces exposed to air. 

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The resistance of aluminium to atmospheric corrosion has been a very important issue, and, since the early 1930s, has attracted a great deal of attention from corrosion experts working with the major aluminium producers in Europe and North America. The first tests of aluminium alloys in outdoor corrosion testing stations were performed in the United... 

Our present knowledge of the resistance of aluminium to atmospheric corrosion has solid foundations, based on two complementary approaches ... 

Like blackening (see Section D.1.5), this is an alteration of the visual properties of the natural oxide layer, due to its progression under the influence of atmospheric humidity. This is not corrosion. Water staining does not affect the resistance of aluminium to weathering. Anodising is a reliable means to avoid water staining of aluminium that is exposed to weathering. 

The relative susceptibility of metals to atmospheric corrosion varies widely with the type of contaminant, e.g. zinc and cadmium, two metals that are used for the protection of steel in exposed environments, are both rapidly attacked by organic acidson the other hand, aluminium alloys resist attack by organic acids but may be rapidly corroded by chlorides, especially at crevices or areas of contact. 

In air, carburising atmospheres and sulphidising/oxidising atmospheres all alloys showed excellent corrosion resistance due to the formation of protective oxide scales. It was found that the concentration of aluminium or chromium does not significantly effect the corrosion behaviour in any of these environments. 10 wt% aluminium, probably even less, are sufficient to enable the formation of protective Al203-scales even at temperatures as low as 650°C. Overdoping with reactive elements (mischmetal), however, causes high oxidation rates in air and should be avoided. 

Although it readily combines with oxygen, aluminium is resistant to atmospheric corrosion because a thin film of closely adherent oxide is formed which protects the underlying metal from attack. Aluminium powder is therefore used as a pigment in anticorrosive paints. 

Common product forms, tensile strength, and resistance to atmospheric corrosion of some wrought aluminium materials are shown in Table 10.11. The strongest grades of the various alloys imply lower ductility, i.e. elongation mainly within the range 2-12%, compared to 15-35% for the softest grades. A number of fields of application of the same materials are listed in Table 10.12. 

Sainte-Claire Deville observed that aluminium had good resistance to atmospheric corrosion, which included the particular atmosphere of gas lamps (used for street lighting in the Second Empire), an atmosphere laden with hydrogen sulphide (H2S). He also recognised the very good resistance of aluminium in contact with water. 

Many decades of experience with its use in buildings, pubfic works, shipbuilding, etc. have confirmed the observations of the 19th century chemists. Aluminium and the alloys of the 1000, 3000, 5000, 6000 and 8000 series have excellent resistance to atmospheric corrosion in the marine, luban and industrial environments (see Part C). 

Contrary to a common misconception, the purity of the base metal does not improve the corrosion resistance of aluminium. Metal with a very low iron and silicon content (1199) does not resist atmospheric corrosion better than 1070 or 1050. Only at much higher concentrations of iron and silicon (Fe > 0.50 and Si > 0.25), which was frequently found until the end of the 1940s, will the corrosion resistance be altered. 

Due to its excellent resistance to atmospheric corrosion, the use of aluminium in construction, civil engineering, electrical power transmission lines [1] and transport has been increasing considerably since 1930. Nowadays, aluminium is the second most common metal, after steel, to be exposed to weathering, in all climate and geographic zones. 

This is why rain is so important. Rain cleans the surfaces and washes away dust and soluble corrosion products. From this point of view, rain has a beneficial effect on aluminium s resistance to atmospheric corrosion, and often it is easy to perceive the difference between surfaces cleaned by the rain and surfaces that are poorly (or not at all) cleaned by the rain. Periodic cleaning of aluminium surfaces of buildings that eliminates dust and deposits (including corrosion products) thus contributes to their resistance to atmospheric corrosion and maintains a clean surface aspect. Likewise, the difference between buildings and equipment that are properly maintained and those that are not at all or only poorly maintained can be easily seen. 

Figure C.5.3. Comparison of the resistance to atmospheric corrosion of aluminium and steel [6],...

The development of applications of aluminium in constructions such as metallic fittings, foldable walls, shop equipment, urban furniture stems from the excellent resistance to atmospheric corrosion of anodic layers. These layers aim at maintaining the surface appearance over a long period of time by preventing blackening and pitting corrosion. 

Aluminium is a very reactive metal with a high affinity for oxygen. The metal is nevertheless highly resistant to most atmospheres and to a great variety of chemical agents. This resistance is due to the inert and protective character of the aluminium oxide film which forms on the metal surface (Section 1.5). In most environments, therefore, the rate of corrosion of aluminium decreases rapidly with time. In only a few cases, e.g. in caustic soda, does the corrosion rate approximate to the linear. A corrosion rate increasing with time is rarely encountered with aluminium, except in aqueous solutions at high temperatures and pressures. 

Since 1980, the zinc-5-aluminium (notably Galfan which has a mischmetal addition) alloys, which are essentially based on the eutectic structure, have been developed commercially. They give 30-200% increase in corrosion resistance in the atmosphere and are extremely flexible. They can be used for sheet, wire and some types of tube galvanising whereas the zinc-55%-aluminium alloy is restricted to sheet. 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

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