Abstract
Third-generation aluminum-copper-lithium (Al-Cu-Li) alloys are attractive for aerospace applications because of their improved strength-to-weight and stiffness-to-weight ratios, fracture toughness, and corrosion resistance compared to legacy alloys such as AA2024. Al-Cu-Li alloys can suffer from localized corrosion during service, and accelerated laboratory testing is an important tool for quickly assessing corrosion properties. Many standardized tests for high-strength aluminum-alloys exist, but these tests do not all correlate well with service results. Furthermore, different tests that are aimed at assessing the same corrosion behavior can produce drastically different results for the same alloy temper.
Exfoliation is a specific form of localized corrosion that occurs in rolled alloys that have an elongated grain structure, and it can be service life-limiting for many high-strength Al alloys. Three accelerated tests for exfoliation corrosion were considered in this study, including ASTM G34 (a constant immersion test), ANCIT (a modified G34 test), and ASTM G85-A2 (a cyclic acidified salt spray test). These tests were chosen because they produce different results for Al-Cu-Li alloy 2060. Each test incorporates various strategies for accelerating corrosion, such as high chloride concentration, low solution pH, elevated testing temperature, increased oxidizing power, and/or relative humidity cycling, but the impact of these parameters on exfoliation corrosion is not well understood. Both the under-aged AA2060-T3 temper (exfoliation-susceptible) and the near peak-aged AA2060-T86 temper (exfoliation-resistant) were considered in this study.
The roles of individual test parameters were isolated by measuring the impact of altered testing conditions on electrochemical kinetics and corrosion morphology. It was found that the low solution pH of ASTM G34 caused an increase in the cathodic reaction kinetics compared to ANCIT testing, due to fast proton reduction kinetics in highly acidic solutions (pH <1). The elevated testing temperature of ANCIT caused a significant increase in anodic kinetics, which made ANCIT more aggressive overall. This increase in anodic kinetics resulted in faster exfoliation formation in AA2060-T3, which was the expected corrosion morphology based on seacoast exposures. Although exfoliation formed on the susceptible -T3 temper during exposure to ANCIT, the resistant -T86 temper experienced an unrealistic severity of attack in ANCIT.
A common method of increasing corrosion rate during accelerated corrosion testing is to add an oxidizing agent with reduction kinetics faster than oxygen reduction (ORR) and proton reduction. The impact of two oxidizing agents, hydrogen peroxide (H2O2) and potassium persulfate (K2S2O) on corrosion kinetics during accelerated testing of AA2060-T3 and -T86 was investigated as a part of this work. H2O2 produced faster cathodic kinetics than K2S2O8 did, but the cathodic kinetics of K2S2O8 could be manipulated by adjusting solution pH and adding aluminum chloride (AlCl3). Faster kinetics does not always result in the desired corrosion morphology, however, and AA2060-T3 and T86 samples exposed to solutions containing both K2S2O8 and AlCl3 could not be distinguished by temper.
Relative humidity (RH) was found to be a critical parameter for cyclic salt spray testing. Dry bottom (DB) and wet bottom (WB) ASTM G85-A2 was performed in two different chamber brands, and exposure results were different for the different brands. Seemingly small differences in chamber operation caused RH to be significantly different during the dry air purge and dwell periods in the two chambers. In chamber Brand 2, the low RH during the dwell period (17% - 20.5%) caused corrosion kinetics to slow, and only mild exfoliation formed after 4 weeks of exposure. It was shown that a more moderated RH (77% - 61%) during the dwell period was more appropriate for accelerating exfoliation. Two modified cyclic salt spray tests were designed to have higher RH during the dwell period, and both tests produced higher corrosion kinetics and faster exfoliation formation compared to the standard test. Additional high-strength aluminum alloys (AA7075, AA2024) were exposed to the modified tests, and both tests could correctly distinguish between exfoliation-resistant and susceptible tempers.
