Doctoral Dissertations

Simon Joshi

Abstract

"This research used electrochemical techniques to characterize the deposition and corrosion protection behavior of cerium-based conversion coatings on AI 7075-T6. Alkaline activation decreased native oxide impedance (5.9 kΩ-cm²) by ~25% promoting deposition of 250-500 mn coatings. Activation in NaOH solutions deposited coatings with large cracks and craters, whereas Na₂C0₃ activation resulted in uniform coatings, i.e., fewer cracks and almost no craters. Uniformly deposited coatings exhibited better cathodic inhibition and higher impedance (~200 kΩ-cm²) than on NaOH activated substrates (~100 kΩ-cm²). Subsurface crevices, caused by Cl⁻ and H₂O₂ in the deposition solution, were found under large cracks and craters. Thus, Na₂CO₃ activation produced fewer subsurface crevices. To reduce subsurface crevice formation, Ce(NO₃)₃ and CeCl₃ were used in different ratios. Coatings made using 100% Ce(N0₃)₃ solutions were ~60 mn thick without subsurface crevices, but the coatings offered little corrosion protection. Despite formation of subsurface crevices, Cl⁻ was necessary as impedance increased linearly with Cl⁻ concentration in the deposition solution. To characterize the different non-uniform features of the coatings, microelectrochemical testing was performed and it showed three distinct regions: active, intermediate, and passive. Humidity experiments were performed to understand the effect of moisture during salt spray testing and showed an increase in coating impedance by making the exposed substrate oxide more passive. However, this passive oxide could not provide corrosion resistance in a chloride environment. Dissolution studies showed that cerium migration was only possible at pH ≤ 2. Overall, deposition of uniform 250-500 nm thick coatings was essential to make it an effective barrier to Cl⁻ attack and prevent subsurface crevices on AI 7075-T6"--Abstract, page iv.

Advisor(s)

OKeefe, Matt
Fahrenholtz, William

Committee Member(s)

Schlesinger, Mark E.
Miller, F. Scott, 1956-
Switzer, Jay A., 1950-

Department(s)

Materials Science and Engineering

Degree Name

Ph. D. in Materials Science and Engineering

Sponsor(s)

Strategic Environmental Research and Development Program (U.S.)

Publisher

Missouri University of Science and Technology

Publication Date

Fall 2010

Journal article titles appearing in thesis/dissertation

  • Alkaline activation of Al 7075-T6 for deposition of cerium-based conversion coatings
  • Effect of alkaline cleaning and activation on aluminum alloy 7075-T6
  • Electrochemical characterization of Al 7075-T6 surface oxide after alkaline treatments
  • Microelectrochemical testing of cerium-based conversion coatings
  • Characterization of cerium-based conversion coatings on Al 7075-T6 deposited from chloride and nitrate salt solutions
  • Effect of humidity on cerium-based conversion coatings on Al 7075-T6
  • Dissolution of cerium from cerium-based conversion coatings in 0.1 M NaCl solution

Pagination

xvii, 200 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2010 Simon Joshi, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Subject Headings

Aluminum alloysCerium -- ElectrometallurgyCoating processesElectrochemical analysisSurfaces (Technology)

Thesis Number

T 9711

Print OCLC #

750048801

Electronic OCLC #

908761588

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