Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Table XI. Troubleshooting Stannate Baths Problem Cause Anodes gray or white Initial current density too low to form film. Anodes brown or black, Low free hydroxide film passive Low temperature High anode current density Remedy Remove anodes and replace one at a time when current is on. Add KOH or NaOH. Increase temperature. Add anodes. If above does not remove film, dip anodes in a 20% hydrochloric acid solution and rinse well before using. Excessive anodic gassing Film lost after first forming No anodic film Low temperature High anode current density High free hydroxide Low anode current density High temperature Poor anode contact Initial current density too low to form film Low cathode efficiency Low temperature Low tin High free hydroxide High current density Low anode efficiency Low temperature Low free hydroxide High current density Narrow plating range Low temperature Low tin Potassium bath Sodium bath Low conductivity Nonadherent deposits Solution crystallizes Low temperature Low free hydroxide Low tin Initial current density too Low to form film High carbonates Rough, dark, or spongy Stannous tin in solution deposits Increase temperature. Add anodes. Add 10% acetic acid. Remove some anodes. Lower temperature. Clean contact. Remove anodes and replace one at a time when current is on. Increase temperature. Add stannate. Add 10% acetic acid. Lower current density. Increase temperature. Add KOH or NaOH. Lower current density or add more anodes. Increase temperature. Add stannate Add stannate. Switch to potassium bath. Increase temperature. Add KOH or NaOH. Add stannate. Remove anodes and replace one at a time when current is on. Freeze out carbonates and then filter. Treat with hydrogen peroxide. quickly. Practically all problems in the operation of a stannate bath are resolved when proper anode operation is achieved. Inert anodes are advantageous in that they avoid the difficulty of filming and they inhibit the formation of harmful stannous tin by producing oxygen at the anode. Furthermore, they do not change shape in use. Their use has previously been some- what limited because of the need for chemical replenishment of the bath as more tin is consumed. Replenishment of the tin content of the bath by a tin oxide solution has made the use of inert anodes practical in potassium stannate systems only. Operation of Stannate Baths Stannate solutions should be light straw or light gray in color. Black solutions indi- cate the presence of stannous tin, and they should be treated with hydrogen per- oxide (2 ml 35% hydrogen peroxide per gallon of solution). If frequent additions 254

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