Letter 1177

Aluminum vs. Lead cathodes for anodizing tanks

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I am looking for advice on using aluminum vs. lead cathodes for anodizing. We currently use lead cathodes, but are considering switching to aluminum, based on supplier recommendations. The case for aluminum cathodes is that less current and less cooling is required do to the greater current carrying capacity of aluminum vs. lead. A more even coating thickness across the workbar and elimination of lead from the waste stream are also given as benefits. Does anyone have experience with this matter ? Any other information (recommended alloy of aluminum, proper anode to cathode ratio, recommended way to connect to copper bussing, etc.) would be greatly appreciated.

Keith Rosenblum
plating

Charlie Grubbs wrote several articles on this subject. Products Finishing or Clarient may have copies of this article.

He recommends 6063 (6061 a fair substitute).

Albright and Wilson Americas has some fairly strong opinions on this also.

James Watts
- Fl

I have 30 years of experience running anodizing lines. I have used both lead and aluminum. I prefer lead by far. I recommend lead cathodes connected to aluminum buss work. I have found that copper corrodes in a sulphuric acid atmosphere.

If you wish to discuss this further, I will be happy to give you pros and cons.

Raymond Hendrix H&H Equipment Troy, Tennessee

As a major producer for the lighting market, we find that aluminum cathodes work for us.

Gerald Janssen

Charlie Grubbs' article on aluminum cathodes can be found in the Nov. 1981 issue of Plating & Surface Finishing.

 
Ted Mooney, P.E.
finishing.com - Brick, NJ

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
Fenis Aluminum

The Sanford Process supply to their licensees the process tank with graphite cathodes.

Leonid Lerner
NATICK, MA

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
Acadian Group

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.

Drew Nosti, CEF Anodize USA Ladson, SC

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