Susceptibility of Aluminium Alloys

Rolling and extrusion leads to a grain orientation parallel to the direction of transformation. Since stress corrosion has an intercrystalline propagation, susceptibility to this type of corrosion is, therefore, not the same in the three directions with respect to the direction of rolling or extrusion (Rgure B.2.13)  

The resistance to stress corrosion of thick rolled or extruded products depends on the sampling direction of the test pieces. It is always lower in the short transverse direction than in the other two directions. 

In general, the resistance to stress corrosion is higher for the following tempers  

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In addition to examining pre-exposure effects, the slow strain-rate testing technique has been used increasingly to examine and compare the stress-corrosion susceptibility of aluminium alloys of various compositions, heat treatments and forms. A recent extensive review draws attention to differences in response to the various groups of commonly employed alloys which are summarised in Fig. 8.57. The most effective test environment was found to be 3 Vo NaCl -F 0.3 Vo HjOj. The most useful strain rate depends upon the alloy classification. 

The susceptibility of aluminium alloys to stress corrosion was mentioned for the first time in 1922 by Rawdon, who established a relation between susceptibility to intercrystalline corrosion and stress corrosion of Duralumin [19]. 

By analogy with failure mechanics, two criteria can be determined for assessing the susceptibility to stress corrosion of aluminium alloys ... 

Testing corrosion of exfoliation corrosion of aluminium alloys Determination of susceptibility to exfoliation corrosion of alloys of the 2000 and 7000 series alloys (EXCO)... 

The comparison of the dissolution potentials of aluminium alloys may reach absurdity, for example, leading to a preference for alloys of the 2000 series, which have a dissolution potential far less negative, about — 650 mV, over those of the 5000 series, which have a more electronegative potential, on the order of - 800 mV (Table B.1.3). And yet the latter show excellent corrosion resistance, while alloys of the 2000 series are highly susceptible to pitting corrosion in natural environments. 

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. 

This example of aluminium illustrates the importance of the protective him, and hlms that are hard, dense and adherent will provide better protection than those that are loosely adherent or that are brittle and therefore crack and spall when the metal is subjected to stress. The ability of the metal to reform a protective him is highly important and metals like titanium and tantalum that are readily passivated are more resistant to erosion-corrosion than copper, brass, lead and some of the stainless steels. There is some evidence that the hardness of a metal is a signihcant factor in resistance to erosion-corrosion, but since alloying to increase hardness will also affect the chemical properties of the alloy it is difficult to separate these two factors. Thus althou copper is highly susceptible to impingement attack its resistance increases with increase in zinc content, with a corresponding increase in hardness. However, the increase in resistance to attack is due to the formation of a more protective him rather than to an increase in hardness. 

Aluminium alloys are susceptible to thermite sparking when dropped on to rusty surfaces. Consequently their use may be subject to restrictions. For example, in ships tanks the weight of the anode and the height that it is suspended are strictly controlled. This is because thermite sparking is dependent on the kinetic energy of the anode. 

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. 

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... 

Method for visual assessment of exfoliation corrosion susceptibility of 5XXX series aluminium alloys (Asset test)... 

Test method for determining the susceptibility to intergranular corrosion of 5XXX series aluminium alloys by mass loss after exposure to nitric acid (NAMLT test) Practice for liquid sodium corrosion testing of metals and alloys... 

A result of these hot salt corrosion and hot salt stress corrosion studies of titanium alloys was the importance of alloy composition in determining a titanium alloy susceptibility to attack. It was shown that alloys high in aluminium were preferentially attacked by hot salt [24, 25], with both A1 and Zr being incorporated in the internal and external oxide scales [26]. This early work raised the importance of aluminium in the hot salt corrosion of titanium alloys. 

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