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Amount of hex-chrome in Conversion Coatings for Aluminum
(to provide context, hopefully helping readers more quickly understand the Q&A's)
Chromate Conversion Coating is done on aluminum both as a corrosion resistant final finish and a pretreatment for painting. MIL-DTL-5541 [on DLA] is the most commonly referenced specification for the process. "Iridite" is the trade name of Macdermid-Enthone's line of chemistry for this purpose ("Alodine" is Henkel's), and these trade name products are so widely used that they are often used essentially generically.
This thread started in 2006, and there has been one very big change in the interim: trivalent chromium processes (TCP) have been developed which meet the MilSpec requirement and are RoHS-compliant. The MilSpec now recognizes two types of coatings, hexavalent chromium and non-hexavalent chromium, and the latter has become commonplace (perhaps predominant).
Q. Hi All,
We have, like everybody else, been getting a lot of questions from our customers regarding RoHS compliance and the like and have answered the majority with simple yes or no answers. However, I occasionally get a customer who is curious about just HOW MUCH hex-chrome is in the finished conversion coatings per MIL-C-5541, so a while back I prepared the following:
The coating is primarily made up of Hydrated Chromium Oxide (CAS # 1308-38-9) with the Chromium in its trivalent state. Other significant compounds include Aluminum trifluoride (CAS # 7784-18-1), Aluminum monofluoride monoxide (CAS #13596-12-8), Aluminum oxide (CAS # 1344-28-1) and finally Chromium ferricyanide (CAS # unknown) which has Chromium in its hexavalent state. Percentages of coating mass and estimated mg/ft2 deposit values are shown in the table below. The values given for trivalent and hexavalent chromium are included in the values for Chromium Oxide and Chromium ferricyanide respectively.
Name and CAS Formula Cr contents % by weight mg/sq ft
Hydrated Chromium Oxide Cr2O3*h20 58.83% 70.59857886
1308-38-9 \-------> Cr III 13.57% 16.285716
Chromium Ferricyanide CrFe(CN)6 3.27% 3.92
\-------> Cr VI 0.23% 0.28
Aluminum trifluoride AlF3 3.12% 3.7485
7784-18-1
Aluminum monofluoride monoxide AlOF 3.12% 3.7485
13596-12-8
Aluminum Oxide Al2O3 31.60% 37.92
1344-28-1
Total % Total applied weight
99.95% 119.94 "
Although I'm not certain, I think that the chromium in the CrFe(CN)6 is in the trivalent state rather than hex; which prompts two questions -
1) Am I correct on that? Does Chromium Ferricyanide have the Chrome in the tri state rather than hex?
2) What compound in the coating DOES have hex-chrome then? And, does anybody know how much there is?
Thanks very much for any input!
Jim Gorsich
Compton, California, USA
2006
A. It's been 25 years since Treverton's studies, and our understanding of chromate conversion coatings (CCCs) has advanced considerably. Studies through early 1998 have been incorporated in revising Chapter 5 of The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn., (2002), and I list some more recent articles below.
1) Spectroscopy indicates there isn't any chromium ferricyanide per se.
"The IR and Raman spectra of the air-dried CCC [chromate conversion coatings] corresponded to those of Berlin green, a Fe+3-CN-Fe+3 polymer, and Fe(CN)6Ú3 physisorbed on Cr(OH)3. No other cyano-containing products were observed." -- L. Xia and R. L McCreery (1999; see below).
They reported that the ferricyanide accelerator acts as a chemical intermediary, being reduced by Al and oxidized by Cr(+6) [e.g., a catalyst].
The ferricyanide may also play a physical role, helping to separate the upper portion of the CCC into granules (as opposed to a barrier layer), thereby enhancing diffusion. The Surface Treatment and Finishing of Aluminum and Its Alloys, 6th Edn., pp. 242-249 (2002), shows that the CCC consists of an upper portion of gelatinous, hydrated chromium oxide granules covered with a monolayer of ferricyanide ions, and a lower portion predominantly of mixed aluminum oxides and fluorides. Total Cr decreases from ~23 at% in the outer portion to 6 at% at the inner surface.
