Pitting corrosion aluminium alloys

The major factors believed to influence the pitting of aluminium alloys are conductivity, pH, and bicarbonate, chloride, sulphate and oxygen content [2.6]. Because of the interrelationship of the composition and service factors, it is difficult to predict the influence of water on aluminium corrosion from a table of water composition alone. A number of studies have been conducted of synthetic waters containing several metal and salt ions alone and in combination [2.15-2.17]. They found that the corrosion of aluminium was accelerated when salts of copper, chlorides and bicarbonates were present together, compared with cases where only a single impurity was present. In some cases where two of the three constituents were present, there was little corrosion, but with the three species present together, nodular corrosion occurred. 

Mears and Brown [10.9] studied the influence of temperature on pitting of aluminium alloys in chloride solutions. They found that as the temperature increased, the density of pits and the probability of pitting increased, while the pitting rate or average pit depth decreased. Consequently it has been observed that it is extremely important to maintain basin water temperatures as low as possible to avoid corrosion. 

Metals which owe their good corrosion resistance to the presence of thin, passive or protective surface films may be susceptible to pitting attack when the surface film breaks down locally and does not reform. Thus stainless steels, mild steels, aluminium alloys, and nickel and copper-base alloys (as well as many other less common alloys) may all be susceptible to pitting attack under certain environmental conditions, and pitting corrosion provides an excellent example of the way in which crystal defects of various kinds can affect the integrity of surface films and hence corrosion behaviour. 

Murray, G. A. W., Artificial Pits for Quantitative Studies of Corrosion of Aluminium Alloys in Natural Waters , Corrosion, 20, 329 (1964)... 

Frazer, M. J. and Langstaff, R. D., Influence of Cupric Ions on the Behaviour of Surface-active Agents Towards Aluminium , Bril. Corrosion J., 2, II (1%7) Szklarska-Smialowska, Z. and Janik-Czachor, H., Pitting Corrosion of 13Cr-Fe Alloy in Na2S04 Solutions Containing Chloride Ions , Corros. Sci., 7, 65 (1967)... 

Most simple inorganic salt solutions cause virtually no attack on aluminium-base alloys, unless they possess the qualities required for pitting corrosion, which have been considered previously, or hydrolyse in solution to give acid or alkaline reactions, as do, for example, aluminium, ferric and zinc chlorides. With salts of heavy metals —notably copper, silver, and gold —the heavy metal deposits on to the aluminium, where it subsequently causes serious bimetallic corrosion. 

Aluminium pipes Aluminium might become an important material for carrying water if its liability to pitting corrosion could be overcome. Very soft waters are difficult to accommodate when normal pipe materials are used, and it is for these that aluminium offers most promise ". The possibility of using it for domestic water pipes, however, appears at present to depend upon finding a cheap and effective inhibitor that could be added to the water, or upon the use of internally clad tube, e.g. Al-1 25 Mn alloy clad with a more anodic alloy, such as Al-lZn. Such pipes are at present mainly used for irrigation purposes. ... 

Recommended practice for examination and evaluation of pitting corrosion Test method for determining susceptibility to stress corrosion cracking of high-strength aluminium alloy products Test method for pitting and crevice corrosion resistance of stainless steels and related alloys by the use of ferric chloride solution Recommended practice for preparation and use of direct tension stress corrosion test specimens... 

The application of a chromate conversion coating resulted in a corrosion potential of about -676 mV, i.e. increased slightly compared with the etched alloy. Pitting occurred at a potential of about 150 mV above Ecorr. Thus, the sol-gel coating with incorporated nanoparticles provides significant barrier protection for the AA2024 aluminium alloy. 

As expected from the data in Table 7.3, galvanic corrosion is one of the major practical corrosion problems of aluminium and aluminium alloys. The reason for this is that aluminium is thermodynamically more active (less noble) than most other common structural materials, and that the passive oxide which usually protects aluminium may easily be broken down locally when the potential is raised due to contact with a more noble material. This is particularly the case when aluminium and its alloys are exposed in waters containing chlorides or other aggressive species (see also Section 7.6, Pitting Corrosion). 

Conversely, for several aluminium alloys, pit initiation can be accepted under many circumstances. This is so because numerous pits are usually formed, and the oxide is insulating and has therefore low cathodic ability, so that the corrosion rate is under cathodic control. However, if the cathodic reaction can occur on a different metal because of a galvanic connection or for instance deposition of Cu on the aluminium surface, the pitting rate may be very high. Since we in other respects can accept pit initiation, the time dependence of pit growth and pit depths is important, and we shall consider this more quantitatively. 

The cubic root relation in Equation (7.4) is not always valid. There is considerable scatter, the exponent may deviate from 1/3, and other deviations may exist too, particularly during the first few time intervals. Figure 7.32 shows an example of pit depth development expressed by functions of the form d = a -F b log t. The results deviate little from the cubic root relationship. Altogether, Equation (7.4) can be considered as reasonably representative, at least for the more corrosion-resistant aluminium alloys (those represented in Figure 7.32 as well as the AIMn alloys). 

Cathodic protection can also be applied to prevent pitting. Regarding aluminium, strong cathodic polarization should be avoided because this can lead to a large increase of pH close to the metal surface, which can cause so-called alkaline corrosion (compare with the Pourbaix diagram for aluminium in Figure 3.11, Section 3.8). Use of sacrificial anodes of Zn or A1 alloys is therefore safer than impressed current. 

Aziz PM. Application of the statistical theory of extreme values to the analysis of maximum pit depth for aluminium alloys. Corrosion, 12, 1980 35-46. 

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]. 

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