Aluminium alloys atmospheric corrosion

Corrosion of metals and alloys— Removal of corrosion products from corrosion test specimens Metals and alloys— Atmospheric corrosion testing—General requirements for field tests Anodized aluminium and aluminium alloys—Rating system for the evaluation of pitting corrosion—Chart method... 

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... 

Table 7.25 Effect of aluminium on the corrosion at 982°C of Fe-35Ni-15Cr alloy in a reducing flue-gas atmosphere containing 2-3 g m of sulphur (100-h test period) ...

The embrittlement caused to aluminium alloys by pre-exposure to moist atmospheres or stress-corrosion environments is thought to be due to hydrogen in the atomic form. Intergranular bubbles of hydrogen, formed in association with certain precipitates, have been observed by HV and... 

In atmospheric exposure to industrial environments its corrosion rate is only about one-third that of zinc and the corrosion reaction is stifled by the tenacious oxide which is produced nevertheless it can frequently function as an anodic coating both for steel and for the less corrosion-resistant aluminium alloys. 

The zinc-aluminium alloys are most important. The zinc-55%-aluminium-1.5 -silicon alloy hot-dip coating was initiated over 20 years ago by the steel industry and has recently become of major worldwide importance (known as Galvalume, Zincalume, Alugalva, Aluzink, Aluzinc, Zincalit or Zalutite). The coating usually has 1(X)-4(X)<% more corrosion resistance than galvanising in the atmosphere, but less cathodic protection and also has the inherent problem of aluminium alloys when in contact with alkalis. 

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. 

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. 

Several aluminium alloys show very good corrosion resistance in various atmospheres. Some pitting occurs, but the pits remain small. Maximum depth seldom exceeds 0.5 mm during 6-20 years of exposure it is usually in the order of 0.1 mm. Some alloys may, however, be attacked by intergranular corrosion or exfoliation corrosion (see Section 7.7). Extensive galvanic corrosion may occur on aluminium in contact with copper, mild steel (in marine atmosphere) and graphite, less in contact with stainless steel, while aluminium is compatible with zinc [8.2]. 

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. 

Corrosion of metals and alloys—Corrosivity of atmospheres—Guiding values for the corrosivity categories Corrosion of metals and alloys—Corrosivity of atmospheres—Measurement of pollution Corrosion of metals and alloys—Corrosivity of atmospheres—Determination of corrosion rate of standard specimens for the evaluation of corrosivity Corrosion tests in artificial atmospheres—Salt spray tests Nickel-based alloys—Determination of resistance to intergranular corrosion Corrosion of aluminium alloys—Determination of resistance to stress corrosion cracking Corrosion tests in artificial atmosphere at very low concentrations of polluting gas(es)... 

Due to the various industrial applications and economic importance of alutninium and its alloys, its protection against corrosion has attracted much attention (Aballe et al., 2001 Cheng et al., 2004 Hintze and CaUe, 2006). Most aluminium alloys have good corrosion resistance towards natural atmospheres and other environments, because alutninium alloy surfaces are covered with a natural oxide film of thickness about 5 nm (KUckic et al., 2000). However, in the presence of aggressive ions, like chloride, the protective layer can be locally destroyed and corrosive attack takes place (Kliskic et al., 2000). Yet, if correctly protected, applications of aluminium alloy may be more reliable and have long service life. 

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