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topic 3651

Chromic Anodizing on Titanium Racks


(1999)

What happens when titanium racks are used in chromic acid anodize? Do they draw current? A lot of current? If so, do the parts get less coating than they would on aluminum racks? Is it a bad idea to use titanium racks in chromic anodize?

Kent Kessler
Seattle, Washington


(1999)

My opinion.

1. Ti racks anodize with a very thin coating.
2. They draw current.
3. The amount is relative. If you have an excess of power supply, it just costs a few cents. If your power supply is marginal, you will occasionally not be able to ramp up the voltage without overloading the power supply.
4. The parts will get identical coating if the cross section area is large enough to carry the load. Ti is a poor conductor compared to aluminum.
5. Aluminum racks are cheaper but do not last anywhere near as long.
6. You generate more waste stripping aluminum racks than Ti.
7. I like Ti racks for all uses except large area parts, which Ti has a problem with its ability to carry the load. The cost and the wt of the rack becomes prohibitive.

James Watts
- Navarre, Florida


First of two simultaneous responses--

(1999)

What happen if I am performing anodizing on titanium parts? What material is most suitable for the racks as also for the cathode) according to AMS 2488 [link by ed. to spec at TechStreet], the electrolyte is of pH 13 or more). Someone told me lead to use titanium rack and lead plate as cathode. What is your opinion?

Pang Choon, TAN
Singapore


Second of two simultaneous responses--

(1999)

I usually value Jim's "opinions" and his answer here once again hits the nail on the head. I'd just like to add that since titanium is a relatively poor conductor of electricity as compared to aluminum, titanium racks will actually draw less current than aluminum racks. Also, unless you are processing your parts through some highly active nitric/HF bath, after the first time through the anodizing cycle, the amount of current draw of titanium will drop off dramatically, as the titanium does develop an anodic oxide while the parts are anodizing. This oxide will get thicker each time through until it reaches a limiting point. This oxide is very brittle and easily scratches off at the contact points. Chromic acid anodizing is almost always done by voltage anyway, so your rectifier will automatically adjust itself to the proper current draw. I will add one comment about aluminum racks. The chromic acid anodic coating is very thin, as compared to sulfuric anodized coatings, and is therefore very easy to strip. Some anodizers make the mistake of putting racks in an etch bath for several minutes to strip, as is required for sulfuric anodize, but a chromic acid coating will strip in less than a minute. Leaving the racks in the etch longer dramatically reduces their life. A chromic-phosphoric strip solution, discussed several times at this site, will prolong Al rack life even more.

phil johnson

Phil Johnson
- Madison Heights, Michigan



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