Corrosion protection mechanisms of trivalent chromium based passivations on γ-ZnNi coated Al6061-T6 alloy

<p>“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.</p>

Name thesis:degree_name

Ph. D. in Metallurgical Engineering

Grantor

Missouri University of Science and Technology

Author dc:creator

• Foster, Kevin

• Characterization
• Cobalt
• Corrosion
• Hexavalent Chromium
• Passivation
• Trivalent Chromium
• Metallurgy
• Nanoscience and Nanotechnology

Repository record dc:identifier

https://scholarsmine.mst.edu/doctoral_dissertations/3146

OAI identifier oai:identifier

oai:scholarsmine.mst.edu:doctoral_dissertations-4151