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Investigation Into the Localized Corrosion of Aluminum-Copper-Lithium Alloy 2099

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2018, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Third generation Al-Cu-Li alloys have improved localized corrosion resistance compared to previous generations and are attractive to the aerospace industry because of the mix of low density and good mechanical properties. Al-Cu-Li alloy AA2099 (Al 2.7Cu 1.8Li 0.6Zn 0.3Mg 0.3Mn 0.08Zr) is a newer precipitation-strengthened alloy with a cleaner microstructure that contributes to increased corrosion resistance. However, there is still a susceptibility for intergranular and inter-subgranular (IGC/IsGC). Because localized corrosion associated with coarse constituent particles is diminished due to alloy cleanliness, intergranular forms of attack are a larger factor in the corrosion profile of this alloy. The susceptibility to localized corrosion in AA2099 was characterized based on the attack morphology after exposure to various NaCl aqueous solutions. Alloy 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, corresponding to time and temperature ranges that are commensurate with commercial practice. The resulting microstructures were analyzed using scanning transmission electron microscopy (TEM), electron back-scattering, and diffraction methods, which characterized the precipitates formed during artificial aging. The formation of the strengthening phase T1 (Al2CuLi) was of particular interest due to its reported anodic behavior relative to the alloy matrix. This particle is prone to corrosion attack and plays a significant role in the evolution of localized corrosion mode and morphology depending on its location within the alloy. The results from the exposure experiments provided a map for the various heat treatments to identify when IsGC susceptibility will occur. Results showed that AA2099 went through several attack categories as samples were aged to under-aged (UA), peak-aged (PA), and over-aged (OA) conditions. The morphology in the cross section progressed from pitting to IsGC to pitting caused by early-stage crosshatch intra-granular attack. The susceptibility to IsGC was determined to follow a similar trend for T phase growth in another Al-Cu-Li alloy heat-treated within the same temperature and time ranges. Electrochemical experiments also concluded that the formation of the T1 particle is integral in the susceptibility to IsGC. The corrosion potentials of a specially synthesized bulk analog T1 and AA2099 matrix samples were measured in identical environments that were used for the exposure tests. The free-corrosion potential of T1 was lower than the matrix in all three solutions, which indicates the particle will preferentially dissolve in the alloy when galvanically connected to its surroundings. TEM characterization showed that samples that displayed a high density of T1 isolated along grain and subgrain boundaries also displayed a large amount of IsGC. Conversely, treating to the OA condition or cold working the sample produced nucleation of T1 into the matrix. This caused the localized attack to shift back to a primarily pitting morphology. The increased formation of matrix T1 was also seen in the breakdown potentials of the alloy, which became more negative and approached the breakdown potential of pure T1. Potentiostatic tests were conducted to investigate how the localized corrosion propagated into the alloy. Held just above the breakdown potential to initiate attack, the alloy experienced a predefined amount of corrosion damage as specified by an induced charge density. The cross-sectional pit/IsGC depths were imaged and measured to determine which factors affect attack growth. It was determined that the characteristics of the grains and sub-grains had a larger influence than heat treatment. Because attack was mostly confined to the grain interior, the orientation and chemical makeup of the boundaries dictated attack propagation, i.e. grains that were elongated into the sample surface and heat treatments that produced the highest density of isolated boundary T1 experienced the deepest attack. Finally, an artificial neural network model was developed as a method of predicting how severe IsGC will be in AA2099 based on the experimental conditions used in the electrochemical tests. Attack morphology and measured depths were used in training the model to learn the effect heat treatment, grain orientation, and imparted charge have on the measured depth and IsGC susceptibility. Trained models could reasonably predict corrosion damage accumulation from known inputs characterizing exposure conditions. A sensitivity analysis was also performed to identify which characteristics were most important for the prediction. It was found that attack depth was primarily affected by grain orientation and imparted charge; IsGC susceptibility was controlled by heat treatment, consistent with conventional understanding of IGC susceptibility for wrought aluminum alloys.
Rudolph Buchheit (Advisor)
236 p.

Recommended Citations

Citations

  • Hanna, B. (2018). Investigation Into the Localized Corrosion of Aluminum-Copper-Lithium Alloy 2099 [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534749550969422

    APA Style (7th edition)

  • Hanna, Benjamin. Investigation Into the Localized Corrosion of Aluminum-Copper-Lithium Alloy 2099. 2018. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1534749550969422.

    MLA Style (8th edition)

  • Hanna, Benjamin. "Investigation Into the Localized Corrosion of Aluminum-Copper-Lithium Alloy 2099." Doctoral dissertation, Ohio State University, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=osu1534749550969422

    Chicago Manual of Style (17th edition)