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topic 34019

Can the chromic acid dip be omitted from MIL-DTL-13924?


A discussion started in 2005 but continuing through 2019

2005

Q. In a recent Supplier Audit I was auditing for compliance of MIL-DTL-13924 [link is to free spec at Defense Logistics Agency / dla.mil] and the supplier stated that they did not complete the Chromic acid dip. They also stated that this is an industry standard as supplementary preservative now takes the place of this part of the specification. My question is what does the Chromic acid dip do for this process, and can it be omitted from the process as this supplier has done?

Jill Pring
Aerospace - Muskegon, Michigan, USA


2005

A. I am not familiar with that spec, but if the spec calls for a chromic dip, then it has to be done or have a waiver from the end user. It has nothing to do with logic or better, you cannot certify the process if it does not follow the spec or be waived. The people in charge of the specs are not necessarily sharp. Years ago, the office that put out a change to the cadmium spec was not even aware that acid cad was a possibility, let alone desirable in some cases.

James Watts
- Navarre, Florida


2005

A. Although it may be an industry standard it is required per the mil-spec. As a general rule we will not perform this as well but with the acknowledgment of our customer.

We do perform the chromic dip for the military jobs processed.

Bill Grayson
- San Jose, California


2005

A. Jill,

Are you carrying out the Oxalic acid test? From my experience you will not pass the Oxalic acid test without sealing in the Chromic acid solution.

Before you omit the Chromic acid stage you had better check with your customer. If all of the parts have a further supplementary treatment (other than oiling up) then they may well allow you to omit this stage as it is really only of benefit to parts that are either left bare or are consequently oiled.

Brian Terry
Westland Helicopters Ltd - Yeovil, Somerset, UK



July 15, 2019

Q. IS A CHROMIC ACID DIP THE SAME AS ANODIZING AND WHAT SPEC IS IT?
I HAVE A PRIMER SPEC.

PAUL SCHMIDT
- RIO RANCHO, New Mexico U.S.A.


July 2019

A. Hi Paul. I don't know what you mean by "I have a primer spec". Are you saying that you have a spec about the primer and it says you must chromic acid dip or anodize before applying it? More words please!

The chromic acid dip discussed on this page and the other page you tried to post on (thread 13797) is a post-treatment after black oxide coating of ferrous materials. Anodizing usually applies to aluminum, magnesium, or titanium. In any case, a chromic acid dip does not mean the same thing as anodizing (although some anodizing does sometimes involve chromic acid).

Regards,

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


July 17, 2019

Q. B/P NOTE STATES TO DO A CHROMIC ACID DIP & 1 COAT PRIMER PER BAC5736. THERE IS NO SPEC LISTED FOR CHROMIC ACID DIP. I DO NOT THINK I CAN USE A CHROMIC ACID ANODIZE SPEC. CAN YOU ASSIST IN THE DIFFERENCE BETWEEN DIP AND ANODIZE AND WHICH SPEC TO USE?

I AM STILL HAVING DIFFICULTY LOCATING A SPECIFICATION FOR CHROMIC ACID DIP. CAN SOMEONE ASSIST? I SEE A LOT ON ANODIZING BUT I BELIEVE DIP IS DIFFERENT.

PAUL SCHMIDT [returning]
- RIO RANCHO, New Mexico U.S.A.


July 2019

A. Hi again, Paul. The problem remains that asking for a spec for chromic acid dip is sort of like asking for "a spec for painting"; a chromic acid dip after black oxide on ferrous materials is a very different thing than a chromic acid dip after anodizing or after zinc plating, or before zincating and tin plating.

However, since you have mentioned BAC5736, you are probably processing and priming aluminum, not black-oxided steel. So I think Mil-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil] ought to be applicable for you. But asking the customer to be more specific about what they want never hurts :-) Good luck!

Regards,

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


October 8, 2019

A. Yes MIL-DTL-5541 is the Chromate, Alodine process the Federal Gov uses to pretreat Aluminum for receiving paint and prevent further oxide formation. The solution of HexaValent Chromate is suspended in Hydrofluoric & Nitric acid. Its quite toxic and probably requires very specific documentation just to obtain the concentrate. It's considered a carcinogen.

Gregory Blank
- Manchester, Maryland, USA


October 2019

A. Thanks Gregory. MIL-DTL-5541 now includes the option to specify trivalent chromium conversion coatings, which are not considered carcinogen. Unfortunately though, anything which is free of hexavalent chromium is also free of chromic acid, which means it's not a "chromic acid dip". The old spec Paul is trying to comply with really needs updating :-)

Regards,

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


October 9, 2019

A. Hi Paul,

Have another look at BAC5736, it should detail either a specification for the chromic dip or the actual solution to be used. If it doesn't, then you'll need to go back to your customer for clarification. Ultimately, if they can't help you, you'll need to contact Boeing for interpretation of their specification.

Brian Terry
Aerospace - Yeovil, Somerset, UK

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