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Chromic Acid Anodize Process Substitution

2007

Hi, I'm conducting internet research regarding anodize verses Alodine processes and have the following question:

A Boeing aircraft Standard Overhaul Practice Manual requires a repaired metal part made of 6061-T6511 Aluminum Alloy to under go a "Chromic Acid Anodize" process prior to application of a protective BMS 10-11 Type 1 Primer. This metal part is part of a Boeing Nose Steering Actuator, Trunion Assembly. How would the alloy /part be affected by substituting the Boeing process with Alodine 1201 ⇦this on eBay or Amazon [affil links] ?

Is the end result equivalent?
If fit tolerance is critical how might it be affected?
In your opinion would safety be compromised?

Regards

Richard Baker
regulator - Alameda, California, U.S.A.

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You may be very close to the situation and be in a good place to evaluate it, Richard, but I couldn't possibly offer comment on it from here except to say that it might be a very bad idea. Boeing has moved many components from chromic acid anodize to their proprietary boric-sulfuric anodizing process, and that is another possibility.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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2007

Richard,

Just to add to what Ted said, there is only one authority that can give you that answer and that is the OEM. You cannot just swap treatments without first consulting the designer, they may have a specific reason for using a particular coating, especially if a lot of work on fatigue life and stress characteristics has to be done if the coating is changed.

Look at it like this, if you were flying on a Boeing aircraft at 36,000 ft would you prefer the aircraft built to design requirements or would you mind if an untested replacement was used? I know what my answer would be!

Brian Terry
Aerospace - Yeovil, Somerset, UK

Ed. note: Finishing.com probably caused some confusion of this letter, for which we apologize. Mr. Baker listed himself as a "regulator" and most of us over-regulated metal finishers probably assumed he is an environmental regulator trying to suggest that a metal finishing shop use a less toxic coating. In fact he is with the agency responsible for air safety. Sorry.

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2007

Richard
I am not clear from your question whether you are investigating a case where someone has already substituted Alodine (Alochrom) for chromic anodising or suggesting that the substitution would be a good thing - possibly to reduce Chrome VI
In either case, I am a little worried if you are relying on the internet for your research. There is much good information out there but there is also much rubbish and you need a good background in metal finishing to separate them.
Brian is quite right, only the Prime can authorise a change and Boeing have some very knowledgeable people.
The direct - but simple - answer to your question is that the two systems are not interchangeable.
Chromic anodising is a thick, hard oxide film with a built in capability for crack detection. I have seen no evidence that the oxide layer contains any chrome.
Alodine (Alochrom in Europe) is a conversion coating used to prepare aluminium for painting.

Geoff Smith
Hampshire, England