Anodizing Problem

I have machined and sanded all the aluminum parts for 5 reels, but have been putting off the anodize process. It’s boring, 1 hour per part, 7 parts per reel. Today I finally started.
The first part, an S-crank, came out wrong. It was much darker than what I got with the first reel (May 4, 2010 blog entry).
So what went wrong? My acid solution was old, saved from last May. But it was stored in a well sealed polypropylene jar, and I would not expect sulfuric acid to deteriorate. The more likely problem is that the solution was cold; it had been sitting in my garage (~40F). I am pretty sure that this was the cause, since I noted that the anode to cathode voltage drop was unusually high during the process.
So I mixed a new acid-water solution, since I was too impatient to do a series of trials to isolate the cause. This time, with room temperature solution, the voltage drop was normal and the coating was much lighter.

Top: dark part, anodized in old, cold solution
Middle: correct part, new solution at room temperature
Bottom: not yet anodized
The top part has a machining mistake, note the square hole. I will use it as a replacement for the less well shaped crank of my first reel, after I strip the anodize and re-coat.

The properly anodized crank (middle) is less reflective than an uncoated part.  If I wanted a bright, shiny reel I would probably have to go to a professional metal finisher.

Update March 18, 2011: When I did my first anodizing (May 4, 2010) it was spring and indoor temperatures were at least 68 deg F. I was running 12 amp/sq ft current, and the acid solution was getting warm. But all anodizing looked OK. I have gone back and read Ron Newman’s instructions again, and can now see that temperature control is important.

A problem with my small anodizing cells is that they have little thermal mass. So I am now putting the cell in a tray of water, and monitoring the water temperature. I add some warmer or colder water when out of range 65 to 70 F. Also, I have cut the anodizing current from 12 to 6 amps/sq ft, so heating is less. Newman says this should produce better color as well.

There are 6 main aluminum parts in my reel: two rings, two spool ends, crank, and foot. But there are also 24 small parts, pillars and screws. These are too small to fiddle with individually, so I made this aluminum bar to hold them while anodizing all together. Of course the bar becomes anodized, and could be an insulator for the next batch. So I strip it with lye between batches.

This entry was posted in Anodizing/Plating. Bookmark the permalink.

4 Responses to Anodizing Problem

  1. Leroy Teeple says:

    Hello, I had the same issues as you when I started to anodizing this winter after being being successful last summer, figured that my acid water solution was to cold. A couple of phone calls solved the problem.
    Very nice looking reels.

    • dave49 says:

      Thanks for the report, Leroy. One other thing I noticed was that a dark deposit develops on the cathodes. It wipes off easily if done soon after ending the process. I now clean all cathodes between runs, just to be sure that they continue to conduct, and to remove the dark substance from the tank.

  2. swellcat says:

    The colour on the “mistake” is the most appealing to me. In the one photo, it seems to approach bronze.

    • dave49 says:

      My photo does not very well depict the color; I think that the yellow tinge is from light reflected from the wood interior of my house. My color perception is below average, and I don’t always realize when the photos don’t look right. The “mistake” part looked dingy. At some point, I may experiment with dyeing the anodize layer to get a bronze appearance.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s