Metal finishing Q&As since 1989

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Tin sludge formation in an MSA solution with a tin reactor

I have having problems with excessive sludge formation in a tin plating line for copper wire. We use inert anodes and MSA electrolyte with an external tin reactor to supply tin to the solution. Any experience or recommendations to solve this problem?

Rick Sisson
- Worcester, Massachusetts
2002

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The sludge is Sn+4. It is a normal reaction of tin. You can reduce it by avoiding indroduction of air. Make sure that you agitation does not introduce air into the solution. By the way, as well as I remember Sn+4 does not cause any trouble, it is more a cosmetic problem.

Sara Michaeli
Tel-Aviv-Yafo, Israel
2002

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Yes, air(Oxygen) is the primary sludge former. If the flow rate past the anodes in the tin reactor is creeping you may throw a bit of Sn(IV) off the anodes as well but this is usually very small compared to the fraction that gets converted by air contact. Since you mention a wire plater, it is probably reel-to-reel, yes? These cells usually are high speed and have a lot of plating solution motion and waterfalls everywhere which mix a lot of air into the plating solution. Anything you can do to close off the bath to the outside air will improve the situation. For example you may be able to close the plating cell and pipe the returns back to the reservoir under the solution level, etc. Only expensive filters can remove the sludge in process, or you can use a centrifugal separator. Most people just let it settle overnight and decant in the morning before shift begins.

Dave Kinghorn
Chemical Engineer
SUNNYvale, California
2002

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Few factors which contribute to the formation of the sludge (Sn4+) are air (oxygen), Agitation and temperature. Oxygen may get evolved from inert anode (ferrite anode used for this purpose have low oxygen over potential) may oxidize Sn2+ to Sn4+, Atmospheric oxygen (from Air) may get into solution by excess agitation (barrel rotation, violent shaking etc), air leak in filter, etc can also cause oxidation. Heat evlolved during electrolysis is also a contributing factor. If your plateout is excess, due to internal resistance in the bath the bath may get heated up it is always preferable to maintain the bath temp between 18-22 'c. These are the few factors to consider for excess sludge formation.

Excess Sn4+ can be settles using some proprietary flocculants which are readily available in market. Use them once in 15 days to keep your bath free from Excess Sn4+.

I was using a MSA bath for Solder plating (Sn and Pb), I was having problem of Excess tin getting into the bath and was finding it difficult to control the composition. It was a barrel plating with small tank capacity. We limited the use of anode and used inert anode (ferrite anode) simultaneously along with soluble Sn/Pb anode. This struck a balance between the metal getting into the solution from the anode to the metal getting plated out. The bath became stable and also the excess sludge formation which we were facing also reduced. But we did regular maintenance by settling the Sn4+ formed in the bath by adding flocculant every 15 days.

Good luck ...

Karthik
- Singapore
2002