hexavalent & trivalent chromate 1

[rubepierre]

is this another layer of coating after zinc plating??
and is this where the coloring comes from??
any/all information will be most welcome...
rubepierre

 

[CoryPad]

Yes, this is an additional layer on top of the zinc layer. It is deposited on the surface by immersion in a chromate ion containing bath. The zinc and chromate layers both change the color - the thicker they are, the darker the color (typical range starts with clear/blue, then becomes yellow, then olive). You can search this site for previous posts on this subject - they include websites for more information, e.g. Products Finishing Online.

 

[kenvlach]

Traditional chromate conversion coatings involve the reaction of chromic acid (Cr+6) with a base metal (such as Al, Cd or Zn) to form a (Cr+3) compound. The coating contains both Cr(+6) and Cr(3) compounds; it is believed that the Cr(+3) is in the form of an oxide while the Cr(+6) is in the form of a chromate. Both contribute to the corrosion protection of the coating.

Recently, solutions containing only Cr(+3) have been developed for Zn that give clear or blue chromates that are claimed by the manufacturers to give protection equal to the yellow chromates from Cr(+6) solutions, although sometimes an additional sealing solution is required. I am not aware of any Cr(+3) coatings yet for aluminum that equal those from Cr(+6).
The driving force for the development of non-Cr(+6) coatings is environmental hazards and regulations associated with Cr(=6). E.g., the European Union has essentially banned the use of Cr(+6) coatings by limiting the total amount of Cr(+6) in automobiles to an infinitesimally small value.

 

[kenvlach]

I noticed 2 new studies on chromate coatings in J. Electrochem. Soc.

http://ojps.aip.org/dbt/dbt.jsp?KEY=JESOAN

"Chromate Conversion Coatings Formation on Zinc Studied by Electrochemical and Electrohydrodynamical Impedances," January 2003 -- Volume 150, Issue 1, pp. B16-B25, A. A. O. Magalhães, B. Tribollet, O. R. Mattos, I. C. P. Margarit, and O. E. Barciad.
This is a rather complex study of the kinetics, with 55 equations. However, Figure 5 shows a schematic of the metal, coating layers & diffusion layer in the liquid:
Sequence is Zn/ZnO + chromate/chromate/boundary layer of liquid which limits diffusion and then the bulk liquid. They used a commercial product containing sodium dichromate and sulfuric acid.

"Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release," Journal of The Electrochemical Society--February 2003,Volume 150, Issue 2, pp. B83-B91
E. Akiyama, A. J. Markworth, J. K. McCoy, G. S. Frankel, L. Xia, and R. L. McCreery.
(I give the abstract only, since copyrighted material):
"The release of chromate ions from chromate conversion coatings (CCCs) on Al alloys was studied, and the effect of aging of CCCs on the chromate release kinetics was investigated. Chromate release from CCCs into aqueous solutions was monitored by measuring the change in the chromate concentration in solution using UV-visible spectroscopy. Heat-treatment of the CCC greatly reduced the chromate release rate. The chromate release rate also decreased with increasing aging time at room temperature. A diffusion-control model was proposed based on the notion that the CCC in an aqueous solution is a porous, two-phase structure consisting of a solid phase with adsorbed Cr(VI) species that is in local Langmuir-type equilibrium with an interpenetrating solution phase. This model results in a concentration gradient of soluble Cr(VI) in the solution phase of the CCC as chromate is released. The concentration and diffusion coefficients of soluble Cr(VI) in CCC were estimated. The estimated diffusion coefficient tended to decrease with aging time, suggesting that the CCC is modified with aging time. © 2003 The Electrochemical Society. All rights reserved."
They studied Al 1100 and Al 2024-T3 coated by immersion in Alodine 1200S solutions at room temperature.