using aluminum as a cathode

[scocklin]

I have some professional aluminum light stands that I want to re anodize and dye. I have broken down the stands into their individual parts. They run between ½ to 1 inch in diameter and about 32 inches long. I purchased a 6 inch diameter 10 foot section of PVC pipe and cut it down into 3 foot sections. I will use two of the sections as a stripping and cleaning tank and the third as my anodizing tank. I also have a 3 foot section of 4 inch diameter PVC capped that I will use as my dye tank.

I have caped off the bottoms and cleaned them out. I purchased some roofing aluminum and scrubbed it with steel wool. I tested it with an ohm meter and it ohms out completely on one side. I want to use the aluminum as a cathode in the anodizing tank but have always used lead cathodes. What can I expect to be different now that I am using aluminum as a cathode?

[sswee]

Shouldn't notice any difference as long as you have any coating that was on the alum. off. Is the SA of the alum. the same or larger than your part? Let us know how it goes.

[scocklin]

The surface area is close to the entire diameter of the 6 inch diameter tube. Two aluminum sheets fit perfectly inside the tube circling completely around and over lapping each other without needing to be physically attached to the tube surface.

My biggest worry is not knowing if I did in-fact completely remove all of the plastic coating from the aluminum. I put 30 volts through the aluminum and used a voltmeter to complete the circuit checking for continuity at various points on the surface. It read 30 volts through out the entire surface. I’m worried about possibly not removing all of the plastic coating and contaminating the acid bath. Do you know if the plastic used to coat the aluminum could possibly contaminate the bath?

[Fibergeek]

I found out the hard way about the plastic on Al roof flashing, it will leave a sticky plastic goop floating in the tank after a few hours. I had to use a belt sander to get all of the plastic off of the flashing. Actually, soaking it (expendable) sulfuric acid might be a good way to get it off.

The effect of aluminum cathodes vs. lead will be slightly less voltage required for a given current density. Your anodizing process remains the same.

How will you provide agitation?

[scocklin]

I could go two ways, one with air or I could use a circulation pump I purchased, I would welcome any suggestions.

Thanks for the heads up on the plastic. I have a large 2 handled orbital vacuum sander that I use on my boat. I’ll try it with some 60 grit paper on the flashing to make sure all of the plastic is off.

[Fibergeek]

I'd use a finer grit, maybe 150 to 220.

Because of the unique shape of your "tank" (very deep for its diameter) air bubbles might be easier, bubbling up from the bottom all the way to the top. The plating guys have been experimenting with large bubbles, roughly 1/4" spherical (rather than small) it seems to work better. To do this you need very low PSI, but a large volume of air.

[scocklin]

I'll give it a try; I've got about 20 stands that break down to 6 tubes each. As I said before the tubes are 1/2 inch to 1 inch in diameter. The tank is 6 inches in diameter and 3 feet long. I know about the 3 inch spacing between pieces and the cathode but I'd like to get at least 2 parts in the tank at a time. Can this be done?

[Fibergeek]

I don't know.

Actually I would be happier if the tank were larger, but I'm not sure just where the limits are. I've never dealt with a shape of tank like this before. The problem you MIGHT encounter would be inconsistant coating thickness.

Believe it or not, if you experience a problem you MIGHT do better with a cathode that does not cover the entire the whole internal circumference. Maybe an Al bar running the entire depth, with LESS than the surface area of the work. I know this sounds weird. Lines of force (what they call it in electroplating) MAY dominate due to the shape.

[scocklin]

I could not go into a larger tank mainly due to size and the amount of chemicals needed. My work area is shrinking as I get more into this hobby. The tank is tall so I'm going to try several pieces at a time at various depths. The use of aluminum bars sounds like a good idea. Would two or three bars running the length of the tank spaced evenly around the tank work?

[Fibergeek]

I'm talking about 1 bar, with maybe 1/3 the SA of the work. I know this goes against what you have learned dealing with more conventional shapes.

In any event, Scocklin is treading on new ground with this project. I'm sure I'm not alone in being very interested in how it turns out.

Please Steve, document everything carefully. A project like this has the potential to payback our investment in you (the time) in spades.

[sswee]

I am definitly interested in how it goes. I am using the big PVC tubes for the other steps besides anodizing because of the container volume issue. If I run into any longer parts, I'll be in the same boat. SS

[scocklin]

I will definitely document everything and share what I have learned with all. I should have some results by tomorrow evening at the very latest.

[scocklin]

Just a quick note it's almost 10:30 PM and I just finished a test run using an aluminum bar as the cathode. I anodized two tubes and they turned out fine. I dyed them electric blue and they took the dye immediatly. I recorded all of the numbers and took many pictures. I see a great deal of benefits to this method. Fewer chemicals, less space for tanks and you reach the proper temperature faster. I will post a complete report tomorrow with pictures.

