tank size vs acid concentration

[Fibergeek]

First off:
Are you're sure the anodizing current is 4.5 A/sq.ft? If the piece you were anodizing has a surface area of 1 sq.ft, then 4.5 A would be right. As an example; if the piece had 0.5 sq.ft. of surface area, the current for 4.5 A/sq.ft. would be 2.25 A. What is the surface area of the piece?

The ammeters on battery chargers are notoriously inaccurate, some as much as 50% off. This is why I want you to use the 0.1 Ohm resistor.

For maximum layer thickness, the voltage should measure 11.25 V or so at 1 aq.ft. surface area. This is the voltage at the end of the process. Again, as an example, if the surface area was 0.5 sq. ft., the voltage at the end would be 5.6 V. This assumes PAR is 2.5 ohms when the max anodization thickness is reached. If you wind up being within about 90% of PAR, you will be fine.

V = I x R. If I is 4.5 A, then 4.5 A x 2.5 Ohms = 11.25V

If you are reading 8.6 V and the piece is 1 sq.ft. you only have 75% of full thickness, which isn't enough for excellent dyeing.

The acid concentration you have is too high for LCD. You have dissolution occuring too fast, you may also have excessive pore size, which is bad for dyeing. This is probably why your anodize isn't thick enough. The anodize layer you're grown is being dissolved by the acid. I don't recommend just dumping more water in to dilute it. This is easy to get wrong. Start with a new mixture which is 1 part battery acid to 3 parts
water by volume.

For reference:

Battery acid is 19.17% acid concentration by volume.
The Caswell standard mix (1:2) is 6.4% acid concentration by volume.
The LCD mix (1:3) is 4.79% acid concentration by volume.

Specific Gravity (SG) measurement is only valid when the mix is new and has not been used for anodizing. As soon as you anodize, a little aluminum get dissolved in the electrolyte. For standard current densities, like 12 -15 A/sq.ft., you get about 1 gram of aluminum dissolved per sq.ft of anodizing done to 0.8 mils (0.0008") thickness. This rate would be less for LCD, but I don't know how much less. The SG of aluminum is 2.8.

(edited to fix wrong number in LCD concentration)
(edited again to fix conc. numbers)

[whistule]

Yeh the part has 1 sqft surface area. I can't get a .1ohm restistor for the moment so I'm going to have to put 10 1 ohm restistors in parallel - maybe that will change things if I get a more accurate reading. Changing the acid concentration is really the last option I want to try. When I mixed up the acid not so long ago I added 20lts battery acid 20lts water but I have weakened it by removing 5lts of that and ,making up with distilled water. If my calculations serve me corectly that would mean I have 11% acid concentration by volume so its strength is proably what's accounting for my low voltage. I think I see what you mean.

Thanks again for the help.

[Fibergeek]

OK, 1 sq.ft.

For the resistor, you want 10 Watts power rating minimum. That means your ten 1 Ohm resistors need to be 1 Watt each minimum.

Reducing the acid concentration is the first thing you should try, your acid concentration is clealy too high for LCD to work properly.

Acid concentration has nothing to do with voltage; you are controlling the voltage, and by doing this also the current.

[Fibergeek]

Whistule,
note the correct concentration by volume for LCD mix is 6.6%, not 4.9%. Sorry for the error.

[whistule]

I was under the impression that if the acid was weaker the resistance of the circuit would increase so therefore a higher voltage would be required to push the same current?

[Fibergeek]

That is correct.

But if we are talking about 1 sq.ft. of area to anodize, and about 40 liters (10.6 gallons) of electrolyte, the change in voltage between LCD and your electrolyte would be maybe 1 Volt at most. Acid concentration has a small effect on the anodizing Circuit, it has a substantial effect on the anodizing Chemistry.

You will find it very valuable to learn to distinguish between electrical and chemical causes and effects, when you can do this anodizing is reduced to one simple DC circuit (easy to analyze) and two chemical reactions; athough more complex than the simple anodizing circuit, they aren't actually that difficult.

