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topic 1419

"Neutralizing" sulphuric anodizing solution with bicarbonate or nitric acid

A discussion started in 1998 but continuing through 2019


Q. We've experienced inconsistency when black anodizing "MIC-6" and "JIG PLATE". The problem is white spots on the parts after the parts are dyed and sealed. We have read that this is caused by acid bleed out from shrinkage voids in the material, occurring after dye treatment. We also read that immersion in sodium bicarbonate prior to the dye treatment serves to neutralize the acid and prevent this problem. We have tried this with some improvement but still do not have a complete elimination of the white spots. Is it possible that current density (we use 15 A.S.F.), coating thickness (our thickness is .6 - .8 mils), or pretreatment could be the problem? We would greatly appreciate any information you have to offer on this problem.

Steve Davidson

Ed. note 1/23/05: You may also want to consult Robert Probert's on-line article "Where do 'white spots' come from"

"Surface Treatment & Finishing of Aluminium and Its Alloys"
by Wernick, Pinner & Sheasby
from Abe Books
info on Amazon


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.

pooky tom pullizi signature
Tom Pullizzi
Falls Township, Pennsylvania


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.

David A. Kraft
- Long Island City, New York


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.

Keith Rosenblum
plating shop - St. Paul, Minnesota


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

Jim Austin
- Goleta, California USA

sidebar 1998

Does your application allow you to use any of the 7000 series wrought tooling plates available? Alca Plus and Alpase k100s for example are also a continuous cast product but are a 7000 series aluminum and are not subject to this spotting issue nearly as much as the Mic-6 products.

Michael D Eheler


I have read the series of responses regarding white spots on anodized cast plate. My company sells both the Alca Plus and Mic 6 product so I consider myself impartial. We receive calls with this concern for both products, it is inherent in the product of cast plate, not in the manufacturer. Both Alcoa and Pechiney producers of Mic 6 and Alca Plus have published procedures for anodizing cast plate. You can find guidelines for Pechiney's Alca Plus on the website under the section titled finishes. You can also find information regarding Mic 6 on under products and alloys. Unfortunately, the Alcoa site does not give the detailed directions that are listed in their brochure. I will type out what they have defined, which I might add is different than Pechiney's procedures.

Anodizing Mic 6 For conventional anodizing, a 15% by weight sulfuric acid solution (or bath) is recommended. Optimum results occur at a current density of 1.2 A/dmsq (12A/ftsq). The voltage required is generally 15.5. However, as the aluminum content of the anodizing bath increases, the amount of voltage needed will increase.

Once the anodizing process has been completed, the plate surfaces should be rinsed. If a black dye is to be used, the plate should be neutralized. Since cast tooling plate may contain micro-shrinkage porosity, entrapment of sulfuric acid can occur during the anodize process.

Simple rinsing is not always sufficient. A dip or submersion in a neutralizer is suggested. A 5% solution of ammonium oxalate, or sodium bicarbonate, is recommended for 5 to 15 minutes.

The temperature of the solution should be 80 to 90 °F. If the entrapped sulfuric acid is not neutralized, white spots can occur at each shrinkage void after a black dye treatment. The acid bleeds out of the voids and oxidizes the dye. In the case of a normal or hard coat anodize, a rinse without neutralizing has been found to be satisfactory, and is the general rule.

Note: in the density equation above, I was unable to type the formula with a 2 indicating squared. I replaced it with sq.

I am certain if you visit either of the websites above, you can request a printed brochure be mailed to you with these instructions.

Hope this helps.

Kim Anderson
- Milwaukee, Wisconsin

Ed. note Jan 2019: Sorry, neither of those websites function anymore, and neither Alcoa nor Pechiney exist as independent companies anymore.


In dealing with the spots and voids in anodized cast plate such a Mic-6, Alca-Plus, and K100 the common cause as stated in some of the responses above is voids or micro-porosity that exist in the cast aluminum structure. This is a very long standing problem with painting or anodizing cast plate. A product called Alpase K100-S Cast Aluminum Plate is cast in 5083 aluminum alloy with virtually no porosity which allows the cast product to be anodized with great success.

