Deconstructing the Electrochemical Basis of ASTM G34 and ANCIT

ASTM G34 and ANCIT are full immersion tests designed to measure exfoliation susceptibility of aluminum alloys (Al alloys). G34 works well for certain 7XXX series alloys such as AA7178and AA7075, but this test fails to reproduce the attack observed in natural environments for many other Al alloys. ANCIT was designed to replace G34 for certain Alalloys, but even this improved test does not correlate well with seacoast exposure results for 3rd generation aluminum-copper-lithium (Al-Cu-Li) alloys such as AA2060. The purpose of the current work is to deconstruct the G34 and ANCIT testing environments and to explain the electrochemical impact of each testing variable on the exfoliation corrosion of AA2060. Understanding the role of each component of the testing environment will allow more intelligent design of future immersion tests, reducing the need for time consuming trial and error. It was found that cathodic kinetics in fresh G34 solution were driven primarily by proton reduction as a result of the low solution pH (0.2 to 0.4), but cathodic kinetics were dominated by reduction in ANCIT solution. The small amount of AlCl₃ present in ANCIT solution seemed to impact the mechanism by which was reduced, but the origin of this effect was not well understood. The presence of slowed anodic kinetics, and this effect was more significant for AA2060-T3 than for AA2060-T86. The elevated temperature during ANCIT exposure increased anodic kinetics, and this was identified as a key difference between the two tests that caused exfoliation to form faster in ANCIT than in G34. Although the solution pH started out much lower in G34 (0.2 to 0.4) than in ANCIT (2.8 to 3.2), the pH increased during the first 24 h of G34 resulting in a similar solution chemistry for the remainder of testing time in both tests. The evolution of solution chemistry had implications for cathodic kinetics as the initial fast H⁺reduction kinetics in G34 was a result of the low solution pH in fresh solution.