Intergranular corrosion in aluminum alloys

The mechanism of intergranular corrosion in 7XXX alloys is believed to be analogous to that in 2XXX alloys, but it has not been studied in as great detail because most of the precipitate phases cannot be grown to a sufficient size to be analyzed. However, addition of zinc and the dissolution of the MgZn2 phase both shift the potential of these alloys in the anodic direction, so that a potential difference of as much as 0.24 V could exist between pure aluminum... 

Kaesche H (1974), Pitting Corrosion of Aluminum and Intergranular Corrosion of Aluminum Alloys, in localized corrosion, proceedings Staehle R, Brown B F, Kruger J and Agrawal A, ed, Houston, Texas, NACE International, 516-525,... 

In aluminum alloys exposed to aqueous chloride solutions, corrosion fatigue cracks frequently originate at sites of pitting or intergranular corrosion. Initial crack propagation... 

Small additions of aluminum slightly decrease the corrosion resistance of zinc, especially if lead is also present. The maximum decrease occurs at 0.3% aluminum (in an alloy containing 0.2% lead). Aluminum appears to increase slightly the risk of intergranular attack in zinc alloys. 

SCC in aluminum alloys is typically intergranular, although transgranular SCC has been observed for a few alloys under highly specific environmental conditions [16,19] and may be part of the propagation mechanism in some 7xxx alloys. SCC requires the interaction of a metalliugically susceptible material, a corrosive environment (water vapor may be sufficient) and sustained (but not necessarily continuous) tensile stress. 

Intergranular corrosion does not necessarily correlate with SCC in aluminum alloys. The 6000-series (Al-Mg-Si) alloys such as 6061 can suffer inteigranular corrosion but never fail by SCC. Examination of fiactuied specimens following intergranular corrosion shows crystallographic tuimeling attack on either side of... 

As a general rule, aluminum alloys, particularly the 2xxx series, are less corrosion resistant than the commercial purity metal. Some aluminum alloys, for example, are susceptible to intergranular corrosion as a result of low-temperature aging reactions and the subsequent precipitation in the grain boundaries. Susceptibility to intergranular attack in these alloys shows up as exfoliation and stress-corrosion cracking (SCO). 

Lifka B.W., Sprowls D.O., Significance of intergranular corrosion in high-strength aluminum alloy products, ASTM, STP, vol. 516, 1972, p. 120-144. 

SCC is a time-dependant intergranular fracture mode (in aluminum alloys) that requires the combined presence of a susceptible alloy, a sustained tensile stress and a corrosive environment. 

Austenitic stainless steels are the most significant class of corrosion-resistant alloys for which intergranular corrosion can be a major problem in their satisfactory use. The problem is most often encountered as a result of welding but also may result from stress-relief annealing or incorrect heat treatments. Intergranular corrosion also can occur in ferritic stainless steels and in nickel- and aluminum-base alloys. 

Fig. 7.90 Effect of stressing direction on the intergranular stress-corrosion crack path in susceptible high-strength aluminum alloy. Dark boundaries are representative of ones favored for cracking for indicated direction of applied stress. Source Ref 97...

---