Dwayne Gordon
- Downey, California, USA


Machining the Mic 6 cast plates with the anodization process brings me to say that Alpace products are far superior to that of the Mic 6 when it comes to the black, or Hard black processes in Anodization. Once I saw the plating hues and deep color relation to that of 6061 and no issues regarding the "Night Stars" condition frequently seen, Alpace Products and its plating successes through the alloys mentioned and its substitutes called Mic 6, Jig plate, are in my professional opinion not an option to me anymore.

If they want it Flat and Black I suggest the change to Alpace K100S get them to change the material call out, most new engineers are not all that abreast of the experiences we all have had with Mic 6.

Andy Eden
precision machining of Alpace and Mic 6 - Morgan Hill, California, USA


MIC6 is the best product I've ever used. It's dimensional stability and flatness characteristics are simply the best.

Jack Samuels
- Cleveland, Ohio


Okay, okay, okay! We've had our Alpace vs. Alcoa vs. Pechiney debate. Thank you for your passion and beliefs folks. But considering how internet forums can draw shills, often using fictitious names and posing as satisfied customers, and considering that we can't post balancing criticism lest we hear from the lawyers, there's no point in continuing this "Ford Trucks!" / "Chevy Trucks!" / "Dodge Trucks" / "Ford Trucks!" / "Chevy Trucks!" / "Dodge Trucks" style debate. Thanks for your understanding.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

December 23, 2008

Q. My company applies a NiTuff coating to the lapped surface of a Mic-6 plate. Should I have similar concerns with post process blooming from the aluminum when using the NiTuff coating? Could the post process blooming affect the coefficient of friction of the plated part surface?

Tim Abrahamson
Industrial positioning systems - Seattle, Washington

May 3, 2012 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread

Q. What is the recommended concentration for the nitric acid bath used in anodize as neutralizer after the sulfuric before the dye?

I have been using 35%-45% nitric and it works good, we have another facility that uses 7-10%, we are concerned about the integrity of the oxide film after using the high concentration of the nitric, but we don't know if the lower concentration is good enough.


Andrea Saulsbury
Process Engineer - Baltimore, Maryland USA

May 4, 2012

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.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

May 7, 2012

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.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide

Maintenance Schedule for Nitric Acid Rinse

January 28, 2019

Q. My company recently started testing a nitric acid rinse in our anodizing line post anodizing and pre dying. I am wondering what sort of maintenance schedule is required of a 3% nitric acid rinse tank that is not recirculated.

Corey Rahn
- St. Louis, Missouri, USA

January 28, 2019

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.

robert probert
Robert H Probert
Robert H Probert Technical Services
Garner, North Carolina

January 28, 2019

? 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.

alaattin tuna
- TURKEY,sakarya

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.


pic of Ted Mooney
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Striving to live "Aloha"

January 29, 2019

Q. Ted, you are correct. We are using the nitric to pull out the sulfuric for additional rinsing before dying.

Robert, that is what I supposed would be the case. We will probably start on a weekly basis and go from there.

We did see a slightly faded color when testing the nitric as well as some blotching of the dye. Is that an expected outcome from the nitric?

Corey Rahn [returning]
- St. Louis, Missouri, USA

January 29, 2019

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.

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont, United States

February 1, 2019

Q. Thanks for the in depth response, Rachel. We will certainly be testing our submersion time and rinsing technique.

Another question I have regarding the nitric rinse, does anyone notice any unique health or safety issues from an operator standpoint associated with that solution. Of course, proper PPE is required but other than that, anything in particular?

Corey Rahn [returning]
- St. Louis, Missouri, USA

February 5, 2019

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.

Rachel Mackintosh
Plating Solutions Control Specialist / Industrial Metals Waste Treatment - Brattleboro, Vermont

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