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Silica in sealing solutions

(-----) February 14, 2008

We operate Dichromate sealing baths for Sulphuric Anodised parts.
The one made up with the least amount of Silica in the demin make up water lasts a long time before the colour of the parts, start to fade. The one with the higher Silica content lasts approx. 8 weeks.
I know acid dragout can cause this problem, and I have increased rinse times accordingly and tested pH of the rinse baths which seem OK.
Has anyone come across written evidence that Silica can cause sealing problems?
I have made the Auto line bath up with brought in demin water containing no Silica to try and get some evidence that this is the reason. This is running at the moment.
Mike Channing

Michael Channing
Industrial Chemist - Hampshire, England

February 14, 2008

I and others here have said many times that many metal finishers believe that no water except demineralized water should be used in a plating or anodizing shop. Trying to prove every fault that every degree of water contamination can cause in every process is a hassle. Certainly we know how deleterious contaminated water is to dye tanks. And we know that some people use steam seals to avoid contamination. And we know that others like yourself use DI water.

Good luck in your hunt for documentation, but I'd take yes for an answer from your experiments, i.e., that the less silica the better.

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

February 15, 2008

What's rate of hot-sealing bath's contaminations as a ppm?especially silica?

Alaattin Tuna
engineer - Istanbul

February 19, 2008


I haven't seen any literature about the effects of silica on sealing ability but it is very common in US specifications to specify a limit of 4ppm.

From my own personal experiences I have not seen any problems with sealing (based on salt spray results) with silica levels upwards of 50ppm.

To remove silica to 4ppm and below will need a supplementary column (at obvious extra expense). Even then, with drag in from other water sources it can rapidly go over the 4ppm limit.

Brian Terry
Aerospace - Yeovil, Somerset, UK

February 21, 2008

I had a problem a couple of years ago, was supplying chemicals to a customer who suddenly started having Q.C. problems with sealing. We were not monitoring silica content in the baths at that time so his answer to was to change chemical suppliers. I was later informed that the municipal water supply had been changed over to bore-hole water. This led to a much higher silica level that the customers DI plant was not able to cope with. While I am unable to give silica ppm's of the tanks involved, I can testify to the fact that higher levels caused havoc in tanks that had been stable for months.

Andrew Farrow
- Johannesburg, Gauteng, South Africa

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