[whistule]

Fibregeek,

I was just going to alter the concentration of the acid but I am slightly confused by the numbers you posted>>
Battery acid is 19.7% acid concentration by volume.
The Caswell standard mix (1:2) is 9.9% acid concentration by volume.
The LCD mix (1:3) is 6.6% acid concentration by volume.

By calculation I make 1:2 as 6.6% acid concentration by volume and 1:3 as 4.9% which seam to be different from your results. Am I making some mistake with the figures?

I assume our (uk) battery acid is the same concentration as your 19.7% acid - ours seams to be identified with an s.g. of 1.26?

[Fibergeek]

No, I made the mistake.

Neilfj informed me that I had it closer to right the first time. These are the correct figures:

NAPA battery acid; 19.17% conc. by volume (not 19.7%)
LCD electrolyte; 4.79% conc. by volume (let's call it 5%)
Caswell standard electrolyte; 6.4% conc. by volume
"Professional" electrolyte (165.3 g/l by weight); 9.375% conc. by volume

I will edit my post above to correct this.
Sorry for the mess.

I would assume that UK battery acid is the same as US battery acid, NAPA claims that the SG of their's is 1.265.

[whistule]

I have diluted the acid down and that seams to have improved things. I am still not convinced I am getting the full anodising thickness. I ran a check at 4.5 A/sqft and 6 A/sqft. The 6A current density provided a thicker layer - it added arouund 0.01mm to the existing surface as opposed to 0.0075mm with 4.5amps per sqft. I would say the colour took better at 4.5 A/sqft but the layer was thinner so it stopped taking earlier.

Working through the maths :-
current density 6
end voltage 13.2
therefore R = 13.2/6 = 2.2ohms
So the layer around 88% of what it could be. ??

The best colour depth I have had so far has actually been with the old method but this was anodising at a high rate for just 20 minutes so the part was pitted - not the best!


I am wondering what would would be the next step to trying to get the optimum LCD anodising thickness using 4.5 A/sqft? I suppose I could dilute the acid down again? what would be the consequences of diluting the acid too far? - Longer anodising time? The only other thing I can think of is the acid temperature which starts at 17 degC and moves to 19 degC (59 > 62.4 degF!)

As always any help is much appreciated!!

[Fibergeek]

Good work!

You did the math right, 2.2 Ohms means you should have anodized for a longer time.

Recall that the entire point of measuring PAR is to be able to determine when to stop, 2.5 Ohms per sq.ft. is the endpoint figure that I picked. If the dissolution rate you have for your setup will allow it, you can anodize to a greater thickness, which you can determine by the greater resistance you measure (calculate from voltage divided by current).

How do you know if your dissolution rate will allow it?
When the resistance stops going up and starts going down, dissolution is overtaking anodic layer growth, and it's time to stop.

Exactly how long it takes to get to that point will vary with almost everything; temperature, acid concentration, current density, etc..
90 min. at 4.5 A/sq.ft. and 60 min. at 6 A/sq.ft. is what it was in my setup. Your setup is different, so the elapsed time will be different. This is OK, since you now can tell when to stop. Don't be suprised if repeated anodizations under the same conditions have longer or shorter elapsed time, this is the process compensating for parameter differences. If you anodize to a particular resistance, the anodic layer will come out almost exactly the same every time. You now have a closed loop process, you can now get extremely consistant anodizing, limited only by the accuracy of your power source and test equipment.

You will discover shortly that you will be doing even better if you use a current source. Not to mention how much less work it would be; the current source adjusts the voltage automatically, and far more accurately than a human can.

Notice how low the temperature rise was in your tank? 2 degrees Centigrade is small enough for you to ignore.

If you dillute the electrolyte further, you will make the pores smaller, and retard dissolution a little more.

If you lower the temperature, the pore size will be the same, but dissolution will be slower, so you can grow a thicker layer.

As to what is "right" for you, you will have to determine that. You now have the tools to control anodization completely, you can adjust things to get exactly what you want.