Exfoliation Corrosion Mechanism: Interplay between Intergranular Corrosion and Stress Corrosion Cracking during Exfoliation Corrosion of AA7449

Abstract

It is generally considered that exfoliation corrosion is due to the build-up of corrosion products that create a wedging stress that lifts up the surface grains. However, the exfoliation mechanism is still under discussion: possible operating mechanisms include intergranular corrosion of in plane grain boundaries accelerated by the wedging effect, or crack propagation by a “purely” stress corrosion mechanism. The sensitivity to exfoliation corrosion of AA7449 in relation to the intergranular and stress corrosion cracking sensitivity has been addressed in a program of controlled quenches followed by thermal treatments. Our observations demonstrate that the quench rate has a strong effect on intergranular corrosion and exfoliation corrosion sensitivity and in a lesser extent on stress corrosion cracking. In the first moments of the EXCO test, the initiation of corrosion follows the same trends as those revealed by the ASTM G110 test. We observe intergranular initiation for the slow quench rate (~5°C/s) and pitting initiation for samples quenched between 50 to 500°C/s. On the contrary, the final EXCO corrosion quotations do not seem to correlate with the intergranular resistance but rather with SCC resistance.