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Localized corrosion and stress corrosion cracking studies of Al-Li-Cu Alloy AF/C458

Mathur, Deepanker
http://rave.ohiolink.edu/etdc/view?acc_num=osu1407401974

Abstract Details

2000, Master of Science, Ohio State University, Materials Science and Engineering.
Corrosion, localized corrosion and stress corrosion cracking behavior of AF/C458 were characterized in this study. Electrochemical properties, namely, corrosion potential, pitting potential and passive current density were determined from anodic polarization in nitrogen sparged 0.5 M NaCl solutions. Corrosion potentials ranged from -0.82 to -0.88 Vsce. Pitting potentials decreased from about -0.650 to -0.700Vsce with increasing aging time, and passive current densities were about 1 to 5 µA/cm2. As a baseline for SCC testing, mechanical properties of the alloy were determined. Hardness testing on Rockwell (RB Brinell) scale was done on the solution-heat treated and artificially aged (SHT + AA) and T3 and artificially aged (T3 + AA) samples. Similar hardness values were obtained for ST and LT oriented samples in SHT + AA condition (75-73 (ST-LT) and 81-82 (ST-LT) at 150 and 190°C, respectively). Uniaxial tensile tests showed a yield strength of 440 MPa (63.9ksi) and 426MPa (61.8ksi) and tensile strengths of 500MPa (72.6ksi) and 473 MPa (68.7ksi) in ST and LT orientations, respectively. Tests to evaluate intergranular attack (IGA) were conducted. SHT + AA samples and T3 + AA samples at 150° and 190°C were exposed to a solution of 57g NaCl and 10ml H2O2 in DI water, for two hours. Optical analysis of the surface showed that T3 + AA samples at 150°C exhibited the least intersubgranular (IGSB) attack, while SHT + AA samples at 190°C showed the maximum intergranular attack. Transmission electron microscopy showed presence of phase TB (Al15Cu8Li2) at subgrain boundaries in alloy, which showed least attack, and T1 (Al2CuLi) which showed most attack. This observation suggests that increased precipitation of electrochemically active T1 on boundaries makes the alloy more susceptible to the corrosion attack. Whether the TB phase is less electrochemically active than T1 has not been determined conclusively. Stress corrosion susceptibility tests -- alternate immersion and constant extension rate testing (CERT) indicated that the alloy has good stress corrosion resistance in 3.5 wt% NaCl. No failures were observed in alternate immersion tests for a period of 60 days. Tensile tests conducted on exposed samples, revealed that loss in ductility was highest in the ST samples aged at 190°C for 36 hrs. CERT tests indicated that only samples aged at 150°C exhibited any trend for SCC susceptibility, and this was comparatively minor. Samples aged at 190°C exhibited some loss in yield and tensile strengths in the environments, but had ductility ratios close to one. SEM fractography revealed intergranular separation, signifying brittle mechanical failures in peakaged and overaged materials, and dimpled rupture, signifying ductile failures in underaged materials. Influence of environment on fractured surfaces was hardly detectable in CERT samples but was present in some alternate immersion tests samples. Fixed displacement SCC tests were conducted to determine if AF/C458 was susceptible to pre-exposure embrittlement phenomenon. Round tensile bar samples were given an initial strain corresponding to a stress of 65% of the yield strength then exposed to 3.5 wt% NaCl for 7 days. They were then placed either in CO2-free atmosphere or in lab-air for a period of 5 days. No failures were detected in any environments suggesting that AF/C458 is not susceptible to this phenomenon unlike 2090, and 8090. Thus, overall the results indicate high stress corrosion resistance for AF/C458.
Rudolph G. Buchheit (Advisor)
Gerald S. Frankel (Committee Member)
122 p.
Materials Science

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Citations

  • Mathur, D. (2000). Localized corrosion and stress corrosion cracking studies of Al-Li-Cu Alloy AF/C458 [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1407401974

    APA Style (7th edition)

  • Mathur, Deepanker. Localized corrosion and stress corrosion cracking studies of Al-Li-Cu Alloy AF/C458. 2000. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1407401974.

    MLA Style (8th edition)

  • Mathur, Deepanker. "Localized corrosion and stress corrosion cracking studies of Al-Li-Cu Alloy AF/C458." Master's thesis, Ohio State University, 2000. http://rave.ohiolink.edu/etdc/view?acc_num=osu1407401974

    Chicago Manual of Style (17th edition)

osu1407401974
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This open access ETD is published by The Ohio State University and OhioLINK.