Aluminium atmospheric corrosion

However, the object of this section is to outline the principles which govern atmospheric corrosion, and the emphasis is placed on metals whose atmospheric corrosion is of economic importance. These include iron and steel, zinc, copper, lead, aluminium and chromium. 

Protection of Iron and Steel by Aluminium and Zinc against Atmospheric Corrosion , Sprayed Metal Coatings, BS 2569 Part 1 (1964)... 

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. 

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. 

As mentioned in Chapter 3, the most significant deviations from the theoretical Pourbaix diagram in practice are that chloride or other aggressive species may destroy the passivity and that impurities in the metal may cause weak points in the oxide, with pitting as a possible consequence (Section 7.6). For pure aluminium, the corrosion resistance decreases considerably when the content of impurities in the metal increases from 0.01 to 1%. However, even 99% aluminium resists neutral atmospheres and chloride-free water very well. In seawater, pitting will usually occur, but the weight loss is low and the pits shallow (Section 7.6). 

Atmospheric corrosion tests were continued for years with number of panels, and to have traceability, each uncoated panel was marked by hole with respect to reference plate made as per standard [3] as shown in Fig 2.5. The reference plate made of Aluminium has identical size of test panels and a hole is made in left hand corner marked as R, which stands for reference mark. Just below this hole, nine holes are made vertically engraving numbers 1-9 and similarly, in the right hand side, again nine holes are made vertically giving numbers 10-90 from top to bottom. At the top of the panel, five holes are made horizontally to represent hundreds. 

Berukshtis, G., and Klark, G. (1965). Atmospheric corrosion of steel, zinc, cadmium, copper and aluminium in different coastal and continental regions. Korroz. Met. Splavov Sb., (2), 332-350 (in Russian). 

Kelley, V. 1. (1975). Atmospheric Corrosion Investigation of Aluminium-Coated, Zinc-Coated and Copper-Bearing Steel Wire and Wire Products. A Twelve-Year Report. ASTM STP 585, ASTM, Philadelphia, 89 pp. 

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... 

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. 

Wood J., Harris D.A., Atmospheric corrosion tests on cast aluminium alloys, British Foundryman, vol. 74, 1981, p. 217-221. 

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. 

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... 

Atmospheric corrosion has been the subject of many publications. The first was by E. Wilson, who reported results of observations made on electrical cables exposed in London over 24 years, beginning in 1902 [3]. Since 1945, many national and international conferences in Europe and in the United States have been devoted to the atmospheric corrosion of aluminium (as well as that of other common metals and alloys such as steel, copper, etc.). 

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

Our understanding of the atmospheric corrosion of metals is based on theoretical foundations that are a special case of corrosion of metals and alloys. Before discussing the specific aspects of aluminium, they need to be recalled here. 

The first theoretical explanation of atmospheric corrosion of metals was given by Vernon [5] and Hudson [6] starting in 1923 and was completed later by Rozenfeld in the 1960s [7] and by Graedel for aluminium in the 1980s [8]. Vernon introduced the concept of the critical degree of moisture, the threshold below which practically no corrosion will occur. The value of this threshold depends on several factors such as the nature and concentration of atmospheric pollutants and the metal s surface condition. 

Figure C.2.1. Influence of moisture on atmospheric corrosion of aluminium (from Rozenfeld).

The influence of rain on the atmospheric corrosion of metals in general, and aluminium in particular, is complex [7]. Rain maintains the level of humidity of air above the critical level at which corrosion starts to develop. 

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