Effect of Humidity on Cerium-based Conversion Coatings on Al 7075-t6

Abstract

Cerium-based conversion coatings and Al 7075-T6 were exposed to 85% relative humidity at 40 °C to simulate ASTM B117 corrosion testing without NaCl. As-deposited coatings with cracks >0.5 µm wide showed a significant increase in impedance from ~5 kΩ-cm2 before exposure to ~95 kΩ-cm2 after 168 hours of exposure compared to phosphate post-treated coatings (from ~60 to 90 kΩ-cm2) with cracks <0.5 µm. the impedance of bare Al 7075-T6 increased from ~10 kΩ-cm2 before exposure to ~275 kΩ-cm2 after 168 hours of exposure. the increase in impedance in all cases was due to an altered layer containing Al and O on exposed alloy surfaces. X-ray photoelectron spectroscopy indicated that exposure to humidity reduced Ce(IV) to Ce(III) on as-deposited coatings, whereas post-treated coatings did not show any change in Ce oxidation state. Despite the increase in impedance, only post-treated coatings were able to inhibit corrosion in salt spray testing.

Department(s)

Materials Science and Engineering

Keywords and Phrases

Corrosion; Passivation; Anodic Films

International Standard Serial Number (ISSN)

1938-5862

Document Type

Article - Conference proceedings

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 2010 Wiley-Blackwell, All rights reserved.

Publication Date

01 Jan 2010

Link to Full Text

Share

 
COinS