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topic 3212

Stripping Cadmium plating and choosing a method




An ongoing discussion from 1999 through 2014 . . .

(1999)

Q. I am wondering why someone would choose to abrasively blast off cadmium plating rather than use ammonium nitrate. What sort of test can you do to make sure the tank is charged at the right strength? Also, if you MUST blast the cadmium plate, what sort of media would you use? Are there alternative chemical strippers for cd plate? Thank you!

Carrie L.
- Seattle, Washington


(1999)

A. Abrasive blasting of cadmium is a bad idea unless all the contaminated air is trapped in a sealed container and filtered before releasing to atmosphere. For released air, OSHA emission limit is <2.5 ug/cu.m. Ammonium nitrate should be inexpensive and adequate in most cases.

Mandar Sunthankar
- Fort Collins, Colorado


(1999)

A. Wow, someone is blasting cadmium! Not a good idea (see above) but isn't it, like zinc powder, explosive? Nitric acid @ 50% vol/vol can also be used to strip cad but you have to watch out for the nitric attacking the substrate.


Megan Pellenz
- Syracuse, New York


(1999)

Q. Thanks for the thoughts. I think some places require at least a light blast prior to replating - doesn't it make the new cad plate "stick" better? Also, I guess cadmium plated parts with Be-Cu or Al-Ni-Br substrates could be damaged by ammonium nitrate (anyone know if this is true?).

Carrie L.
- Seattle, Washington


(2001)

A. Anytime you blast a substrate prior to plating it enhances the bond strength. This strip will not affect copper or aluminum as a direct substrate, however, alloyed materials are always a difficult call. The best way is to run a test sample part to determine the outcome.

Q. I am searching for the contents of MIL-8-175, it is the mil-spec for the stripping process, can anyone help me get or know where to get a copy?

Thanks.

Paul Arato
- Toronto, Canada


(2001)

A. See our FAQS about mil-specs, Mr. Arato. The address has changed many times, so we keep the correct and constantly updated address in one spot rather than littering the site with broken links :-)

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


March 20, 2009

A. STRIPPING USING ABRASIVE METHODS IS THE BEST IDEA WHEN THE MATERIAL IS HIGH STRENGTH STEEL.
This takes away any chances of the process damaging or chemically altering the material.
Many Aerospace OEM's require baking after chemical stripping. This just to ensure that hydrogen pick up is bleed out

Terry Martin
- Toronto, Ontario, Canada


July 9, 2013

A. A simple chromate solution, with 13-15% nitric acid, will strip cadmium off of 4340 type steels without any damage to base material.

Larry Wasowski CEF
- Queen Creek, Arizona, United States



June 4, 2014

Q. A supplier of ours stripped cadmium plate from a lot of NAS [National Aerospace Standards] bolts, and re-coated with Black Oxide. When chemically stripped do the parts require embrittlement baking prior to recoat?

Bob Newkirk
- Valencia, California USA


June 2014

A. Terry Martin is certainly correct that acid stripping of high strength steel can cause hydrogen embrittlement and these bolts may now be defective. Considering that these are supposed to meet an NAS specification, I think it is worthwhile to acquire the particular spec covering the bolts in question and see what it has to say on the subject of hydrogen embrittlement. Good luck.

Regards,

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


June 10, 2014

A. We use plastic bead blasting when type 2 cad is applied and needs stripping. This to ensure the chromate is removed prior to cad strip in ammonium nitrate according to Mil-Std-871. Use breathing protection anytime when blasting whatever material with whatever media - any dust is bad for your health! Never use abrasive blasting to remove cadmium. You can't tell when all cadmium has been removed and you'll probably damage the base material.

Ammonium nitrate works perfectly. However, make sure you have thorough air agitation on your tank. Embrittlement relief baking per Mil-Std-871 is not required with this method. However, quite some CMM's (in the MRO business) require you to bake anyway. Make sure you use an alkaline soak then prior to baking to prevent bad corrosion.

By the way, one of the posts above states that blasting will always improve adhesion; that is not true. Al2O3 grit blasting will cause embedded grit in the base material, even in ultra high strength steels, which will cause adhesion failures in hard chrome or nickel plating without rigorous pretreatment (HF pickling) after blasting prior to plating.

Jeroen Visser
landing gear - Geldrop, The Netherlands



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