Keywords and Phrases
Characterization; Cobalt; Corrosion; Hexavalent Chromium; Passivation; Trivalent Chromium
“The role of cobalt in trivalent chromium passivations (TCPs) to improve corrosion resistance of γ-ZnNi coated steel and aluminum is based on its effect on hexavalent chromium content in the passive layer. Investigations of both a cobalt-containing and cobalt-free TCP on SAE 1008 steel indicated that both passivations protect well for up to 1000 hours in neutral salt spray exposure (SSE). A repetition on Al 6061-T6 alloy indicated that TCP performed much better than cobalt-free TCP implicating the underlying substrate. Optical and electron micrographs indicated physical changes such as crack thickness, crack density, passivation porosity, and passivation thickness existed between the TCP and cobalt-free TCPs but had contradictory results on corrosion performance. Electrochemical differences between the TCPs on both substrates were consistent and scribed specimens on the Al 6061-T6 specimens showed active protection from TCP and not cobalt-free TCP indicating a chemical rather than physical difference. Confounding factors of electroless nickel (EN) between the substrate and γ-ZnNi coating and heat treatments led to Al 6061-T6 panels that were heat treated and steel panels with EN layers. The EN layer had no significant effect and heat treatments had inconsistent performance. Direct measurements of Cr(VI) content found some correlation between the amount of Cr(VI) and corrosion performance. XPS analysis of the surface Cr(VI) content revealed that Cr(VI) is needed for corrosion protection but that there must be an interaction with physical aspects of the coating to explain the inconsistent results. The TCPs were found to perform better because the divalent cobalt in TCPs facilitated production of Cr(VI) during corrosion”--Abstract, page iv.
Moats, Michael S.
O'Malley, Ronald J.
Switzer, Jay A., 1950-
Materials Science and Engineering
Ph. D. in Metallurgical Engineering
Missouri University of Science and Technology
Journal article titles appearing in thesis/dissertation
- Characterization of cobalt containing and cobalt-free trivalent chromium passivations on γ-ZnNi coated steel substrates
- Characterization of cobalt containing and cobalt-free trivalent chromium passivation on γ-ZnNi coated Al6061-T6 substrates
- Effect of heat treatment on the chromate content and performance of trivalent chromium passivations on γ-ZnNi
- XPS and 1,5-diphenylcarbazide analysis of chromate content and performance of heat-treated γ-ZnNi coated steel
xvi, 149 pages
© 2022 Kevin Lee Foster, All rights reserved.
Dissertation - Open Access
Foster, Kevin, "Corrosion protection mechanisms of trivalent chromium based passivations on γ-ZnNi coated Al6061-T6 alloy" (2022). Doctoral Dissertations. 3146.