Flow-type hard-anodizing of cylinder bores?

We manufacture rather sophisticated aluminium hydraulic cylinders for special app's (bore dia approx. 30-40 mm). The cylinder bores need to be hard-anodized, but all sealing surfaces must not be affected.

At present, we have a very time-consuming process of covering all sealing surfaces prior to a conventional batch-type hard-anodizing process.

The idea is to substitute the anodizing bath with some kind of fixture where the acid circulates through the bore, having an aluminium bar inside as cathode.

Does such a process exist? Any hints & comments welcome, thanks in advance! Markus Kramer
- Hanau-Grossauheim, GERMANY

I have personally never heard of any standardized system for such purpose, but this sort of thing is done regularly for plating, electroless plating, electropolishing. It sounds like a mechanical problem to solve with no conceptual roadblocks in the way.

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

It should be possible, but will require a lot of trial and error. Where high speed plating requires turbulence in the flow, at some point turbulence will be detrimental to anodizing as it will accelerate the acid eating the formed anodize surface faster than it is being formed. Flow rate will need to be optimized, mechanical (piping) turbulence minimized and quite possibly voltage/amperage optimized. There also needs to attention to having the temp be the same thruout the entire cylinder. I am sure that there are other factors that will come into play as you solve one factor. James Watts
- Navarre, Florida