Why it is necessary to add boric acid to acid tin bath plating

A. I looked at two references and it appears that it is only used in a fluoborate bath.
The common reason for adding boric acid is to maintain the pH at the interface of the part-liquid. A bath that evolves hydrogen gas will leave two OH radicals for each hydrogen gas molecule evolved. This will momentarily lower the pH at the part and normally cause a darkening of the plate.

A. I was in error with the purpose of boric acid. I had it confused with another constituent in the bath. James and Trevor were right in the well house. Thanks!

A. Boric acid is a good buffer agent in order to control the ph decrease due to OH formation in the part-electrolyte interface as my colleagues stated. But be very careful about solubility of boric acid in your specific bath constituents and at the temperature utilized! Excess amount would precipitate and you would have to change entire bath solution.

A. Not totally true Cemil. You can always lower the temperature to ambient and scoop out the precipitated material. We used a pan that looked like a dustpan on a long PVC handle. Keep it clean and it is reusable.
The evil of precipitated boric is that it can cause shelf roughness on the part if the tank is saturated and you put a cold part in the solution.

A. As we know , we use stannous sulphate as the main resource of Tin. And the stannous salts are easily to be oxided to Tin(4+) when the pH value is not acidic. So we must keep the solution in acid circumstance. But when the content of the sulfuric acid is high, the color the surface will turn gray. So we must choose some weak acid to keep the pH value.

A. James, you are right about the solubility of boric acid, but only to a certain limit. Most nickel platers use boric acid in the range of 25-40 g/l, but I knew of one very well known record company who used concentrations of 95 g/l and a bath temperature of 80 °C. Furthermore, the bath was sulphamate based and they claim they had no problems with stress! They also said that they needed to keep the bath hot to prevent the boric acid precipitating out (surprise surprise!). They also reckoned that when it precipitated, it would block the pipes and that would cause them a lot of problems. The company was really good at stating the obvious, but they made excellent vinyl records! I know from personal experience that if boric acid does precipitate out, it becomes more difficult to re-dissolve; I suspect it is due to a change in crystal form or some other structural effect, but I have no real evidence for that.

A. The roll of boric acid in an acid fluoborate solution is to help prevent the formation of HF. Fluoboric acid is made from HF and Boric acid. Adding excess boric acid makes sure that HF does not form and if there is an excess of boric acid makes fluoboric acid.

A. I suspect contamination with other metals, more than pH. If it is pH, then just adding some fluoboric acid would help.

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