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Letter 1177
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Raymond Hendrix H&H Equipment  Troy, Tennessee |
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As a major producer for the lighting market, we find that aluminum cathodes work for us.
Gerald Janssen
aluminum coil anodizing - Streamwood, Illinois
Charlie Grubbs' article on aluminum cathodes can be found in the Nov. 1981 issue of Plating & Surface Finishing.
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As stated above, 6063 T6 or T5 is the recommended alloy for aluminum cathodes. Alloy 6101, NOT 6061, is a substitute! Do not use the overaged 6063 T52 temper as it will dissolve rapidly. All welds should be made with 5356 alloy welding rod, not 4043, which is a common welding material.
Anode:cathode ratio should be approx. 3:1.
Lead cathodes or SS cathodes are not as good conductors as aluminum (Al = approx. 60%). Lead has a conductivity of approx. 9%; SS has a conductivity of approx. 4.5%, therefore, much of the energy from the power supply is utilized overcoming the high resistance of lead or SS, whereas the aluminum conducts the energy to the anode (parts) for better anodizing response.
In many instances, a 2-3 volt saving is noted when aluminum is used. Depending upon the total amperage being used, this could add up to considerable energy savings.
I would suggest that you bring the aluminum cathodes out of the tank and connect to either flexible copper cable or aluminum bus conductor (if the distance from the tank to the rectifier is not too far). Keep the copper below the top of the tank to minimize the possibility of copper dissolution into the anodize tank.
Hope this answers some of your questions.
Charlie Grubbs
Dear Gerald,
We have been using aluminum cathodes and busbars for 25 years. Purer the aluminum, better the conductivity for the busbars. AA6063 with minimum alloying elements concentration recommended. For the cathodes, even EC grade extruded sections can be used. Cathode/Anode ratio is 1:2 to 1:4 (not so important, my opinion). If busbars in aluminum, 1 amp/sq.mm is the min. section area for good conductivity. Connection of aluminum to copper needs care to prevent corrosion by cell-effect later. There are some proprietary compounds to put between Cu and Al for such purposes.
Sincerely,
Timur Ulucak
aluminum extrusions & finishing - Istanbul, Turkey
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The Sanford Process supply to their licensees the process tank with graphite cathodes.
Leonid Lerner
NATICK, MA
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To choose a cathode material for an anodizing system we have to take into consideration a number of factors, such as conductivity, reactivity, corrosion resistance, maintenance convenience, cost and mechanical properties, and so on.
Each kind of cathode has its own advantages and disadvantages. In general, the voltage drop on the cathodes, resulted from electric resistance, is negligible in the order of millivolts, relative to the whole cell voltage in the order of volts to 100 volts.
The voltage drop on the cathodes is principally attributed to the cathodic reaction resistivity. In the case of anodizing aluminum in sulfuric acid solution, for example, major cathodic reaction is hydrogen evolution. As indicated in many references, the hydrogen evolution resistivity on lead, stainless steels, or graphite is much larger than that on aluminum. As a result, the voltage drop on aluminum cathodes is the least among the cathodes mentioned above.
From the point of view of energy saving, it is natural to recommend aluminum acting as a cathode material for aluminum anodizing. To further reduce the voltage drop on cathodes, one can design the configuration of cathodes to increase their specific surface areas or develop alloys containing catalytic elements.
Ling Hao
- Grand Rapids, Michigan
Keith,
6063 T6 isn't a readily available alloy in the T6 form. To get certified material usually you have to have it extruded at a minimum of 1000 lbs. That's a lot of material for most anodizing operations. The good news is we have our special extrusion certified to 6063 T6 in stock at all times. No minimum. We also have it in 1.5" and 2" X 4" as header bars to attach the extrusion to.
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Drew Nosti, CEF |
Well, this is an old thread, but I have a new question. I replaced some stainless cathodes with aluminum cathodes. I followed Charlies advice for the materials. I used 1/4" x 4" 6063 T5 flat bar stock and welded it into a grid/lattice pattern with 5356 rod. The aluminum dissolved completely in 2 months. What gives? The only thing that I can think of is that the tank wasn't used much which means that it didn't spend much time cathodically protected. I guess I'm forced to go back to stainless or lead, but I really wanted aluminum to work. Comments?
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Jon Barrows |
Strange that you would get only 2 months life time. What
concentration are you running?
I use cathodes supplied by Drew Nosti (the poster above yours) and I
get close to 2 years life.
With that being said, I have 2 tanks that I operate at similar
concentrations, but at different temperatures, and have noted that I
get a much quicker dissolve rate of the cathodes in the tank at the
higher temperature. What I've done to combat that, is when that tank
is going to be idle for a period of time, I will keep it chilled to
40 F to slow down the dissolve rate.
Another possible solution for you, depending on your use (which
sounds very infrequent), would be to store your acid in drums, then
pump into the tank as needed.
Lastly, are you sure you don't have any stray current from your
rectifier going to the cathodes while the tank is idle? That would
also cause them to dissolve quicker.
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Marc Green |
Thanks Marc. The tank is "thin film sulfuric anodize" and the concentration is much more dilute than type II at around 45 g/L. Temp is 78 degrees. Even though the temperature is higher, I expected less attack because of the dilution. That was probably wrong. The bath is used infrequently because we are waiting on auditors to qualify the process. I can't really store the tank solution in drums because it is 2200 gallons. Energy costs to chill that much solution to 40 degrees during idle periods would be too costly. The life of the anodes was so short, that I have no choice but to switch to either lead or stainless and try to design for the poor conductivity. My curiosity about the aluminum failure is mainly academic at this point.
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Jon Barrows |
Jon
Double check your alloy/temper. I have seen your issue happen twice,
both times due to the wrong temper being provided.
Willie Alexander
- Colorado Springs, CO
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June 28, 2009 I still feel the material you received was NOT 6063 T6.
But I have always said that I put my money where my mouth is
SOOOO !
July 1, 2009 Drew and I might have to agree to disagree on this. I use Drew's cathodes, but am a proponent of welding the cathodes to the header bar, then coating the top with a fluoropolymer powder coating to prevent corrosion, and an unsightly anodizing tank. This also minimizes cleaning, and eliminates the need to check bolt torque on a regular basis.
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