A Comparison of Potentiodynamic and Potentiostatic Electrochemical Test Methods for Resolving the Localized Corrosion Response of Heat Treated Al-Li-Cu Alloy 2099

Abstract

The corrosion mechanisms for 3rd generation Al-Li-Cu alloy AA2099 were characterized based on the localized attack on the microstructure. Samples were subjected to a series of artificial heat treatments conducted at temperatures ranging from 120°C to 180°C for times ranging from 12 to 168 hours. The varying microstructures were analyzed using transmission electron microscopy (TEM) and electron diffraction spectroscopy (EDS), which characterized the secondary phase precipitates grown during artificial aging. The formation of the strengthening phase T1 (Al2CuLi) is of particular note due to its anodic behavior relative to the alloy matrix. This particle is prone to selective dissolution and dealloying and can dictate the morphology of localized corrosion depending on how it is distributed within the alloy. Localized corrosion was induced via individual immersion tests that utilized a highly oxidizing NaCl/H2O2 aqueous solution. The extent and morphology of the corrosion was assessed by optical microscopy. Results showed that pitting attack was apparent on samples aged at lower temperatures and/or for shorter times, when growth of secondary phase precipitates is hindered. Intergranular corrosion (IGC) was observed for the time-temperature combinations that promoted such precipitate growth. As revealed from TEM analysis, these samples showed T1 precipitates localized along grain boundaries. However, as samples continued to age and T1 formation became prevalent in the matrix, IGC was reduced. This is attributed to a smaller potential difference between grain boundary T1 and the surrounding area. Potentiodynamic experiments were then performed to identify any change in the localized breakdown potentials among the heat treated samples. It was shown that the specimens susceptible to IGC displayed a lower breakdown potential than those susceptible to only pitting. Additional electrochemical tests were performed to capture the transition from pitting to IGC and determine whether the transition is potential-dependent, time-dependent or a combination of both. Unlike other Al-Cu alloys scanned potentiodynamically through the region of passivity breakdown, AA2099 did not display a double breakdown potential that would have indicated separate breakdown events for pitting and IGC. However, potentiostatic tests conducted after the onset of pitting showed that this transition can be captured based on the total charge imparted on the alloy. Samples that exhibited the most severe IGC via the immersion tests required the smaller amount of imparted charge to initiate IGC from pits. This demonstrates that the progression from pitting to IGC is marked by a distinct temporal component.