"Neutralizing" sulphuric anodizing solution with bicarbonate or nitric acid

A. Have you tried hot-and-cold rinsing in conjunction with the bicarb treatment?

Have you analyzed the white spots? Do they contain appreciable amounts of alkaline cleaner/etch components? Could the voids be filling up with these solutions in their relatively hot solutions? Can you reduce the temperature in these solutions to below the hot portion of the hot/cold rinse? Perhaps that would prevent these solutions from getting into recesses that the subsequent steps cannot reach.

A. We have tried the bicarbonate step also & have found a better alternative. When black anodizing odd material, i.e., jig plate, castings, etc we use a quick (10-20 sec) dip in nitric acid after the anodize. This thoroughly cleans out the pore so that it will more uniformly take the dye.

A. We have found that 20% to 40% nitric acid does a better job of neutralizing the sulfuric acid anodizing solution than the sodium bicarbonate. Also, (just a guess), try avoiding an etch and a strong desmut in your pretreatment cycle if you can. These could open up the pores more on the surface of the aluminum, making absorption and consequent bleed-out of the anodize solution more likely.

A. Try a vacuum bake of the cast material prior to anodize.

A. Hi Andrea.

Further up in the thread, Keith R mentions 20 to 40%. In letter 35084, Jeffrey Holmes suggests 5%. Dilute nitric acid in Jeffrey's range is all that I am familiar with. Aluminum is resistant to nitric acid, so I doubt that a high concentration will much hurt it, but I personally subscribe to the philosophy that no more of any process chemical should be used than proven necessary.

Regards,

A. Sodium Bicarbonate WILL go into the voids and pores of a casting, however, the sodium bicarbonate molecule is too large to go into the pores of the anodic film. 10% Nitric Acid which does go deep into the anodic pores, does not "neutralize", instead, due to its lower surface tension than the "oily" sulfuric acid, it helps flush out the sulfuric acid from the pores hence making more room for dye and the hydrating seal. A 2 minutes dip in 10% Nitric Acid WILL NOT strip enough aluminum oxide that you can EVER measure. There are two commercial specs out there (one in aircraft and the other in Ohio) that prohibit the nitric dip because they have never accepted the proven FACT that 10% nitric acid will not remove any measurable amount of aluminum oxide.

A. I have been having to answer that question to many of my clients. Since you drag sulfuric acid into the nitric acid post rinse, a simple titration will not separate the two acids.The analysis could, theoretically be done, using a centrifuge and barium nitrate, however, the cost of the barium reagent is prohibitive. So I say guess at it, and dump weekly or bi-weekly.

? This is the first time I've heard of nitric acid bath after anodizing and before dye. What's your aim? What's your expectation in putting nitric acid bath after anodizing and before dye?. What kind of dye do you use?

You say you are testing. If I were you I would cancel this testing. Nitric acid is a very dangerous chemical for tin/nickel coloring baths if you use these baths. If you use nitric acid you will get iridescent color.

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January 2019

A. Hi Alaattin. This is probably organic dye, where it is fairly widely practiced, not two-step anodizing. The purpose is to improve the rinsing, i.e., to drive out the remnants of sulphuric acid.

Regards,

Ted Mooney, P.E. RET
finishing.com - Pine Beach, New Jersey
Striving to live Aloha

