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Ted Mooney, P.E. RET
Pine Beach, NJ
The authoritative public forum
for Metal Finishing since 1989
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Mixing stainless steel parts in anodize process?
Q. I have an assembly consisting of two aluminum pieces that are held together with a stainless steel pin. Originally, I planned on getting the aluminum pieces anodized and then drilling them and pinning them in the assembly process. It would be simpler to drill and pin before the parts are anodized.
If I pinned the parts together before processing, and then the entire assembly is anodized (including the stainless steel pin), will it cause any problems? If the stainless steel pin goes thru the tanks, will there be any negative effects (bad adhesion, chemical mixing, etc)?
Thanks very much for any assistance you folks may be able to give.
consumer, engineer - Berkeley, California, USA
2007
|
2007
"The Surface
Treatment & Finishing of Aluminium and Its Alloys" by Wernick, Pinner & Sheasby 
(note: this book is two volumes) on eBay or AbeBooks or Amazon (affil links) 
Aluminum How-To"Chromating - Anodizing - Hardcoating" by Robert Probert Also available in Spanish You'll love this book. Finishing.com has sold almost a thousand copies without a single return request :-) A. The stainless will vanish. Poof!  Jeffrey Holmes, CEF Spartanburg, South Carolina A. If the stainless pin is not completely isolated from the anodizing bath, it will burn up (dissolve) during processing. If I were you, I'd assemble after anodizing. You can pre-drill your pin hole(s), and have your coater mask it during processing.  Marc Green anodizer - Boise, Idaho 2007 A. The stainless steel would be destroyed during the anodizing process and take a fair chunck of the aluminum with it. Compton, California, USA 2007 A. The stainless steel, since it does not coat and hence insulate, will draw all the current until it completely dissolves and falls out !  Robert H Probert Robert H Probert Technical Services  Garner, North Carolina  2007 |
A. See MIL-A-8625 / MIL-PRF-8625
[on DLA] ANODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS, Para. 3.3.1.2. It's not a good idea to anodize assemblies. The electrolyte wicks into joints, is very difficult to rinse and causes corrosion. Chromic acid anodizing is an option where assemblies must be anodized, as residual chromic acid rather inhibits corrosion.
Dissimilar metals, if exposed, are typically destroyed during anodizing. As anodic oxide forms on the aluminum, current preferentially flows through the non-anodizing metal which electrochemically dissolves ('burns'). Stainless also gets hot due to ohmic heating (SS conductivity 2-3% IACS vs. 40% for Al 6061), which accelerates the electrochemically attack.
It may be possible to mask off the SS, but also make sure that it doesn't carry significant current between the aluminum pieces. A titanium pin might work, as titanium also anodizes, but it also has low electrical conductivity. Why not design an aluminum alloy pin to avoid galvanic corrosion issues?
- 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.
2007
A. Try with paint of nails, if you cover well there's no problem.
Oscar Gtz- Jalisco, Mexico
2007
Anodizing aluminium with steel pin
Q. I want to anodize an aluminium brake caliper for an oldtimer. The problem is that the caliper has a pressed in steel pin for guiding the brake piston. And it's not removable. What is the best method of masking/insulating this pin? What can survive in the acid?
Laszlo Rogacsrestorer - Hungary
August 28, 2018
A. Hi Laszlo. A liquid maskant, probably applied in more than one layer, probably offers the best chance of survival, especially if you start at the hole the pin goes through and make sure there can be no wicking into it.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2018
Q. Good Day,
I have some automotive clamps, the main body is aluminum and the clasps are stainless steel. The aluminum is purple , I know I can strip the purple anodizing , Can I get the clamps anodized in black without affecting the stainless in strength or color?
Spring Valley Motors - Owings Mills, Maryland USA
April 3, 2020
A. Hi Jay. We attached your inquiry to a thread on the subject which makes the point that while it may not be strictly impossible, it's probably impractical; the masking would have to be perfect and failsafe.
Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
April 2020
A. To be clear, if you try to anodize aluminum which is connected to stainless steel, the aluminum will immediately insulate and then the current will go to the stainless steel and start dissolving away. As Mr Mooney implied, the stainless steel will have to be 100% masked.
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina
April 6, 2020
A. Bob is spot on with his comment. Just give it up if you can not remove the stainless from the parts
Drew Nosti, CEF
Anodize USA
Ladson, South Carolina
April 7, 2020
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