Most heat treatable aluminum alloys are, to some degree, susceptible to intergranular corrosion, depending upon the metallurgical treatments that have been given to the metal. Susceptibility to intergranular attack results from a localized decomposition of solid solution at the grain boundaries and the formation of identifiable precipitates in the grain boundaries. Hence, any alloying addition, thermal treatment or mechanical treatment that influences precipitation of alloying constituents can influence the susceptibility to intergranular attack. The exact nature of the localized corrosion path and the specific roles of grain boundary precipitates and the parent solid solutions in the grain margins depend upon alloy composition. The actual significance of the intergranular corrosion tendency with regard to the serviceability of an aluminum alloy product varies with the alloy and the temper.
Products of alloys such as 2024-T3, 7075-T6, and 7178-T6 become susceptible to intergranular attack usually as a result of heat treating procedures that are less than the optimum, and in such a condition they may be susceptible to stress-corrosion cracking or exfoliation corrosion. Such products can be artificially aged to produce special tempers, 2024-T8, 7075-T76, and 7178-T76 that provide a high resistance to stress-corrosion cracking and to exfoliation. Although products in these tempers may show some susceptibility to intergranular attack in an accelerated corrosion test, such as specified in MIL-H-6088E, and in some natural environments, this intergranular attack does not indicate a poor service performance.
Hence, it should be understood that predictions of the serviceability of a high-strength aluminum alloy product can be made on the basis of an intergranular corrosion test only in certain instances, chiefly for thin sections of aluminum-copper alloys in the naturally aged temper (T3, T4) and aluminum-zinc-magnesium-copper alloys in the maximum strength temper (T6). The most reliable predictions can be obtained from appropriate stress-corrosion cracking and exfoliation tests; for some alloys other criteria, such as electrical conductivity and potential measurements, are feasible.