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-----Can't get rid of sealing smut
Quickstart:
In the anodizing of aluminum the components are immersed into a tank of acidic solution and connected to the anodic (+) positive pole of a power supply. The current separates some of the water in the solution into positively charged hydrogen and negatively charged oxygen. The oxygen is attracted to the positively charged components and converts the skin of the aluminum to aluminum oxides.
Readers new to anodizing of aluminum may wish to view our "Intro to Aluminum Anodizing".
'Sealing' is usually a necessary step in the process and involves immersing the parts in a hot solution (usually with an additive such as nickel acetate) after the anodizing to swell the surface of the aluminum to close off the pores of the anodizing.
This sealing step can leave a 'smut' on the surface.
Read on, join in ...
Q. I just had a recent conversation with a scientist who works in a research lab and he told me that at one time he had sealing smut on a part/test panel that he could not remove easily. I was surprised to hear that because I have read several articles where they stated that sealing smut/chalk/bloom can be easily wiped off the piece. In the specific articles that I read I am quite sure they were talking about the smut that forms from over-sealing. Have any of you had this problem where the sealing smut that forms was not easily removed?
Rade Savija- Seaside Heights, New Jersey
November 5, 2025
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Hi Rade. You're welcome to try to survey the industry via this forum.
But our experience is that hypothetical questions draw less response that actual problems being confronted (type of seal, its operating conditions, your actual situation. etc.) ... so details usually draw more enthusiastic participation.
Luck & Regards,
Ted Mooney, P.E. RET
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finishing.com - Pine Beach, New Jersey
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A. Hi Rada
Your conversation with the scientist was correct, that is depending upon what "difficult to remove" means. Nickel seal comes in powder and liquid forms. There is cold seal, and hot seal. Cold does not actually mean cold but rather it means lower temp. ( I will skip the parameters). Anyhow, the liquid is single component product, basically incorporate it into to bath, adjust pH and you are ready to go.
The powder on the other hand is a dual component product. The powder is green and there is an additive that goes along with it which aids in absorption of the seal into the pores. I had always heard you needed the second component but wanted ti see for myself why. ( some of the best lessons are learned through pain and agony.)
Anyhow, we made the seal bath not using the additive and when the parts were taken out, there was a green powdery residue on top, but, it did not wipe off with a rag. It took lots of elbow grease, lots of scotch brite pads and lots of time to clean the parts. Lesson well learned. Also, we knew when the tank was getting low on the additive because parts began to display this same smutty condition.
Take his word for it.
Retired - newport, tennessee
November 13, 2025
Ed. note: Great answer Phil. Thank you very much!
⇩ Similar, related, Q&As -- oldest first ⇩
Q. Hi,
We are experiencing heavy smut on our finish even our sealing and rinses are new.
We try to reduce the sealing time until it fails the sealing test to resolve this problem, but smut is still there on our finish. We are using sealing additive and D.I. water for our sealing but the occurrence of smut is still there. Any suggestion to help us solve this problem? Thanks.
aluminum company - Rizal, Philippines
2006
A. Eugenie
I would suggest you check or adjust the pH of your sealing bath (nickel acetate). We have found that the mfgr's specification for the pH does not always produce a smut-free finish. But don't cut back on your seal time -- that is critical with regard to coating thickness.
Follow-up with seal quality testing.
- Colorado Springs, Colorado
? I must ask a few questions to see if I can figure out what you are asking.
1. Are the parts dyed or clear? I'm thinking dyed.
2. What alloy are you plating? 2XXX series in my experience smuts after dying if there is a deficiency of aluminium in the plating tank.
3. Are you deoxidizing the parts correctly after etching?
After etching the aluminum the parts will come out a grey-to-black depending on the alloy and duration spent in the etch tank; they will then need to be deoxided/desmutted. The parts should come out a white/silver out of the deox/desmut and then be rinsed thoroughly before insertion into the anodizing bath.
I hope this helps.
Ryan Cook
Toccoa, Georgia
Aluminum How-To
"Chromating - Anodizing - Hardcoating"
by Robert Probert
You'll love this book. Finishing.com has sold a thousand copies without a return request 🙂
A. Is this a nickel acetate seal? Smut may be due to high pH. Adjust the pH to about 5.8 using acetic acid
⇦ on
eBay
or
Amazon [affil link] or ammonium hydroxide as necessary. Ask your additive supplier for technical support.
For further assistance, there is a chapter on sealing in Mr. Probert's book:
ALUMINUM HOW-TO -- The Chromating - Anodizing - Hardcoating Handbook ⇨
- Goleta, California
Finishing.com honored Ken for his countless carefully researched responses. He passed away May 14, 2015.Rest in peace, Ken. Thank you for your hard work which the finishing world, and we at finishing.com, continue to benefit from.
A. Without knowing the additive and tank parameters, it's hard to answer your question. There are numerous causes for sealing smut.
I would contact the supplier of your additive for advice, especially if the additive is designed to reduce/eliminate smut.
Marc Green
anodizer - Boise, Idaho
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