Chemistry question regarding anodising Aluminium

[stix]

This question is actually pyro related, but in an abstruse way.

 

So, I've been making a test stand for measuring rocket performance. There are many aluminium pieces which I've decided to anodize for longevity reasons - also a nice colour looks pretty.

 

My understanding of Anodzing Aluminium goes like this. The surface of the Al gets a layer of aluminium oxide deposited on the surface, it's porous, therefore can suck up a coloured dye if that takes your fancy. Then it's sealed using boiling water. This results in a hard wearing surface - and coloured pretty.

 

My full understanding may be incorrect, but I've managed to get a good result.

 

The main method for anodizing aluminium uses Sulfuric Acid as the electrolite (which is difficult to come by these days), but there is another way using Sodium Bisulphate which is a ph Decreaser used in pools.

 

It has worked well:

 

But the problem is that it is not consistent. The Al doesn't take up the dye as it did when first mixed.

 

I did read that Sodium Bisulphate gets weaker over time - like over a few weeks. There is not much info out there, so I was wondering if someone had some experience in this area.

 

Cheers.

 

https://soundcloud.com/user-566434468/fading-away-vox-ideas

Edited June 27, 2021 by stix

[MadMat]

I worked a number of years in metal finishing laboratories, so I can help you out with this! So to answer your question lets delve a little deeper into what is going on during anodizing process. You re actually growing aluminum oxide crystals on the surface of the aluminum. These crystals are not actually porous, but there is a single pore down the center of each crystal formed as it grows. This pore is how the electrical current gets to the surface of the metal underneath the oxide coating on the surface, since aluminum oxide is also an excellent insulator (the ceramic part of a spark plug on a car is aluminum oxide). One of the problems with using sodium bisulfate is it adds sulfate ions to your electrolyte and there are a number of sulfate salts that are insoluble. Add to this fact that the sulfate ion carries a negative charge and is attracted to the positive surface of the aluminum and you create a condition where that pore can become "clogged". This will not only limit the dye absorbed into the pore, but will also severely limit the thickness of the oxide coating. You say the problem gets worse as the solution is used? This stands to reason, as the more you use it the more impurities are added to the anodizing bath which gives more of a chance for insoluble sulfate salts to be formed. Your best bet is to use a 10% solution of sulfuric acid. Get sulfuric acid of good purity and use distilled water for making your solution. Use lead for your cathodes. The surface of the aluminum must be perfectly clean before attempting to anodize it! Lastly, the quality of the aluminum alloy used for your parts will significantly affect the anodizing process. The two major impurities in aluminum are copper and silicon. Both of these will ruin anodizing. Copper is the easiest to deal with as a dip in nitric acid will strip away the copper from the surface of the part. The nitric acid bath doesn't even need to be all that strong; 25% should be enough if the part is left in a little longer. Nitric acid doesn't attack aluminum so there is no problem leaving the part soak for a little while. Silicon IS a problem! The only way to remove silicon from the surface of aluminum is with fluoride. Unfortunately, hydofluoric acid (HF) is extremely dangerous! I worked at a place where they used HF and every employee had to go through a 2 hour training seminar on the dangers of HF. Apparently a splash of HF on the skin the size of a 3x5 index card can be fatal. HF has a very strong affinity for the calcium in your body and will absorb through the skin and bind with it. Since your nerves rely on calcium for nerve conduction, if too much calcium is bound with HF your heart can stop. On top of all this HF creates horrible chemical burns and even if you neutralize the acid, the remaining fluoride salt will still burn you. If you come across aluminum with too much silicon to successfully anodize I wouldn't bother trying to anodize it at home.

[MadMat]

Oh yeah, I almost forgot. Have you ever heard of "hard coat" anodizing? This is a very thick, very hard coating. It is very durable and protective of the aluminum part. The only thing needed to create hard coat is cool the anodizing bath below 38F. Apparently, they now have a room temperature process for making hard coat anodizing. The chemicals used are proprietary and those chemical companies are VERY secretive of their proprietary formulas

[stix]

Holy Mary Mother of Jesus!!! Thanks Mat for your detailed reply.

 

I have done most things right, ie. carefully cleaning the pieces, a final clean using Sodium Hydroxide (Caustic Soda) and mostly in the beginning, it worked very well.

 

I can get Sulfuric Acid. In fact I have some old car batteries in my garage that I could use. But, the reality is that I didn't want to do that - I do not trust myself with it, that's why I went down the Sodium Bisulphate path.

