Hardcoat Anodizing by Voltage or by Current?

A. Hi Mark. We appended your inquiry to a thread on that subject. You will see that everyone does not agree on the precise optimum ramp-up method & values, but all agree on the necessity of generally what you saw :-)

Luck & Regards,

A. I have tried desperately to find an "artful" way to hard coat by voltage, many do it, but not with consistent results. If you do it artfully by stepping the voltage, then when you cross section and look at the photograph you will see a step in the pore structure for every bump you gave the voltage.
Ideally, you must know the surface area (or use the "Parallel Circuit Method"), know the "target" amperage, spend 15 minutes from zero the target, switch over to current control and let the voltage automatically increase to hold the amperage.

A. Hi Travis

One of the best ways to avoid burning is by using pulse anodizing. I have written a couple of papers and a book about this.

One of the rectifier companies sells a controller which doesn't need to have a calculation of surface area from the operators.

Running Hard anodizing by Voltage is as bad an idea as it is for conventional anodizing. You use too much energy and the process time is often prolonged so a soft coating is found on the surface instead of a hard.

Sunny regards
Anne

A. 36 Amps per square foot is fine for production and lab development jobs of all same size where you can tune the free acid, temp, current density for high current; but, a job shop with varying sizes and alloys needs to stick to 24 ASF to stay out of trouble.

There are three ways to get 24 ASF on the load. (1) Measure the surface area, (2) Use the "parallel circuit method" published by Dr Harold Weisner in the 1958 AES Proceedings and plagiarized by me in Aluminum How-To, (3) Using 16 volts for all alloys except 21 volts for 2000 alloys, spend 7 minutes slowly ramping up to 16 volts, now look at the ammeter, note the total amps at 16 volts, spend the next 6 minutes slowly ramping up to EXACTLY TWO TIMES THAT NOTED AMPERAGE, set the automatic current control, walk away; while you are gone the voltage will automatically increase to hold that current steady at 24 ASF.

A. Travis

As mentioned, running by volts is not the best way to anodize.

If your operating parameters are sound...

Manually ramp (3-5 minute duration) to 24 volts and hold for 5 minutes. After 5 minutes, increase voltage 2 volts every 5 minutes and continue. Check your thickness when you see fit. Current density will start out < 36 ASF, increase to > 36 ASF, and drop off substantially towards the end of a 2-mil thick run on 6xxx. You can play with the later intervals.

Also consider that burning is typically caused by localized overheating. Do you have enough agitation?

A. Mask the 6061 with platers tape or paint-on maskant. Try heavy anodizing of the 6061, perfect seal, trim off the bare contact marks, rack after clean and deox then quick dip back in the deox. Small parts CAN be hard coated on titanium. The two leading rack manufacturers in the USA do not agree with me on titanium, but there is a limit as to how much it will carry and how wide the contact area must be to get twice the amps.

A. Hi Travis

You could mask the 6061 racks, or try a titanium rack. I know a lot of people don't like to use titanium racks because it doesn't conduct the current as well as the aluminum.

Titanium racks are useful in a hard anodizing process when 2024 alloys are processed. You just need to be very sure that the contact from rack to part is good.

Sunny regards
Anne

Thank you guys for your helpful responses. I am going to try to find some plastisol and mask off that way on our existing 6061 racks.

A. Hi Nilesh.

So the area per load is 3.5 sq. meters (37.7 sq. feet)? If so, the amperage seems low.

I'm not personally familiar with your idea of starting out at very low amperage. Low voltage to prevent burning, yes. But a very long, very slow ramp-up of current seems wasteful of potential productivity. Perhaps those with more hard anodizing experience will correct me.

Regards,

A. Ted is correct, the amperage is way too low. You should be more in the 950-1000 amp range, at minimum (assuming Ted's math is correct). You'll want to ramp to that final current setting amps in about 5 minutes total. That should put your process time at a little over an hour to produce a 50 µm coating (typical hardcoat thickness). You say your cycle time is 65 minutes, at the current settings you mentioned, that should produce approx. a 25 µm coating.

Your temperature is fine. You'll want to keep your dissolved Al content at around 8 gr/lt max.

Those are just the basics.

A. Hi Daniel. Pending getting a response to your situation specifically, you can read Anne, Robert, and Willie's suggestions on the subject earlier in this thread, and also see letter 13351, "Suggested ASF for hardcoat anodizing". Good luck.

Regards,

A. Hi Pradeep. 15% by weight 66 °Baumé sulfuric acid is used for general anodizing and can, with luck, we used for hard coating. However, many if not most people add glycolic acid and glycerin-based proprietary additives to this.

We appended your inquiry to a thread that may help, but I suspect you "burned" the aluminum with too much current. Although the hard anodized coating is of very high electrical resistance, bare aluminum has almost none. So -- while you need a high voltage near the end of the cycle to keep the current flowing despite the high resistance -- at the beginning of the cycle you need a low voltage. Read the earlier responses for specific instructions, but most people solve the problem by limiting the amperage rather than the voltage. Are you sure you have very good local agitation to keep the temperature down to about 32 °F (0 °C) right at the parts?

Please introduce yourself and your situation so we can understand what you probably already know vs. what you might not. Although I don't think someone with no plating or anodizing experience can be taught how to do hardcoating in the few paragraphs appropriate to this public forum, you might already know 99+% of what you need to know, but we can't tell yet. If you have no experience, a 15 foot x 3 foot x 3 foot tank is quite large for introductory experiments.

Luck and Regards,