Beitrag zur Wachstumskinetik der interkristallinen Korrosion von ausgehärteten Al-Cu-Legierungen – Teil I: Ergebnisse der Folien-Durchbruchsmethode

Contribution to the growth kinetics of the intergranular corrosion of age-hardened Al-Cu alloys – Part I: Results of the foil penetration technique The growth kinetics of the intergranular corrosion (IC) of age-hardened Al-Cu-alloys in aqueous chloride solutions under potentiostatic conditions have been investigated using the foil penetration technique originally developed for pit growth measurements. The two tested systems were a pure binary Al-4% Cu-alloy (sheet thicknesses 0.2 and 0.5 mm) in 0.01 m NaCl, pH 11, and a commercial AA 2024 type alloy (various sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.1 m NaCl, pH 7, respectively. Both alloys were tempered to maximum IC susceptibility and have been tested at potentials where selective anodic dissolution of the grain boundary regions occurs. As a reference system, the pit growth kinetics in commercial pure aluminum (sheet thicknesses ranging from 0.2 to 1.0 mm) in 0.01 m NaCl + 0.01 m Na₂SO₄ (as pit initiation inhibitor), pH 11, have been investigated at an applied potential considerably higher than the pitting potential. The experimental results have shown that, for the IC in the finegrained commercial alloy, a uniform growth kinetic valid for the whole range of sheet thicknesses, as for the case of pitting, cannot be formulated. The correlation of the current-time-curves and the attack morphologies (after penetration) for different sheet thicknesses with the penetration times leads to the conclusion that the growth kinetics of intergranular attacks are related to the number of sites of active attack (cracks) per metal volume. This specific number of actively growing cracks depends on the grain size and on the electrochemical conditions and, for given parameters, on the exposure time and therefore on the sheet thickness. For the commercial alloy examined, the following three stages of attack with decreasing penetration velocity of the IC could be distinguished:

• activation stage
• transition stage
• stable, macroscopic grain boundary dissolution.

The coarse-grained pure binary alloy showed a markedly higher penetration velocity of the IC under milder electrochemical conditions. This alloy is suitable for a model investigation of the first stage of attack but no quantitative kinetic information could be obtained from the only two disposable sheet thicknesses. The pit growth measurements in pure aluminum showed that the square root-of-time growth law previously found for thin foils and sheets is valid for deep pits too. The discussion explains that the electrochemical mechanisms of pitting of aluminum and of the IC in Al-Cu-based alloys are identical and that the basic difference lies in the geometry of the sites of attack. The ohmic control of the aluminum dissolution and therefore of the growth kinetics of pits and intergranular cracks is governed by the total anodic area and its different time dependence during the growth of the sites of local attack.