 

Thanks for the good explanation. Dunno where to go from here.

 

https://soundcloud.com/user-566434468/what-good-vst-mix

Edited June 27, 2021 by stix

[pyrokid]

Excellent explanation MadMat, thank you!

[stix]

Should I give up?

[stix]

There is something I didn't mention, and that it to do with Dyes. It seems you can get away with using cheaper dyes like RIT which is made for clothing. Sort of Ok, but I've also use the "proper" dyes, that is "Caswell" dyes that are made for Aluminium. The Electric Blue seems to work fine as per my image posted above. The RIT black seems to die in the arse. (pardon the pun).

 

So, I still don't know if it is the dye or the chemistry.

Edited June 27, 2021 by stix

[stix]

When I look at my attempts at black, this is what happened:

 

On the left is a nice black, but the lighting is not correct. On the right should be the same black, but it is an awful greenish colour.

 

 

[stix]

The "Caswell" dye performed much better - pretty much the same as far as I could tell.

 

So, is it the Dye, or using Sodium Bisulphate? or BOTH?, or EITHER, or NIETHER?

 

I appreciate what people are saying here. What I am asking is at what point does using Sodium Bisulphate fall away?

[stix]

https://soundcloud.com/user-566434468/the-west-final-more-bass

[Arthur]

All forms of electrochemistry have a replenishment rate to replace chemicals used up and to displace impurities put into the tank solution. Perhaps preparing a fresh solution regularly would help.

 

A solution with waste battery acid will be impossibly challenging to get good results. Don't go there, likewise fluorides, a few ppm in water and toothpaste is enough for life, any more is a potential problem

[stix]

All forms of electrochemistry have a replenishment rate to replace chemicals used up and to displace impurities put into the tank solution. Perhaps preparing a fresh solution regularly would help.

 

A solution with waste battery acid will be impossibly challenging to get good results. Don't go there, likewise fluorides, a few ppm in water and toothpaste is enough for life, any more is a potential problem

Thanks Arthur. That is the point I have not understood. Replenishment, I can understand - I have done film photography, so I get it. But leaving it for 5 weeks or so with no usage whatsoever, means it is degraded? It seems so. Perhaps depending on the Dye solution?. I suppose with film you probably are best to not leave the developer for too long either. I probably have forgotten some basics.

[stix]

I think I now know what the problem is.

[stix]

Me

[stix]

This is a problem when you get older. Things that seem like one week become 5 weeks. Things go off. Like Milk for instance.

 

So I should get it together quickly. It's not the first time. eerrgghh... oooppps...

[MadMat]

Dye is consumed as you color anodized parts, and even though the solution may not look lighter to the eye, the concentration will go down and your parts will not be colored as well. In an industrial setting, dye tanks are usually tested with a spectrometer weekly or bi-weekly and additions made. With that being said, all dye tanks were dumped and made up fresh after 5 or so cycles. A single cycle was completed when enough dye was added to make up a completely new solution.

[MadMat]

Opps I went back and read over what I had written about sodium bisulfate and realize I had a serious senior moment. You see, sulfuric acid would also put sulfate ions into the solution.... Heh. Either way sodium bisulfate is still not the best choice for anodizing. I would use sulfuric acid. I have never seen a sodium bisulfate anodizing bath in a commercial setting. There are however; phosphoric acid baths and chromic acid (chromium trioxide) baths, just to name a few.

Edited June 28, 2021 by MadMat

[stix]

Thanks Mat. I appreciate your interest.

 

Yes, I agree that the dye is consumed, but I didn't use it much at all. Maybe a surface area of 12 inches. Then went back to it 4 weeks later, and it died in the arse. The colour strength was gone.

 

The main conclusion I've come to is the Dye I was using for the black (RIT clothing dye) is not up to it compared to the "real" dye used for anodizing Al. Works fine for a few small pieces, for a short period of time.

 

I was being cheap and cutting corners. I don't think the issue is with using Sodium Bisulphate instead of Sulphuric acid - in this case. I think it is more to do with not using the proper dye. In fact, the "blue" has worked well.

 

So, I'm about to do another test using a proper anodizing dye (Caswell Deep Red). It takes around 2hrs, so I will report back then.

If it doesn't work properly, then I will conclude that the chemistry (NaHSO₄) is at fault.

 

btw. I'm using the 720 rule calculator for the anodizing.

[stix]

Now going through the "sealing" process. Just off boiling water. Not looking too bad.