[scocklin]

I have included pictures in my personal folder of the entire process.



I’m in the process of re-anodizing my professional lighting stands. All of them are made of aluminum but I am unsure of what type of alloy they are. The stands break down into four aluminum tubes and some straight pieces for legs. The tubes fit inside one another so they telescope out.

The tubes I tested out are 23 and 24 inches in length and ½ and ¾ inches in diameter. This gave me a measurement of 92 square inches for both tubes. I used 6 amps per square foot and calculated my current density at 3.8 amps.

Because of the length of the tubes I decided to try to strip, clean, de-smut, anodize, dye and seal the tubes in schedule 40 PVC pipe. I purchased one 4 inch diameter x 10 foot long PVC pipe and one 6 inch diameter x 10 foot long PVC pipe. I cut the pipes into 3 foot sections, cleaned them out and capped the ends using purple primer and PVC cement. I secured the pipes to my work table using eye hooks and bungee straps.

The 6 inch diameter pipes hold a little less than four gallons each and the 4 inch diameter pipes hold a little less than 3 gallons each. I used the smaller diameter pipes for cleaning with hot SP degreaser, de-smutting, dyeing and sealing and I used the larger diameter pipes for stripping and anodizing.

I proceeded normally with stripping, cleaning and de-smutting the two tubes. I used my sput welder to make aluminum wire connections to the inside of the tubes.

I set up my anodizing tank differently form my normal Caswell 6 gallon anodizing tank. I was going to anodize the tubes using aluminum roofing flashing as my cathodes. Fibergeek than suggested that I use a single piece of aluminum bar stock 1/3 less the surface area of the pieces being anodized. I went with a piece of aluminum bar stock 1 ½ inch wide x 25 inches long. This gave me a surface area of 37.5 square inches a little more than a third. I later realized that I could use a much larger piece of bar stock with various square inches etched on the surface. You could than choose the surface area needed and submerge only that part of the bar stock into the tank clamping it in place with a plastic clamp.

The use of bar stock as a cathode was an absolutely brilliant idea by Fibergeek. I’m hoping Fibergeek will go into detail the differences in flux patterns between a normal setup and this one. For power source connection I welded two aluminum wires to the bar stock using my sput welder.

The physical differences are apparent in as much as I was able to push the tubes up close to the far wall of the tank and not have to worry about the pieces touching a cathode. Remember I only had one piece of bar stock in the tank. I will further try to determine how many pieces I can anodize in one tank and what spacing is needed between pieces.

My tank temperature was between 72 and 73 degrees. I used air agitation using a Boyu S1000 air pump connected to a homemade PVC aerator sitting at the bottom of the tank. This gave me a lot of agitation with large bubbles coming up to the surface.

I noticed hydrogen bubbles appearing quickly I assume because of the smaller size tank. But the agitation is much greater for that same reason. Also because of the smaller size tanks and the thickness of schedule 40 PVC pipe the tanks heated up very quickly. If you use this method you will have to monitor temperatures at all times or you will miss your mark. I walked away from the sealer tank with two heaters immersed and when I came back 10 minutes later 1/3 of the sealer had already boiled over the top of the tank.

My voltage readings at a CD of 3.8 amps on my constant current power supply for this 90 minute run where 10.9 volts at the start, at 10 minutes the voltage dropped to 10.8 and stayed there until the 50 minute mark when it dropped to 10.7 Volts and finished off at the last ten minutes at 10.6 volts. Again my tank temperature was 73 F.

I removed the tubes, rinsed them and dropped them into electric blue dye. They started to absorb the dye immediately. I left them in the dye tank for ten minutes and sealed them.

Fibergeek and I are not sure if conventional cathode arrangement would provide the same results. I will find out when I have more time by using conventional cathodes on my next run. However I prefer this method over the conventional method

The benefits of this process are smaller tanks that take up less room, fewer chemicals and the added benefit of tank temperatures coming to mark sooner.

Disadvantages are the tanks cannot support themselves. You have to be careful when handling the tanks with chemicals in them especially acids. You have a greater probability of splashes and tank spills with this method. I am also not sure how the PVC cement will hold up under heat. If anybody has any thought on this please pass them along.

I have included pictures in my personal folder of the entire process.

[Fibergeek]

Thanks Scocklin, very good work indeed!

To explain further:

The unconventional (to us anyway) cathode arrangement is not my idea. I would credit Drew Nosti at Anodize-USA for it. There is something useful here, we need to check this out thoroughly.

The user of this had BETTER take agitation requirements seriously, fortunately the shape of the "tank" will promote good agitation efficiency.

Hot liquids in PVC pipe is definately a worry. PCV pipe and PVC cement are impervious to sulfuric acid in the concentration we use as electrolyte.