2a) Hexavalent Cr species: Chromic acid, dichromate & chromate ions are present. The hydrated chromium hydroxide gel seems to be full of them, plus there is trapped liquid.
D. Chidambaramet al. (2004, II, below) reported
"The hexavalent chromium content of the CCC formed on AA2024-T3 varied with the method of pretreatment. The hexavalent chromium content formed about 31(+-4)% of the total chromium content of the coating."
>From the Cr 2p spectra given for 10 samples, the following Cr species are present: CrO3 = or > Cr(OH)3 > Cr2O7(-2) > CrO4(-2) > Cr2O3 >> CrO2 [I've estimated relative proportions from peak heights; not very accurate].
2b) Hexavalent Cr content. Total Cr is about 36-37 wt%, so 31 % of that gives 11 wt% Cr(+6), or 13 mg/ft2 for a coating of 120 mg/ft2 total weight.
Selected recent articles from J. Electrochem. Soc.:
"Formation of Chromate Conversion Coatings on Aluminum and Its Alloys," H. S. Isaacs, K. Sasaki, C. S. Jeffcoate, V. Laget, and R. Buchheit, J. Electrochem. Soc. 152, B441 2005.
"Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings. II.," Devicharan Chidambaram, Clive R. Clayton, Martin W. Kendig, and Gary P. Halada, J. Electrochem. Soc. 151, B613 (2004).
"Surface Pretreatments of Aluminum Alloy AA2024-T3 and Formation of Chromate Conversion Coatings. I.," Devicharan Chidambaram, Clive R. Clayton, Gary P. Halada, and Martin W. Kendig, J. Electrochem. Soc. 151, B605 (2004).
"Chromate Conversion Coating on Aluminum Alloys. II.," P. Campestrini, H. Terryn, J. Vereecken, and J. H. W. de Wit, J. Electrochem. Soc. 151, B370 (2004).
"Chromate Conversion Coating on Aluminum Alloys. I.," P. Campestrini, H. Terryn, J. Vereecken, and J. H. W. de Wit, J. Electrochem. Soc. 151, B359 (2004).
"Study of the Chemical Conversion of Aluminum Alloys by Coupling CFDE and EQCM," B. Vuillemin, R. Oltra, H. Perrot, C. Sauvage, and O. Néel, J. Electrochem. Soc. 151, C229 (2004).
"Synchrotron Radiation Based Grazing Angle Infrared Spectroscopy of Chromate Conversion Coatings Formed on Aluminum Alloys," Devicharan Chidambaram, Gary P. Halada, and Clive R. Clayton, J. Electrochem. Soc. 151, B160 (2004).
"Interactions of the Components of Chromate Conversion Coating with the Constituents of Aluminum Alloy AA2024-T3,"
Devicharan Chidambaram, Clive R. Clayton, and Gary P. Halada, J. Electrochem. Soc. 151, B151 (2004).
"Chromate Conversion Coating on Aluminum Alloys,"
P. Campestrini, G. Goeminne, H. Terryn, J. Vereecken, and J. H. W. de Wit, J. Electrochem. Soc. 151, B59 (2004).
"The Role of Chromate Conversion Coating in the Filiform Corrosion of Coated Aluminum Alloys," N. Le Bozec, S. Joiret, D. Thierry, and D. Persson, J. Electrochem. Soc. 150, B561 (2003).
"Structural Analysis and Photocurrent Spectroscopy of CCCs on 99.99% Aluminum," F. Di Quarto, M. Santamaria, N. Mallandrino, V. Laget, R. Buchheit, and K. Shimizu, J. Electrochem. Soc. 150, B462 (2003).
"Dehydration-Induced Loss of Corrosion Protection Properties in Chromate Conversion Coatings on Aluminum Alloy 2024-T3," V. Laget, C. S. Jeffcoate, H. S. Isaacs, and R. G. Buchheit, J. Electrochem. Soc. 150, B425 (2003).
