Milky Tin Plating Solution

 

Question:

We use a bright tin sulfate plating bath. After approximately six months of use the bath turns milky white. When this happens we start having problems with the appearance and roughness of the deposit. In the past we have dumped the plating bath, but I don't think this is a good solution. Can the plating bath be salvaged? H.X.

Answer:

Before we tackle your problem, here are a few words about chemistry. What you are observing is the oxidation of Sn+2 (stannous) to Sn+4 (stannate). This oxidation occurs when the Sn+2 is exposed to oxygen (air). It is a natural process that can be controlled but not prevented. Stabilizers are almost always added to the stannous bath to slow this process. You are probably using one right now (your plating bath vendor calls it an additive). Over time this oxidation will still take place even with the presence of a stabilizer. The oxidized tin (stannate) forms a finely dispersed solid that usually precipitates from solution. Filtration can remove some of this material.

The filtration process does not remove all of the solids from the bath. The surfaces of the solids are good at absorbing the organic additives that are present in the bath. This results in increased consumption of these materials.

An approach to removing the stannate tin compounds is to treat the plating bath with phosphoric acid in a separate tank. After the stannate compounds have precipitated out of solution, decant the clear plating bath solution and put it back into your clean plating tank. The better way to do this is to use chemicals that are specifically designed to precipitate the stannate compounds. They are usually organic in nature and can be obtained from your plating bath vendor.

The bottom line is this: You do not have to dump your stannous tin plating bath every six months.