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Precipitate froms in Cold Sealing Bath after Anodizing

Q. Scenario:
Our Anodizing Line has converted the sealing from Hot Seal to Cold Seal on October last year due to cost saving activities. After 3-4 months running with cold seal, we found a white clear crystal has settled at the bottom of the sealing tank. We've collected and kept for disposal. Again, the same thing happened 3 months later after cleaning. Recently, the rate of crystallized formation has increased suddenly to 7-10 days, and we have to clean it more frequently now and it has covered the air agitation and cooling pipes. We have analyzed the crystal and found that 90% of the crystal is Ammonium Aluminium Fluoride; NH4AlF4, and it's not soluble.


1. Why is the crystal formed in the cold sealing bath?
2. Why did the rate of crystal formation increase suddenly with no major changes of the process parameters before or after sealing?
3. How to prevent it from becoming crystallized like this?


A. Dear friend,

As I understand you are using a nickel fluoride cold seal. You are also probably using NH3 in order to adjust pH. NH3 is your problem, because when added you have precipitation. The salt is being formed and precipitated in conditions of high alkalinity. As I can imagine the precipitation rate increases because you are using more NH3 than before (when your bath was new). If you want to avoid precipitation you should make sure that NH3 is being added pre-diluted in your bath (dilute it in water or in a quantity of the sealing bath).

Good luck!

K. Apostolopoulou
Aluminium Finishing Industry - Athens, Greece


Q. Dear All,
we use cold sealing at anodizing line. We have no problem about quality. Also if I compare hot and cold sealing, I can say cold is better from every angle, especially operating. But I have realized that precipitation rate in the bath is increasing. So we decant it every month, then we can see a huge amount of precipitation. So, what's the reason for it? Is it normal? How can we explain the chemical mechanism? By the way, we filter it constantly; we change filter's disk (plate) every weekend.


Alaattin tuna
- turkey-sakarya
November 5, 2014

Hi Alaattin. We appended your inquiry to a similar earlier letter which seems well answered to me. Does that earlier response sound reasonable and helpful to you? Good luck.


Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey
December 2014

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