"A Duplex Mechanism-Based Model for the Interaction Between Chromate Ions and the Hydrated Oxide Film on Aluminum Alloys," Devicharan Chidambaram, Clive R. Clayton, and Gary P. Halada, J. Electrochem. Soc. 150, B224 (2003).
"Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings on Al Alloys: Kinetics of Release," E. Akiyama, A. J. Markworth, J. K. McCoy, G. S. Frankel, L. Xia, and R. L. McCreery, J. Electrochem. Soc. 150, B83 (2003).
"A Galvanic Corrosion Approach to Investigating Chromate Effects on Aluminum Alloy 2024-T3," William J. Clark, Jeremy D. Ramsey, Richard L. McCreery, and Gerald S. Frankel, J. Electrochem. Soc. 149, B179 (2002).
"Nucleation and Growth of a Chromate Conversion Coating on Aluminum Alloy AA 2024-T3," G. M. Brown and K. Kobayashi, J. Electrochem. Soc. 148, B457 (2001).
"The Use of Infrared Spectroscopic Ellipsometry for the Thickness Determination and Molecular Characterization of Thin Films on Aluminum," T. Schram and H. Terryn, J. Electrochem. Soc. 148, F12 (2001).
"Formation of Chromate Conversion Coatings on Al-Cu-Mg Intermetallic Compounds and Alloys," W. R. McGovern, P. Schmutz, R. G. Buchheit, and R. L. McCreery, J. Electrochem. Soc. 147, 4494 (2000).
"Storage and Release of Soluble Hexavalent Chromium from Chromate Conversion Coatings Equilibrium Aspects of CrVI Concentration," Lin Xia, Ejii Akiyama, Gerald Frankel, and Richard McCreery, J. Electrochem. Soc. 147, 2556 (2000).
"Chromate in Conversion Coatings: A XANES Study of Its Concentration and Mobility," Carol S. Jeffcoate, Hugh S. Isaacs, Antonio J. Aldykiewicz, Jr, and Mary P. Ryan, J. Electrochem. Soc. 147, 540 (2000).
"In Situ Raman Microscopy of Chromate Effects on Corrosion Pits in Aluminum Alloy," Jeremy D. Ramsey and Richard L. McCreery, J. Electrochem. Soc. 146, 4076 (1999).
"Structure and Function of Ferricyanide in the Formation of Chromate Conversion Coatings on Aluminum Aircraft Alloy," Lin Xia and Richard L. McCreery, J. Electrochem. Soc. 146, 3696 (1999).
"Chemistry of a Chromate Conversion Coating on Aluminum Alloy AA2024-T3 Probed by Vibrational Spectroscopy," Lin Xia and Richard L. McCreery, J. Electrochem. Soc. 145, 3083 (1998).
"Nondestructive Optical Characterization of Chemical Conversion Coatings on Aluminum," T. Schram, J. De Laet, and H. Terryn, J. Electrochem. Soc. 145, 2733 (1998).
"Corrosion Protection of Untreated AA-2024-T3 in Chloride Solution by a Chromate Conversion Coating Monitored with Raman Spectroscopy," Jun Zhao, Gerald Frankel, and Richard L. McCreery, J. Electrochem. Soc. 145, 2258 (1998).
"Nucleation of Chromate Conversion Coating on Aluminum 2024-T3 Investigated by Atomic Force Microscopy," J. R. Waldrop and M. W. Kendig, J. Electrochem. Soc. 145, L11 (1998).
[Ref. 91 of Chap. 5 in Sheasby & Pinner, 6th Edn. (2002)].
Abstracts are available at scitation.aip.org
Ken Vlach [deceased]
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.
Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
Ed. note: Thanks greatly for the yeoman search work and the informative preface, Ken. That URL, https://scitation.aip.org, appears to be a great route to searches of the scientific literature, and readers are urged to try it.
Thank you very much for your response Ken. I've already updated the letter I send to customers (seeing how my calculations were apparently off by a factor of 50 or so) and - if you don't mind - have noted the fact that I'm essentially quoting your answer.
Again, thank you!
Jim Gorsich
Compton, California, USA