Recent anodizing failures

[captmorgan]

I have been anodizing for nearly 2 years with fairly good results. The last several attempts have failed to produce an ano layer sufficient to dye. The voltage begins to drop after only 10 minutes. Thinking the problem was bad electrical connections, I purched a Sput Welder. No help. Has anyone encountered (and solved) this problem?

My "hardware" is:
Electrolyte: 5% H2SO4 (battery acid and distilled water at 1:3).
Electrical conections: 14 ga alum wire, previously bolted, now welded.
Power: Leader regulated DC power supply.
Tank: One with lead cathode, one with aluminum. 160 sq in surface area.

I have been following the LCD methed as follows:
1. Clean / degrease with Simple Green until parts pass water break test.
2. Etch in lye solution.
3. De-Ox/de-smut at 100 deg for 3 min. (Thermometer regulated)
4. Ano at 4.5 amps per square foot.
5. Neutralize acid with baking soda solution.
6. Dye at 120 degrees for 15 min with Caswell dyes.
7. Boil in Caswell sealant for 15 min.

Rinsing steps as well as all solutions use distilled water. The last venture involved 6 pieces of 6061 alum totaling 112.5 sq in surface area. Constant current was applied at 3.52 amps (4.5 amps/sq ft). Expected ano time was 102 min. The voltage peaked at 9.79 V and dropped steadily after 11 min without reaching the expected peak of 11.28 V for a .5 mil layer.
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Does the electrolyte need to be changed periodically? I try to prevent contamination but I have been using the same stuff for 2 years. I hope I've provided enough details for someone to spot a possible problem.

Morgan

[jaggy13]

What temp is your acid at durring the ano process?

total newb here, so I am just saying what I don't see supplied for details.

[captmorgan]

jaggy

The instant I hit send, I realized I forgot to mention tank temp. I keep the electrolyte between 65 and 75 degrees, monitoring it with an aquarium thermometer and regulating the temp when necessary with a space heater.

Also although agitation of the electrolyte seem to be recomended by most, I always get wild fluctuations in the monitored voltage level when I try it.

Morgan

[sswee]

Since it initially appears to be an electrical problem, have you pulled, cleaned and retightened the cathode attachment points? Unless contaminated the acid should still be good. Maybe Fibergeek or M_D have a better knowledge of electrolyte life for anodizing. I got all the same numbers you gave in my calculations with the exception of an 80 min. run for .5 mil coating.

[M_D]

From your description, all I can do is offer some general observations. Is the part taking dye at all? Is it even or spotty? What is the appearance like after sealing and drying?

The acid will last a long time, if it isn't contaminated with certain substances. Dissolved aluminum will build up in the acid from the parts and cathodes. If the cathode(s) are left in, they will dissolve with time. A new fresh electrolyte solution will behave differently than older used solution, until it gets severe. Once a certain level is reached, there may be problems you can’t work around. We decant the solution occasionally by removing about a 1/3 of the old and replacing with new, to keep it more consistent than going from well used to completely new solution. This issue may or may not be contributing to your problem.

Also, due to drag out where you loose both acid and water, and evaporation where only water is lost, the actual acid concentration may drift out of range with time. If it gets too weak for the current density, the pores may get too small (not enough dissolution, accompanied by a higher voltage). If the concentration is too strong, there may be too much dissolution (accompanied by lower voltage).

I would recommend starting with either all-new solution, or decanting the existing solution. If you aren’t very certain of the purity (free of contamination) and concentration, a complete change out may be best. That way, you know for certain where you are.

It is correct that all connections need to be checked and cleaned on occasion, as acid will eventually corrode them. In you case, if the voltage number is reasonably accurate, then it doesn't show a bad connection, as it is low. Agitation affects the voltage because without it, the parts and the boundary layer heat up significantly, reducing the required voltage. When the parts are cooled from occasional manual agitation or any other agitation method, the voltage rises. As soon as the agitation ceases, the voltage begins to noticeably rise almost immediately.

So:

Lower acid concentration = Higher voltage
Higher part/electrolyte temperature = Lower Voltage

Higher acid concentration = Lower voltage
Low part/electrolyte temperature = Higher voltage

In many cases where the voltage drops so soon after beginning, I believe lack of agitation is responsible.

One other thing is the dye, it may be old and worn out, or have a ph imbalance. What color is giving problems, and have you tried another dye mix or other color?

[acidrain]

Yes, exactly as M_D has described.
I had the exact same problems... low voltage, bluish looking black instead of deep black, red didn't want to dye at all. I noticed that the level of my ano tank had gone down about an inch over the months (~45 gal. tank). I guessed that evaporation had caused the concentration to become elevated, so I added water to get it back up to the original level.
Ta Da... voltages returned to normal, and parts dyed nicely.

[captmorgan]

I was hoping for a simple easy to fix solution. It doesn't look like that will be the case.

M_D
The dye color, when it takes at all is very faint and splotchy, which tells me that the ano layer is not there or too thin

The acid/water ratio should not be too far off. I pour the electrolyte into sealed glass jugs in between uses to prevent evaporation.

I have not been agitating the solution. Any periodic disturbance causes the voltage to immediately drop 3 or 4 volts before returning to it's upward climb. It makes it hard to determine PAR with an eratic voltage curve. Perhaps a constant agitation will avoid this. I'll try a pump or something in some new solution.

sswee
The electrical attachments are all tight. I always double check them.

You're right about the time. I went back to my logbook and discovered that I used the current setting figure instead of the current density in my calculation.

Thanks. Morgan

[M_D]

I don't know why the agitation, which cools the part, would drop the voltage in your case. That's a new one on me.

Obviously, something is wrong as the poor dye results show. I have had dye get to the point (from age and use) it no longer worked well. Some colors are ph critical. Red is one color, some blacks and other colors use red mixed with other colors and they are even ph sensitive, the color will change due to ph fluctuations. It is possible for a properly anodized part to not dye well if the dye is not good.

If you use the 720 rule, you don't have to rely on PAR to determine when the part should be done. Most successful anodizing is done with agitation, and in our case the voltage curve is always quite predictable. With 6061, our voltage usually either peaks or plateau's just prior to being done, or continues to rise slowy until the power is cut. As I said earlier, if the agitaion is not present or adequate, the voltage is apt to peak low and early will a higher potential for problems.

I wonder if perhaps your power supply and or instruments are inaccurate and thus the current is incorrect?

Another thing I wondered about is if your parts that gave trouble are different in some way than ones previously done. If they were vibratory tumbled for instance, I know for a fact most amature and many pro anodizers will have problems because of the increased oxide present on the sufaces.