When Al-Cu-Mg (AA2024-T351) alloy plate is immersed in 0.53 m NaCl solution, intergranular corrosion occurs, which initially takes the form of narrow fissures at the surface. Although growth of these fissures is initially rapid, it soon slows and the penetration depth is limited. However, these intergranular fissures can give rise to intergranular subcritical crack growth and eventual catastrophic fracture if specimens are strained slowly in laboratory air after pre-exposure to the NaCl solution. In our experiments, tensile specimens of the alloy were strained at strain rates in the range 10-7–10-4/s, and the cracking propensity was found to increase with pre-exposure times up to around 300 h. However, cracking was much reduced with longer pre-exposure times and became negligible after 500 h. This rather unexpected result can be explained by the observation that the intergranular corrosion fissure becomes increasingly broad and un-crack-like with extended pre-exposure times, failing to meet the prerequisite initiation conditions required for intergranular stress corrosion cracking. A further unexpected result was that straining specimens in the same 0.53 m NaCl solution after pre-exposure produced no intergranular crack growth. To investigate this further, tensile tests were conducted under electrochemical control, and this showed that the free corrosion potential in the test solution was too low in 0.53 m NaCl to allow cracking to proceed.
An extensive program of slow-strain-rate stress corrosion testing of aluminum alloys in a wide variety of susceptibility conditions and environments produced results fully consistent with the expected behavior from service experience and the results of other established test methods. Tests were conducted both on laboratory alloys in sheet form and on commercial alloys in wire, sheet, and plate forms; the systems studied included Al-Zn-Mg, Al-Zn-Mg-Cu, Al-Cu-Mg, and Al-Mg. Suitable strain rate regimes for testing, standard environments, and optimum stress corrosion susceptibility parameters were established. In general, a rate of 10−6/s was found to be appropriate for 2000 and 7000 series alloys and 10−4/s for 5000 series alloys. The most effective test environment was found to be 3% NaCl + 0.3% H2O2 for all alloys tested, and in general either relative elongation or reduction in area was the most meaningful susceptibility assessment parameter.
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