Stress Corrosion Cracking of Al-Li-Cu-Zr Alloy 2090 in Aqueous Cl⁻ and Mixed CO₃²⁻/Cl⁻ Environments

A comparison of the short-tranverse SCC behavior of 2090 in pH 5.5 Cl⁻ and alkaline CO₃²⁻/Cl⁻ solutions using a static load smooth bar SCC technique was made. In the alkaline CO₃²⁻/Cl⁻ solutions, E_br for the α-Al matrix phase was 0.130 V more positive than the E_br of the subgrain boundary T₁ phase. In this environment, stress corrosion cracking test specimens subjected to potentials in the window defined by the two breakaway potentials failed along an intersubgranular path in less than an hour. In the Cl⁻ environment, the E_br values for the two phases were nearly equal and this rapid SCC condition could not be satisfied; accordingly SCC failures were not observed.

Rapid SCC failure of 2090 in CO₃²⁻/C1⁻ in our static load, constant immersion experiments appear to be related to recently reported "pre-exposure embrittlement" failures induced by immersing stressed specimens removed into ambient laboratory air after immersion in aerated NaCl solution for 7 days. In those experiments, specimens failed in less than 24 hours after removal from solution. Our polarization experiments have shown that the corrosion behavior of T₁, residing at the crack tip, is relatively unaffected by the alkaline CO₃²⁻/Cl⁻ environments, but the α-Al phase crack walls, is rapidly passivated. X-ray diffraction of the films which formed in simulated crevices suggests that this passivating film belongs to a class of compounds known as hydrotalcites.