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topic 1419

"Neutralizing" sulphuric anodizing solution with bicarbonate or nitric acid


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
Wernick, Pinner & Sheasby
from Abe Books



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 bleedout 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


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


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 call K100-S Cast Aluminum Plate in 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, we've had our Alpace vs. Alcoa vs. Pechiney debate. Thank you for your passion and beliefs Dwayne, Andy and Jack. But considering the anonymity of internet forums, and how they draw shills with hidden vested interests, often using fictitious names and posing as satisfied customers, there just isn't a point in continuing a "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

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, MD 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 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

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 measureable 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

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