How much acid in bath?

[rick201]

Hi,
I have looked on the caswell anodising lcd pdf download and it says for the anodising tank use 3 gallons distilled water and 1 gallon of battery acid but how does this come to a 12% solution ? surely they mean the 1 gallon in the 3 gallon mix total not end up with 4 gallons because that would be much lower than 12% which i understand is used for general anodising. ps the battery acid i buy is 33% solution?
Any ideas
Best Regards
Richard...

[sswee]

I think where your confusion is is the way battery acid is mixed. The 33% you speak of is mixed by weight or 19.5% by volume. The LCD electrolyte is 1:3 by volume, giving a final concentration of 4.88% by volume. Not many end users mix by weight anymore.
SS

[rick201]

Hi,
Thankyou for your reply i think i understand what you say about weight and volume but im still a little confused?
I have a book that says the following and i suppose this is whats weird i will quote it and see if you can make any sense of it for me ?
Quote:using battery acid sold at garages (33% dilution of sulphuric acid) is used then dilute 1 volume of acid to 2 volumes of water this would give a 10% solution.
The 10% solution is used for hard anodising.
If battery acid is diluted 1 volume of acid to 1.5 volumes of water then this would give a 12% solution
The 12% solution is used for general anodising.


So this is where im getting confused the LCD has a solution of 4.88% can this work? obviously it must but im just really confused by it all.
Thankyou in advance for any help.
Regards
Richard...

[sswee]

From my notes.

The traditional electrolyte concentration of 165 grams H2SO4 to 1 liter (1 liter = 1000 grams) of water is 165/1000 = 16.5% by weight. This corresponds to a 9.4% concentration by volume. Since no one commonly weighs sulfuric acid, water, or other liquids, its usually measured by volume in practice.
To sum it up:
Concentrated sulfuric acid; 100% by weight and volume.
Commercial battery acid; 35% by weight, and 19.2% by volume.
"Professional" electrolyte; 16.5% by weight, and 9.4% by volume.
Caswell's 1:2 electrolyte; 6.4% by volume.
LCD electrolyte; 4.8% by volume.

The book quote looks to be prior to the introduction of LCD (Low Current Density) anodizing. It also appears to me to mix the ratios of by weight and by volume. It's confusing enough as it is without the mixing of the different methods. Mixing dry acids by weight is a more accurate method since you don't have to concern with material density but converting ratios takes a bit of figuring. Another way to think about it is a 16 ounce Coke doesn't weigh a pound.
Any questions you have, I'll try to help. I hope I didn't add to the confusion.
SS

[rick201]

Hi,
Well thanks that explains alot I was wondering if I was going mad with the acid figures.
I have one more question that is not really related but im waiting for my air pump for my ano tank to come but I got a bit impatient and started to have a go at anodising without it.
The problem I have found is the 720 calculator works great it tells me what constant current and voltage to set up, but when anodising I leave it untill the required time it says to do 1mil thick by the calculator but my power supply never reaches the max volts setting? and I have even run the power supply for an extra half hour and no difference,it just sits in current limit?
Can you shed any light on that for me as I would say the part is sort of anodised but im not happy as it dont seem right I can scratch of the colour easy ie its not durable at all.
Best Regards
Richard and thanks for your help

[acidrain]

If your acid ratio, or bath temperature (both really important) are not right on, the volts will be off.
For your first runs, don't don't worry if your volts are too high or low. Just get your bath mix (1:3 battery acid/water by volume), and the temperature correct (~68F-70F), and while running in CC mode (set at the calculated amps), make notes of the volts at timed intervals. The volts curve will give you valuable clues if something goes wrong, so get in the habbit of making notes.
Do not ano longer than the calculated time, and never longer than for a 1mil build. Desilution will occure (which is probably what happened to you), and you'll end up with a chaulky, fragile finish.
So let's re-cap:
1:3 battery acid/water mix.
68F bath temp.
Good, strong electrical coonection to the part.
Cathode at least as big as the part.
Vigorous agitation.
LCD current densities (I use 6 amps per sq. ft.)
Timed ano run per 720 rule.
CC power supply.
Good notes.

[sswee]

Another tidbit, the anodize coating takes 24 to 48 hours to dry and harden to full strength.
SS

[rick201]

Hi Guys,
Well thats my own fault for having a go half baked eh! well thanks for the ideas and the notes I will do that for sure did not think of that to be honest.
I think the air pump will be coming the weekend so I hope to have a go at doing it properly then.
I will do it as scientific as I can so if it goes wrong maybe it will give clues.
Thankyou again for all your help I really cant praise this forum and the people enough.
Best Regards
Richard...
(newbie anodiser!)

[archimedes]

Greetings Rick,

I was curious about your air pump setup.
Do you have something in mind? There are a few
tank setups available for viewing here and in
the gallery. SS has made his tank available and
RClint has his tank photos available as well. There
may be others, but I don't recall them at this time
or know of any other photos or submissions.
How have you thought about plumbing your air?
I may have missed this in your posts, but how big is
your tank?

archi

[sage]

I haven't analyzed the peak voltage process mentioned here to any great extent but I too think there is something incorrect with the practicle aspects of it.
I have experimented with waiting for the voltage to peak and then start declining and ended with crazy times and bad results. I even logged the voltage with a Fluke logging voltmeter and there was no appreciable drop in voltage over any reasonable time with 6061 alloy that I could see.
I just stick with my tried and true method of calculating amp-minutes per square inch - which is my way of calculating the same thing as the 720 rule.
If any body cares it goes like this.

Take square inches and multiply by 5 amp-minutes.
This gives the amp-minutes required for the part.
Divide by a current your supply and connections can handle - say 2 amps.
This gives the time required in minutes to complete the anodizing.

If the time is too long (only becasue your impatient) adjust the current until your happy with the time. I usually try to get things done in less than 120 minutes if possible.

eg.
20 square inches x 5amp-minutes per sq.in = 100 amp-minutes.
100 amp-minutes divided by 2amps gives 50 minutes.

So the bath is set for a regulated 2amps for 50 minutes exactly.

You might find (like I have) that the LCD acid being very weak means that the voltage to get the 2amps flowing is quite high, maybe higher than your supply can deliver. This problem gets worse for small parts. It's a simple matter of adjusting the current to get a usable voltage and re-doing the calculation to find the time. Use the constant CURRENT setting of the supply.

If you juggle these figures in the 720 calculator you get the same results. I just find it easier (always have) to think about the amp-minutes required to get the job done. No calculators required. Forget the peak voltage thing.

PS> I've had good results from 5 amp-minutes per square inch quoted here to over 10 amp-minutes per square inch. I did a bath of red last week. Dye was very obviously getting into the part in about 30 seconds, was a dark shade after 20 min, gave off almost nothing in the boiling stage and the finished coating was hard and durable as a rock.


Dave Sage