Doctoral Dissertations

Abstract

"Phosphate post-treatment of cerium-based conversion coatings (CeCCs) on high strength aluminum alloys can significantly improve corrosion resistance. As-deposited CeCCs exhibit corrosion pits and salt tails across the specimen surface after 3 days of exposure, but post-treated CeCCs have withstood 14 days of salt spray exposure without visibly corroding. The morphology, phase, and electrochemical properties of spray deposited CeCCs were affected by post-treatment parameters such as immersion time, solution temperature, and phosphate source. The best performing coatings were post-treated in aqueous orthophosphate solutions for at least 5 min at temperatures of at least 85⁰C. These conditions converted cerium hydroxy/peroxy species in the as-deposited CeCC to hydrated CePO₄ and minimized cracks in the coating. Despite demonstrating the kinetic dependence of processes active during post-treatment, these results suggested that the corrosion resistance of CeCCs was dependent on the coating phase and morphology. Using an aqueous precipitation technique, hydrated CePO₄ coatings were directly deposited onto Al 2024-T3 substrates and compared to as-deposited and post-treated CeCCs. After salt spray exposure, analysis revealed the formation of pits in the alloy where the substrate was exposed by cracks in the directly deposited CePO₄ coating. Post-treated CeCC specimens did not exhibit corrosion at crack/substrate interfaces, indicating that CeCCs can provide electrochemical protection. Post-treated CeCCs also formed an interfacial reaction layer at CeCC/substrate interfaces, a response not observed for directly deposited CePO₄ coatings or as-deposited CeCCs. These results demonstrate that post-treated CeCCs are not static barrier coatings, but respond actively to corrosion"--Abstract, page iv.

Advisor(s)

Fahrenholtz, William

Committee Member(s)

Stoffer, James O.
Fair, Geoff E.
Miller, F. Scott, 1956-
OKeefe, Matt

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

Summer 2010

Journal article titles appearing in thesis/dissertation

  • Effect of post-treatment time and temperature on cerium-based conversion coatings on Al 2024-T3
  • Effect of phosphate source on post-treatment of cerium-based conversion coatings on Al 2024-T3
  • Cross-sectional analysis of as-deposited and post-treated cerium-based conversion coatings on Al 2024-T3
  • Directly deposited cerium phosphate coatings for the corrosion protection Al 2024-T3
  • Chemical and structural analysis of subsurface crevices formed during spontaneous deposition of cerium-based conversion coatings

Pagination

xii, 201 pages

Note about bibliography

Includes bibliographical references.

Rights

© 2010 Daimon K. Heller, All rights reserved.

Document Type

Dissertation - Open Access

File Type

text

Language

English

Subject Headings

Aluminum alloys
Cerium
Corrosion and anti-corrosives -- Research
Phosphate coating
Rare earths

Thesis Number

T 9652

Print OCLC #

747616507

Electronic OCLC #

747723486

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