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Corrosion Fatigue Performance of In-service and Laboratory Accelerated Sensitized AA5456-H116 for Marine Applications

Akman, Allison Michelle
http://rave.ohiolink.edu/etdc/view?acc_num=osu1610085108883445

Abstract Details

2021, Master of Science, Ohio State University, Materials Science and Engineering.
Aluminum-magnesium 5xxx series alloys are utilized in marine applications due to good weldability and generally superior corrosion resistance where Al alloys are concerned. Sensitization becomes an issue in these high magnesium content (> 3 wt%) alloys as Al3Mg2 (β phase), which is anodic to the α-Al matrix, precipitates on grain boundaries after prolonged thermal exposure leading to decreased resistance to intergranular corrosion (IGC), intergranular stress corrosion cracking (IGSCC), and corrosion fatigue (CF). In research, sensitization is typically achieved by accelerating the sensitization process by placing samples in a 100⁰C to 200⁰C oven. In service, naval ships experience sensitization from elevated temperatures ranging from 40⁰C to 50⁰C over a time period of ten or more years, which results in structural performance deterioration. In this work, CF crack growth kinetics (da/dN) as a function of loading frequency (f) for laboratory and in-service sensitized AA5456-H116 microstructures were measured to understand if current lab-based testing reflect CF behavior when material is sensitized in-service. CF growth measurements were obtained on eccentrically-loaded single edge crack tension specimens loaded in the T-L orientation under full immersion in 0.6 M sodium chloride solution. Slow displacement rate testing determined a threshold stress intensity factor, K1SCC, and was used to better inform CF loading parameters. CF testing was conducted at a constant stress intensity range while varying f and applied maximum stress intensity (Kmax). Sensitization exhibited little to no effect on da/dN for both sensitization methods. The da/dN of laboratory sensitized microstructures was just outside the ASTM E647 defined ±50% scatter compared to the in-service sensitized microstructures implying that there is no sensitization method effect. One exception to this observation was the increased da/dN of the in-service intermediate sensitization level over both laboratory and the highest in-service sensitized microstructures. Overall, crack growth rates remained f independent for both laboratory and in-service sensitized microstructures even at the highest Kmax examined. Conversely, the literature establishes that the same lot of AA5456-H116 as the current lab sensitized material exhibited inverse f dependence for highly sensitized microstructures when loaded in the S-L orientation. Based on comparisons with the SCC and CF literature, the lack of f dependence when loading in the T-L orientation can be attributed to the high SCC resistance of this loading orientation. Specifically, loading conditions for the T-L orientation in this study had Kmax values below K1SCC. Thus, an inverse f dependence by SCC superposition would not be expected for the loading conditions utilized in the current work. Due to thickness limitations of the in-service plate, further investigation into the SCC contribution to CF da/dN, in terms of Kmax approaching K1SCC, for the T-L orientation is hindered. Work on thicker material could facilitate a continuation of this research to better understand the correlation between lab-based CF sensitization studies to real world applications where sensitization occurs over an extended period of time at lower temperatures. Scanning electron microscopy (SEM) of fracture surfaces showed differing microstructures across the different variations of AA5456-H116 used in this study. This is likely the result of differing manufacturers and thermomechanical processing paths, which may explain the slight differences measured in da/dN. Specifically, the fracture surface of the in-service intermediate sensitized microstructure exhibited primarily intergranular (IG) fracture, while other microstructures in the T-L orientation revealed predominantly transgranular (TG) cracking with a few IG regions. The received recrystallized microstructure of the in-service intermediate sensitization level combined with the highly susceptible grain boundaries resulted in IG fracture, which likely explains the higher da/dN over the other microstructures that exhibited TG fracture. TG cracking caused the advancing crack to interact with less β phase and resulted in slower da/dN. Fractography showed delamination fissures present perpendicular to the crack plane when loaded in an aqueous sodium chloride solution for both in-service and laboratory sensitized T-L microstructures. Delamination fissures segment the crack front into smaller regions that decrease the constraint of the crack and effectively increase the fracture toughness, which can result in lower da/dN. EAC resistance, in terms of low da/dN, seen for the T-L orientation is likely a result of the combination of TG cracking and delamination fissures. Overall, AA5456-H116 in the T-L orientation is more resistant to cracking than the S-L orientation due to the occurrence of delamination fissures and TG cracking. In the current study, da/dN is independent of sensitization method when material is sensitized in a laboratory (100⁰C) compared to in-service material sensitized at lower temperatures over longer time period.
Jenifer (Warner) Locke (Advisor)
Eric Schindelholz (Committee Member)
105 p.
Materials Science
corrosion fatigue; 5xxx series alloys; sensitization

Recommended Citations

Citations

  • Akman, A. M. (2021). Corrosion Fatigue Performance of In-service and Laboratory Accelerated Sensitized AA5456-H116 for Marine Applications [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1610085108883445

    APA Style (7th edition)

  • Akman, Allison. Corrosion Fatigue Performance of In-service and Laboratory Accelerated Sensitized AA5456-H116 for Marine Applications. 2021. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1610085108883445.

    MLA Style (8th edition)

  • Akman, Allison. "Corrosion Fatigue Performance of In-service and Laboratory Accelerated Sensitized AA5456-H116 for Marine Applications." Master's thesis, Ohio State University, 2021. http://rave.ohiolink.edu/etdc/view?acc_num=osu1610085108883445

    Chicago Manual of Style (17th edition)

osu1610085108883445
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