A. If you leave the parts in the Nitric too long, there can be problems such as a scorched appearance on the finished parts as the Nitric acid has been given time to attack the anodic layer. The goal is to rinse them in the Nitric dip only long enough to clear the Sulfuric residue, and then rinse them very well to remove the Nitric. This also protects your dye tanks from pH shifting, which in turn may cause a color shift. We use a 2 step rinse after Nitric and before dye: pre-rinse in running city water and then final rinse in hot DI water prior to dyeing in an organic azo dye and do not have discoloration problems.
With a reasonably heavy throughput, our 4% v/v from 42 °Be/67% Nitric tank takes on a dirty appearance pretty fast, but is still functional after extended use- up to a year; maybe more but that's about how often we dump ours. I test it weekly for total acid by titration and only make adds to ensure the rinse remains at 4% as Nitric equivalent in order to maintain predictability for the line technicians.
You can certainly have it tested by an outside lab for Sulfur and Aluminum contamination to monitor how badly it is being affected by drag-in. We have run this tank up to 2 g/L each Sulfur (calculated as Sulfate) and Aluminum before we ceased testing because we never saw issues with process failure and the levels never climbed high enough to worry us before the annual dump anyway.
We run monthly salt spray on dyed TyII and III as part of our process controls and have not experienced pitting failure on these parts, even using an old Nitric bath.
Certainly a frequent dump and remake is prudent as a way to ensure that your process is stable and predictable over time, and if you have the ability to do frequent changes, it can only help! However, in our own experience, the main variable for how well the final product comes out is immersion time in the Nitric, and the age and contamination status comes in second.
This is my observation from my own experience and if you are having discoloration on dyed parts that you suspect is being caused by the Nitric bath, look carefully at the immersion time and rinse process before dyeing.

A. Hi Corey,
At that low concentration and ambient temperature, the tank itself does not offgas or fume to a point where it is noticeable walking by, but standard ventilation precautions should always be taken. For normal operation, gloves, safety eyewear, and rubber boots are important; a rubber apron in addition for splashes is ideal.
For tank adds from 42° Be/67%, there IS fuming, and you need a respirator. A 3M half-mask with the Olive multi-vapor cartridge is cheap and works great. I wear a full PVC suit and face shield and double gloves (nitrile liner under neoprene abrasion-resistant outer glove).
Remember to use pumps that are compatible with an oxidizing acid; the PVC-Viton hand operated drum pump you can get through Cole Parmer is pretty durable. My biggest word of advice is not to melt the pump. When you're done with the transfer, drain it in a pail, rinse it by taking a few strokes IN THE ACID BATH, then drain again and rinse in plain water. The step down in concentration reduces dissociation heat and prolongs the life of the pump HUGELY.

A. Hi Corey,
Our spec don't allow the use of extra baths to neutralise acid after anodising, so to ensure acid removal from the anodic film we rinse the parts for up to 10 mins. in order to be certain of acid removal.
You have discovered the risk associated with using other chemical solutions before dyeing to remove the acid.
Anodic dying can be very susceptible to even lower concentrations of contamination as any trapped acid, etc. -- leaches back out of the film during dying and sealing.
Adding a bicarbonate step in your process just means you have then to remove that from the anodic film as well.
So, it's best just to rinse, rinse, rinse.
Best regards
Mark

A. Some specs will require a post dip in sodium bicarbonate to be sure to neutralize the acid in recesses, blind holes, and folds. But the sodium bicarbonate molecule is too large to get into the pores. Over 10 minutes in plain water can start some sealing. What you need is to get the acid and alum sulfate out of the pores,in today's science that is mostly done by post dipping in 5 to 10% nitric acid which is less viscous than sulfuric acid and helps flush the pores, avoids sulfate impurity in the dye/seal, and makes more room for dye and seal. Some specs written by inexperienced school boys denies nitric as a post dip fearing stripping, but you cannot measure weight loss after a 45-75 second dip in 5% nitric acid and you have prevented impurity sulfate in the dye and made a better seal.

A. I USUALLY RECOMEND THE NITRIC DIP AND/OR SPRAY RINSING. I AM NOT IN FAVOR OF THE NEUTRILIZING RINSE -- COULD CAUSE PROBLEMS.

A. Just one more comment on removing the sulfuric acid from anodizing. Bob Probert is right on with the nitric dip. In addition, I recommend spray rinse after anodizing and the nitric might be incorporated there. Then a counter-flow rinse. That set up is the ultimate and what we recommend when setting up a new line.