Anodizing aluminum using various acids
This site here:
describes the substitution of sulfuric acid for the anodization of aluminum. Sulfuric acid is the most commonly used acid, but the author shows that he can get similar results using sudium bisulfate (NaHSO4) commonly sold as a pool pH adjuster.
A little bit of research shows that various acids have been used in the process. For example,
George E. Totten, D. Scott MacKenzie "Handbook of Aluminum Alloy Production and Materials Manufacturing" (2003)
mentions "There are various anodizing processes and patens, using chromic acid , sulfuric acid, oxalic acid, phosphorus acid, boric acid and some of those acid mixes. Among those electrolytes three typical processes
are commonly used in industry. They are (1) chromic acid; (2) sulfuric acid; (3) oxalic acid;"
It is a good book, and some significant pieces of information are:
1. There are many bath compositions that work well enough to be used in production somewhere:
2. Some baths are alkaline, even though most are acidic.
3. Each bath is not just a single step, but has its own process steps associated with it, for example:
4. Different anodizing baths have different properties, such as corrosion resistance. It is not just about coloring. For example:
5. Even without coloring with dyes, the baths result in a different color. Example:
Ok, so I decided to give it a try. I have Sodium Bisulfate, Oxalic Acid, Boric Acid and Sulfamic Acid on hand. So those are the ones that I decided to use.
All sample plates are 6061 Alloy and I am not all that careful with process control, because I am mainly interested in getting a feel for the process.
The dye I used was Rit clothing dye.
Experiment 1: Sodium Bisulfate
45g of NaHSO4
300 ml filtered water
target current somewhere around 0.04 A/in^2
Approximate area of sample 10 in^2 ->0.4A
09:42 : start
09:50 : 19.1V
? : 15.1V; water is warming up...
10:33 : stop. put in dye bath; looks like it's taking on the dye nicely.
Repeat of experiment 1, but I cleaned the sample by immersing it in household ammonia solution to see if surface prep is an issue.
Repeat of experiment 1, but I cleaned the sample by immersing in bleach to see if surface prep is an issue.
Repeat of experiment 1, but I cleaned the sample by immersing in a drain cleaner called Punclin, which I believe to be sodium hydroxide, to see if surface prep is an issue.
The punclin attacks the aluminum quickly, leaving a gray residue behind that can be wiped off easily.
So experiment 4 will be repeated on two samples, one with the residue cleaned off and one with the residue left on.
Result: The residue does not affect the outcome.
Experiment 5: Boric Acid
45g Boric Acid
300 ml water
Three samples are prepared, using ammonia long immersion, ammonia short immersion, and punclin (immersion time not important as per experiment 4.
Generally, I tried to leave all samples in the anodizing bath for 30-45 minutes and in the dye for 30 minutes, but sometimes my schedule interfered. One of the NaHSO4 samples was immersed over night, for example. That sample came out much darker, so even when it looks like the sample has absorbed all the dye it is going to take, leaving it in for a few hours makes a difference.
Experiment 6: Oxalic Acid
25g oxalic acid
300 ml water
again, I did samples using different surface preparation....
The oxalic acid produces pretty noxious fumes in operation....
Experiment 7: Sulfamic Acid
40g Sulfamic acid
300 ml water
and again with different surface preparations.
Because the samples were done with varying process parameters, they are not 1:1 comparable. But to achieve that multiple samples would have to be done for each parameter set. The purpose here was mainly to feel out the process.
So, this is what the samples look like:
It appears to me that except for the boric acid, they all worked in the sense that we have an anodized surface. The different acids produce surfaces with different pore sizes. But without an electron microscope, I can't tell for sure.
The oxalic acid did not seem to produce the golden color that was talked about, but maybe it has something to do with the alloy used. On a piece of MIC-6 aluminum I got a fairly golden color:
(I should note that I tried to dye that piece with some coffee, but it took very little of the 'dye')
And also on an unknown alloy piece:
Here, I place the piece on top of a chemically passivated part to be able to gauge the color. For both parts, there is certainly a hue of gold, which is more visible when holding it in the hand than in the pictures. With the unknown alloy pice, one of the screw holes can be seen to have a more golden yellow tinge than the rest of the part. This is the color of the rest of the part when held at just the right angle when looking at it. Since the color does change with viewing angle, I can certainly see why this is used in architectural trim.
Finally, I tried to dye the unknown alloy with sodium bisulfate, and I think the result is pretty nice.
I have some actual anodizing dye on the way to see if the dye is part of the issue with the different degrees of absorption.
Update: I did perform a number of experiments with the commercial red dye and I cannot see any real difference between the best results I got from it and the best results I got from the Rit dye. But I am getting better at obtaining consistent results, so maybe then I can give a better answer. It sure seems like the best thing to do is to slosh the sample around in the dye right after immersion. Letting it sit in the solution by itself does not appear very effective.
One thing I have learned is that the outcome is dependent on the anodizing bath temperature, so for consistent results, you have to keep the temperature consistent.
This is a known effect. in http://www.ewp.rpi.edu/hartford/~leblar2/FWM/Project/Archive/Research/hardcoat/sdarticle-1.pdf
There is a nice figure showing the nanostructure resulting from different temperatures, though in that case the acid was H2SO4.