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Oxidation from Tin II to Tin IV

In tin electroplating, Tin IV is desirable for the formation of oxide film on the anode. Is it going to generate a lot of sludge? Is this oxide film essential for electrorefining?The accumulation of sludge somehow contaminate the cathodes.

Brigitte Peck Ki Laou
- Montreal, Quebec, Canada

The formation of the Sn(IV) film is critical to good tin plating when using an alkaline tin bath. The film should look slightly greenish yellow and is not very thick. If the anode returns to looking metallic, re-work the anode. This film maintains the dissolution of the tin as stannate (Sn(IV)), but if the stannite (Sn(II)) is allowed to form, the as-plated deposit becomes rough and dark. Formation of the film should not create much sludge as the anode should be pure tin; if it is low grade tin (grade T2 or T1), there is an increased risk of impurities in the tin and this can result in sludge formation.
If you intend to just electrorefine the tin, then the appearance of the cathodic deposit is not important since the pure tin should be remelted and cast into ingots, so the formation of the film is not so critical. Furthermore, it may be more difficult to maintain because of the impurities in the "to-be-refined" tin anode. The more impure the initial tin is, the more sludge will be generated at the anode, so electrorefining can result in higher levels of sludge formation.
However, if you are using an acid tin bath, then the tin anode should be kept bright and metallic.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

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