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Drawing Callout-Trivalent Chromate Coatings for Aluminum

2005

After reading several posted letters, Trivalent Chromate Conversion coatings seem to be colorless when applied over zinc. (Unless the coating is dyed). Is the same true when the coating is applied over 6061-aluminum alloy?

MIL-C-5541E, section 6.6, Visual Appearance allows for a "range in color from clear to iridescent yellow or brown". MIL-DTL-81706B on the other hand, which specifies materials for both Trivalent and Hexavalent coatings, does not describe the visual appearance of the coating. From an engineering drawing callout standpoint, would it be necessary/correct to specify the applied finish as: (Base material: 6061 Aluminum Alloy).

Chromate Conversion Coat per MIL-C-5541E, Class 1A.
Color to be I.A.W. Color Chip #________.
Materials used for coating shall be I.A.W. MIL-DTL-81706B, Type II. (No Hexavalent Chromium)

May color be specified when the coating is to be I.A.W. MIL-DTL-81706B, Type II?
May MIL-DTL-81706B stand alone as a finish callout, or must it be accompanied by MIL-C-5541E?

Thanks!

Al Toney
microwave - Tampa, FL, United States

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Al,

I don t have any experience of the Trivalent systems but my understanding of the specifications is as follows.

MIL-DTL-81706 details the qualification of chemical systems, which are used to produce chromate conversion coatings on Aluminium. The specification allows for the qualification of products such as Iridite, Alodine etc for inclusion in QPL-81760 (qualified products list). This specification does not deal with the application of chromate conversion coatings in a production environment.

MIL-C-5541 details the application and maintenance of the chemical systems in order to produce an acceptable conversion coating in production. Only chemical systems or products appearing on the QPL may be used to produce chromate conversion coatings to MIL-C-5541.

As such if the system you are intending to use appears on the QPL then it may be used in conjunction with MIL-C-5541 to produce the conversion coating. As to the colour range achievable, the data sheet for the particular product you are interested in should give you the answers you require. When calling out a finish on a drg I simply call out MIL-C-5541 (81706 is called out by default).

As to the colour, MIL-C-5541 allows for a range as you have correctly stated thus a sub contractor could legitimately supply a coating within that range. I would suggest that tying the colour up to a particular colour chip could, potentially, give you some headaches, for instance different contractors may run differing conversion systems, or run their baths slightly differently, giving differing colours within the allowable range. Colour will vary from alloy to alloy etc etc. I would suggest that it should be born in mind that the conversion coatings are to give corrosion protection and / or to provide a base for painting. The coatings are not intended as a stand-alone cosmetic finish and any tying up of the colour should be discussed at length with sub contractors to avoid problems.

Rich Mosley
- UK
2005

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Just to add another complication to the matter - While tri-valent coatings CAN be added to the QPL now, to the best of my knowledge none HAVE been added yet. This pretty much makes it impossible to get a non-hex chrome finish that is compliant to MIL-C-5541 with the current revisions written the way they are.

Jim Gorsich
Compton, California, USA
2005

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While none have been added to QPL, product's are out for evaluation. This process in the past was supplier driven and may have received low priority with some very sketchy results on tests, therefore no QPL. Now the emphases is big industry driven and political, with a product that actually works, therefore this stands a much better chance of genuine change. I expect some movement very soon. For those on the sidelines, it may be time to get ready for the change,possibly a second tank for work that doesn't require QPL for now.

Drew Nosti, CEF
Anodize USA

Ladson, South Carolina

2005