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Trivalent Chromium Conversion Coatings on Al and Al-Cu Alloys

Qi, Jiantao

[Thesis]. Manchester, UK: The University of Manchester; 2015.

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Abstract

Trivalent chromium conversion coatings formed on Al and Al-Cu alloys has been investigated using high-resolution, analytical electron microscopy, atomic force microscopy, ion beam analysis, glow discharge optical emission spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, potentiodynamic polarization and electrochemical noise analysis. These coatings on the electropolished Al and sputtering-deposed Al consist of a chromium- and zirconium-rich outer layer and a thinner, aluminium-rich inner layer. Zirconium and chromium are presented in chemical states consistent with ZrO2, Cr(OH)3, Cr2(SO4)3, CrF3 and CrO3 or CrO42-. However, negligible amounts of hexavalent chromium species occurred in both coatings formed in de-aerated solution. On AA2024-T351 alloys, the coating above the second phase particles was thicker than that on the matrix due to the increased localized alkalinity. Moreover, the localized corrosion and copper enrichment of the matrix occurred at the coating base. The presence of copper resulted in a thinner coating at the matrix compared with superpure aluminium. An Fe(III)-containing D30 desmutter can effectively remove the protruded particles generated by alkaline etching. Moreover, a thinner oxide film on the D30-treated surface was evidenced to promote the coating initiation on the matrix. Further, the coating comprised the concentrated zirconium oxyfluorides and decreased contents of aluminium and copper. Coated alloys displayed a significantly enhanced corrosion protection, especially the cathoidc inhibition. In terms of coating post-treatments, the water immersion (40 ºC, pH 5) soon after conversion treatment revealed a simple but effective process to improve the fluoride enrichment in coatings and to enhance corrosion protection of freshly-developed coatings. Increasing concentration of solid-solution copper in sputtering-deposited alloys, in a range of 2, 24, 40 and 64 at.%Cu, significantly promotes the coating growth kinetics and an evident and a thicker corrosion layer by substrate dissolution was observed on Al-40 at.%Cu alloys. In addition, the relationship of copper-rich deposits with Cr(VI) transformation was demonstrated by Raman spectroscopy. 

Layman's Abstract

Conversion coating process is a surface pre-treatment for aluminium and aluminium-copper alloys applied in the aerospace and automotive industries to provide corrosion resistance and appreciable durability for the coating system. However, the eco-friendly concern of the current chromate conversion coating motivates our research of the next-generation of promising replacement whist maintaining corrosion protection. Trivalent chromium conversion coating is of great importance and significance in the candidate processing.The formation, composition, chemistry and corrosion protection of the trivalent chromium conversion coatings on aluminium and aluminium-copper alloys have been investigated using the high-resolution electron microscopy, ion beam analyses, Raman spectroscopy and XPS. The fundamental studies worked out the mechanism of Cr III / Cr VI reaction in the coating formation on Al, and the possible eliminating methodologies were under discussion. Furthermore, the optimal trivalent chromium process includes the appropriate pre-treatment to produce a cleaner surface, conversion treatment for 5 min, water immersion post-treatment and air ageing for 24 hours.

Bibliographic metadata

Type of resource:
Content type:
Form of thesis:
Type of submission:
Degree type:
Doctor of Philosophy
Degree programme:
PhD Materials
Publication date:
Location:
Manchester, UK
Total pages:
330
Abstract:
Trivalent chromium conversion coatings formed on Al and Al-Cu alloys has been investigated using high-resolution, analytical electron microscopy, atomic force microscopy, ion beam analysis, glow discharge optical emission spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, electrochemical impedance spectroscopy, potentiodynamic polarization and electrochemical noise analysis. These coatings on the electropolished Al and sputtering-deposed Al consist of a chromium- and zirconium-rich outer layer and a thinner, aluminium-rich inner layer. Zirconium and chromium are presented in chemical states consistent with ZrO2, Cr(OH)3, Cr2(SO4)3, CrF3 and CrO3 or CrO42-. However, negligible amounts of hexavalent chromium species occurred in both coatings formed in de-aerated solution. On AA2024-T351 alloys, the coating above the second phase particles was thicker than that on the matrix due to the increased localized alkalinity. Moreover, the localized corrosion and copper enrichment of the matrix occurred at the coating base. The presence of copper resulted in a thinner coating at the matrix compared with superpure aluminium. An Fe(III)-containing D30 desmutter can effectively remove the protruded particles generated by alkaline etching. Moreover, a thinner oxide film on the D30-treated surface was evidenced to promote the coating initiation on the matrix. Further, the coating comprised the concentrated zirconium oxyfluorides and decreased contents of aluminium and copper. Coated alloys displayed a significantly enhanced corrosion protection, especially the cathoidc inhibition. In terms of coating post-treatments, the water immersion (40 ºC, pH 5) soon after conversion treatment revealed a simple but effective process to improve the fluoride enrichment in coatings and to enhance corrosion protection of freshly-developed coatings. Increasing concentration of solid-solution copper in sputtering-deposited alloys, in a range of 2, 24, 40 and 64 at.%Cu, significantly promotes the coating growth kinetics and an evident and a thicker corrosion layer by substrate dissolution was observed on Al-40 at.%Cu alloys. In addition, the relationship of copper-rich deposits with Cr(VI) transformation was demonstrated by Raman spectroscopy. 
Layman's abstract:
Conversion coating process is a surface pre-treatment for aluminium and aluminium-copper alloys applied in the aerospace and automotive industries to provide corrosion resistance and appreciable durability for the coating system. However, the eco-friendly concern of the current chromate conversion coating motivates our research of the next-generation of promising replacement whist maintaining corrosion protection. Trivalent chromium conversion coating is of great importance and significance in the candidate processing.The formation, composition, chemistry and corrosion protection of the trivalent chromium conversion coatings on aluminium and aluminium-copper alloys have been investigated using the high-resolution electron microscopy, ion beam analyses, Raman spectroscopy and XPS. The fundamental studies worked out the mechanism of Cr III / Cr VI reaction in the coating formation on Al, and the possible eliminating methodologies were under discussion. Furthermore, the optimal trivalent chromium process includes the appropriate pre-treatment to produce a cleaner surface, conversion treatment for 5 min, water immersion post-treatment and air ageing for 24 hours.
Thesis main supervisor(s):
Thesis co-supervisor(s):
Language:
en

Institutional metadata

University researcher(s):

Record metadata

Manchester eScholar ID:
uk-ac-man-scw:277523
Created by:
Qi, Jiantao
Created:
8th November, 2015, 19:05:15
Last modified by:
Qi, Jiantao
Last modified:
16th November, 2017, 12:38:54

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