Mixing stainless steel parts in anodize process?
A discussion started in 2007 but continuing through 20182007
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
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.
anodizer - Idaho
A. The stainless steel would be destroyed during the anodizing process and take a fair chunck of the aluminum with it.
Accurate Anodizing Inc.
Compton, California, USA
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
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide
Mil-A-8625 [link is to free spec at Defense Logistics Agency, dla.mil] ANODIC COATINGS FOR ALUMINUM AND ALUMINUM ALLOYS, Para. 188.8.131.52. 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
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A. Try with paint of nails, if you cover well theres no problems.Oscar Gtz
- Jalisco, Mexico
Anodizing aluminium with steel pinAugust 28, 2018
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 Rogacs
restorer - Hungary
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.
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"
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