[stix]

I'll post the images tomorrow. Looks good, really good.

 

2:15am here. Time for beadybyes.

[MadMat]

Something else I just thought of; those dyes are organic materials and they will degrade over time (hmm... maybe a month). The pH will also affect how well the dyes work. With some colors a change in pH will actually change the color.

[Arthur]

Some processes require progressing material to carry over some solution, and some require material to be washed twice or more to remove carried over contaminants.

[MadMat]

Some processes require progressing material to carry over some solution, and some require material to be washed twice or more to remove carried over contaminants.

I tried thinking of a process that would require carry-over. I can't think of any. Now, my knowledge is dated since I haven't worked in the field since the 80's. While I was working in the field, they were just starting to do away with cyanide plating baths and "acid zinc" (based on zinc chloride) plating was still somewhat new.

Edited June 30, 2021 by MadMat

[justvisiting]

In the bright-dipping of aluminum prior to anodizing, carryover is a critical part of the process. The carryover and replenishment keeps the aluminum levels in the bright dip from climbing too high and creating dullness in the finish. Bright dipping is less common these days. Operating a bright dip bath on a commercial scale is very dangerous.

 

 

Carryover from the anodizing tank to the dye bath can take the pH out of range for that dye, and the contamination from sulfate ions negatively affects the tinctorial power of the dye.

 

Black is the hardest color to produce in sulfuric anodizing. The coating must be quite thick to have a rich black. Ideally, the anodic film is around 25 microns for black dyed parts. More current makes the coating build faster, but also increases bath temperature. The anodic film is under attack as it's being produced, so the central pore mentioned earlier develops a funnel shape. This attack makes more surface area to accept dye, but makes the parts more difficult to seal. The sealing process involves growing pseudo-boehmite on the pore walls, which ultimately closes them off. Black dyes are tinted with other colors. If a part loses connection during the anodizing process, pore termination can result, and pore shifting can occur when the connection is restored. In effect, this makes the depth of the area to be dyed much less, resulting in poor depth of color.

 

A poorly sealed part will feel sticky. That's the trillions of pores grabbing your fingers like suction cups. A poorly sealed part will screech when rubbed on another poorly sealed part. Since black parts require the longest anodizing cycle, black parts are the most likely to have leaching and sealing issues.

 

Sodium hydroxide solution is used to etch parts prior to anodizing, and to strip off the anodic film for re-working a part. In both cases, the aluminum content of the bath governs the smoothness of the attack on the aluminum. Too little, and the aggressive attack causes localized over-heating and grainy looking finish. The anodizing bath should have a few grams per litre of dissolved aluminum also, to prevent burning at the contact points. A typical anodizing bath would have about 18% by weight of sulfuric acid and a minimum of 5 grams/litre of dissolved aluminum. The bath needs to be replenished regularly to have consistent results.

 

All contacts should be solid, and no parts should be able to wiggle on the racking. If parts must be exactly the same color (for instance a left and right appliance handle set), they should be anodized and dyed together. Aluminum jigging is the most conductive, but it's consumable and it also gets anodized. Titanium is what's commonly used to secure the parts and conduct the electricity. Lead cathodes are not as conductive as aluminum cathodes. Hollow lead cathodes are often used to carry water to cool the bath, as anodizing raises the temperature. Ideal temperature might be 70-72 degrees Fahrenheit. All baths should have proper agitation when in use to assure uniformity of finish.

 

Etching and anodizing aluminum generates hydrogen gas. That's what the bubbling and fizzing is. Combining air from agitation with freshly generated hydrogen can create a very powerful explosion if a spark is introduced. Lots of sparks are produced on an anodizing line

 

This is all from memory from decades ago, so I kept details vague.

[MadMat]

One of the places I worked at had a bright dip for aluminum and we wanted carry-over at a minimum. In fact, the operator was required to rinse the parts in D.I. water before putting them in the bright dip. Our bright dip bath consisted of phosphoric acid and nitric acid. Copper, instead of aluminum, was used as an etch inhibitor. When a new bath was made up, we literally placed pieces of copper in the bath until a concentration of 200 ppm minimum was obtained. Copper, as an impurity, in the parts maintained the copper in the bath as it was used. The analysis of the bright dip was specific gravity for phosphoric acid concentration and a rather involved prep procedure before a simple titration for nitric acid concentration. I can't remember for certain, what the nitric acid concentration was kept at, but it was low; around 2%? It was a finicky bath and nitric acid concentration as well as temperature were critical