Microstructural characterization of AA2199-T8E74 plate revealed large grains elongated in the rolling direction. 5 different compositional domains were found among coarse inter-metallic particles (% of total): Al7Cu2Fe (20%), Al37Cu2Fe12 (20%), Al6(FeMn) (30%), Al13(FeMn)4 (15%) and Al2Cu (15%). Lithium was absent within coarse inter-metallic particles, suggesting the limit for Li incorporation into coarse second phases lies somewhere between 1.4% - 1.7%, given it has been found to be present on Al-Cu-Li alloys of a lower Cu/Li ratio (with 1.8% Li content). Spherical β (Al3Zr) type dispersoids and rod-like Al-Cu-Mn-Li dispersoids were also identified. The strengthening precipitates within the alloy were found to be θâ (Al2Cu) precipitates on {1 0 0} planes, T1 (Al2CuLi) precipitates on {1 1 1} planes and δ (Al3Li) precipitates, found as small, lenticular-like precipitates; flanking θâ precipitates or co-precipitating with the T1 phase of specific orientation relationships with the matrix. Caustic etching removed surface inter-metallics, preferentially removing Al2CuFe, Al2Cu and Al37Cu2Fe12 and results in the development of a surface Cu- enriched nano-layer, both contributing to higher anodizing efficiencies by reducing the amount of current spent on anodizing second phases. Surface heterogeneity impacts the anodic film morphology, giving raise to three defect types. IMP removal during caustic etching reduces the defects in the flaws by up to 50%, but the Cu-enriched layer causes irregular morphology and lateral porosity. This lowers the protection provided by anodic films on practical aluminum alloys, and hinder the ability of hydrothermal sealing to successfully passivate the alloy against corrosion in the presence of aggressive electrolytes. The shortcomings of HTS in successfully protecting TSAA aluminum alloys and the need for Cr6+ removal from aerospace corrosion protection schemes drives the interest for investigating alternatives. Two modified sealing procedures were designed and assessed. These consisted in adding an intermediate step between tartaric-sulfuric acid anodizing and hydrothermal sealing, where the alloy is immersed in a solution based on environmentally benign i) Ce(NO3)3 or ii) a commercial tri-valent chromium solution, SurTec 650V. Assessment of the modified sealing processes was performed via: i) a morphological and compositional characterization of the resulting sealed films, using a combination of SEM, STEM, EDX and glow discharge optical emission spectroscopy (GDOES); ii) electrochemical analysis of polarization curves on the anodized and sealed films, and electrochemical impedance spectroscopy (EIS), monitoring the change in corrosion parameters during immersion in 3.5% NaCl; and iii) visual inspection of the surface after immersion. Equivalent circuits consisting of two RC time constants were used to fit the data. Two formulations are proposed that provide effective corrosion protection during prolonged immersion in Cl- containing media.
Date of Award | 1 Aug 2020 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | George Thompson (Supervisor) & Xiaorong Zhou (Supervisor) |
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- Aluminium; Aluminium Alloys; Anodising; Hydrothermal Sealing
Improving the corrosion performance of tartaric-sulfuric acid anodised AA2199-T8E74 via modified sealing processes
Taylor, A. (Author). 1 Aug 2020
Student thesis: Phd