Localized corrosion susceptibility of Al-Li-Cu-Mg-Zn alloy AF/C458 due to interrupted quenching from solutionizing temperatures

Isothermal time-temperature-localized corrosion-behavior curves were determ ined for the Al-1.8Li-2.7Cu-0.6Mg-0.3Zn alloy AF/C458, to understand the ef fect of slow or delayed quenching on localized corrosion susceptibility. Al loy samples were subject to a series of systematic interrupted quenching ex periments conducted at temperatures ranging from 480 degreesC to 230 degree sC for times ranging from 5 to 1000 seconds. Individual samples were then e xposed to an oxidizing aqueous chloride solution consisting of 57 g/L NaCl plus 10 mL/L H2O2 to induce localized attack. The localized corrosion mode was characterized by optical microscopy. Additionally, the microstructure o f selected samples was characterized by transmission electron microscopy (T EM) to relate the corrosion mode and morphology to microstructural features . Results showed that only pitting attack was exhibited by samples subjecte d to isothermal treatment at temperatures greater than 430 degreesC. At tem peratures ranging from 280 degreesC to 430 degreesC, isothermal treatment t ended to induce susceptibility to intergranular attack (IGA) and intersubgr anular attack (ISGA) for all treatment times investigated. For isothermal t reatments at temperatures lower than 280 degreesC, only pitting was observe d for treatment times less than about 30 seconds, while IGA and ISGA were o bserved for longer treatment times. Comparisons showed that the time-temper ature domains for IGA and ISGA were virtually coincident. Based on this fin ding and the results from TEM characterization, IGA and ISGA appear to be r elated to the precipitation of a Zn-modified T-1 (Al-2(Cu,Zn)Li) precipitat e, which can occur both on low-angle and high-angle grain boundaries in thi s alloy. When the alloy is resistant to IGA and ISGA, the grain boundaries are decorated by theta ' (Al2Cu), and T-B (Al7Cu4Li) phase particles, or su bgrain boundaries are populated by a comparatively low density of T-1 preci pitates. It is, therefore, speculated that theta ' and T-B are more corrosi on-resistant precipitate phases than T-1, and that a critical concentration of boundary T-1 must exist for IGA or ISGA to occur.