Skip to Main Content
 

Global Search Box

 
 
 
 

Files

File List


osu1160670830.pdf (6.41 MB)

ETD Abstract Container

Abstract Header

Role of surface active layers on localized breakdown of aluminum alloy 7075

Zhao, Zhijun
http://rave.ohiolink.edu/etdc/view?acc_num=osu1160670830

Abstract Details

2006, Doctor of Philosophy, Ohio State University, Materials Science and Engineering.
Potentiodynamic polarization curves for AA7075-T6 in NaCl solution exhibit two breakdowns. The second breakdown is dominated by sustained localized corrosion and the first one is believed to be associated with transient dissolution that precedes the onset of stable localized corrosion. This study is aimed at understanding the details of the transient dissolution phenomenon.The first breakdown mechanism of AA7xxx was studied by an in-situ observation system. As-polished (to 1 μm) AA7075-T6 clearly exhibited dissolution of a thin surface layer corresponding to a sharp increase of current just above the first breakdown. No surface layer dissolution was observed for samples that were either ion milled or chemically etched to remove the effects of polishing. This susceptible surface layer was apparently the result of the mechanical polishing process. The surface microstructure of an as-polished sample was analyzed by TEM and several distinct features were found: (1) a unique thin layer with thickness of 100 nm on average; (2) many fewer fine hardening particles in the thin layer compared to bulk matrix, which means that the fine particles were destroyed and eliminated by polishing; (3) high aspect ratio nano-grains elongate along the final polishing direction; EDS analysis in STEM mode revealed a higher concentration of Zn at the nano-grain boundaries. The attack of the surface layer might initiate at the active nanograin boundary followed by nano-grain dissolution.Samples in other tempers were also examined. No surface layer dissolution and no first breakdown peak were observed for the solutionized + quenched alloy because no comparable Zn enriched bands were present and the composition of the surface layer was not substantially different than the bulk matrix. The overaged samples also exhibited no layer attack and only one breakdown potential. Zn-rich bands were observed at the nano-brain boundaries for the overaged temper, but the localized attack propagated into the bulk matrix rather than laterally as layer attack. Underaged samples showed thin layer dissolution and current peaks associated with a first breakdown phenomenon. However, the magnitude of the current peaks changed with the extent of aging as a result of differences in the hardness, and thus layer thickness and dissolution rate of the thin layer. A study of filiform corrosion (FFC) was performed on T6 and T7 temper samples either as-polished or chemically etched. FFC kinetics were higher on as-polished samples than on chemically etched samples, which confirmed higher activity of the altered surface layer induced by polishing. FFC on T6 sample of all the conditions was worse than T7 tempered samples.
Gerald Frankel (Advisor)
221 p.
AA7075; In-situ observation; microstructure; surface layer; polishing; dissolution

Recommended Citations

Citations

  • Zhao, Z. (2006). Role of surface active layers on localized breakdown of aluminum alloy 7075 [Doctoral dissertation, Ohio State University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=osu1160670830

    APA Style (7th edition)

  • Zhao, Zhijun. Role of surface active layers on localized breakdown of aluminum alloy 7075. 2006. Ohio State University, Doctoral dissertation. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=osu1160670830.

    MLA Style (8th edition)

  • Zhao, Zhijun. "Role of surface active layers on localized breakdown of aluminum alloy 7075." Doctoral dissertation, Ohio State University, 2006. http://rave.ohiolink.edu/etdc/view?acc_num=osu1160670830

    Chicago Manual of Style (17th edition)

osu1160670830
1,544
© 2006, all rights reserved.
This open access ETD is published by The Ohio State University and OhioLINK.