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Characterizing the Localized Corrosion of AA7075-T6 and AA2024-T3 by Optical Profilometry

Neeley, Alexandra
http://rave.ohiolink.edu/etdc/view?acc_num=osu1330391114

Abstract Details

2012, Master of Science, Ohio State University, Materials Science and Engineering.
The objective of this work was to use optical profilometry to extend methods of characterizing localized corrosion damage accumulation to support the development of both better predictive models and mechanistic understanding. In regards to predictive models, previous accelerated corrosion testing has shown weak correlations to field exposures. The first part of this work characterized pit distributions for bare AA2024 coupons from atmospheric exposures ranging from sub-tropical beach to desert climates (3-12 months). Three lab exposures (static immersion, ASTM B117, GMW 14872) were also evaluated for AA2024 and AA7075 with a range of three pretreatments (bare, deoxidized, and chromate conversion coated) over 1-7 days. Both field and lab exposures generated maximum pit depths that 10 and 4 times faster than average pit depths, respectively. The difference in pit depths is thought to arise from the occurrence of early pit death and/or delayed pit birth. Cumulative distribution functions revealed notable variation at pit depths greater than the 99th percentile, often characterized by a 'tail' in the distribution. Meaningful interpretation of pit growth kinetics using these pits was not possible. Distributions of pit depth, diameter, and density from atmospheric exposures were compared to those of bare AA2024 lab exposures, generating acceleration factors using a power law fitting approach. Acceleration factors were generally greatest for GMW 14872, followed by ASTM B117 and static immersion, and usually increased with pit depth. Results suggest that the 95th percentile may be the best compromise for eliminating distribution tails while maintaining the focus on the deepest pits. The second part of this work explored optical profilometry in regards to developing a better understanding of the mechanism behind localized cathodic corrosion, following up on the work of Leclere. Pit depth distributions were characterized for AA2024 and AA7075, exposed by static immersion in dilute chloride solutions with and without borate buffer. The presence of buffer yielded less circumferential trenching, typically associated with cathodic corrosion. However, there were no significant differences in pit geometry between buffered and unbuffered exposure solutions. The most notable distinction was that the buffer drastically reduced pit number density, acting more in suppressing nucleation sites than altering pit growth kinetics. Both lifetime prediction modeling and mechanistic studies revealed challenges in monitoring other corrosion modes that may be occurring simultaneously, such as uniform corrosion, subsurface pitting, surface roughness, and pit coalescence. Although optical profilometry revealed complexity of localized corrosion in high strength Al alloys, it does appear to compliment traditional electrochemical techniques that do not consistently discriminate damage accumulation.
Rudolph Buchheit, PhD (Advisor)
Gerald Frankel, PhD (Other)
188 p.
Materials Science
corrosion; optical profilometry; aluminum alloy; topography; localized corrosion; cathodic corrosion; pitting; borate; accelerated testing

Recommended Citations

Citations

  • Neeley, A. (2012). Characterizing the Localized Corrosion of AA7075-T6 and AA2024-T3 by Optical Profilometry [Master's thesis, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1330391114

    APA Style (7th edition)

  • Neeley, Alexandra. Characterizing the Localized Corrosion of AA7075-T6 and AA2024-T3 by Optical Profilometry. 2012. Ohio State University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1330391114.

    MLA Style (8th edition)

  • Neeley, Alexandra. "Characterizing the Localized Corrosion of AA7075-T6 and AA2024-T3 by Optical Profilometry." Master's thesis, Ohio State University, 2012. http://rave.ohiolink.edu/etdc/view?acc_num=osu1330391114

    Chicago Manual of Style (17th edition)

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© 2012, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.