Copper anodes in sulfate fluoborate baths usually become coated with films containing finely divided copper and copper oxide particles. A copper oxide film is believed to cause inter-granular corrosion, which promotes disintegration of surface layer into the copper particles. Some, however, have attributed the films to copper particles formed by the disproportionation of copper (I) ion to copper (II) ion and metal. The films on the anodes in the sulfate bath also contain insoluble impurities such as arsenic, selenium, tellurium, lead, and silver.
Anode film particles often become detached from the anodes. Air agitation promotes the detachment, causing some particles to be dissolved by the free acid. If the work is racked so that cathode shelves lie in a horizontal plane, particles will settle out on these areas and roughen the plate. In such cases, the anode sludge can sometimes be decreased and the deposits made smoother by raising the bath temperature or increasing the acid concentration. In on series of test, less copper was found in anode films formed in flouborate baths than in those form sulfate baths.
Fine copper particles can be prevented from reaching the cathode by bagging anodes with woven Dynel or polypropylene. To allow good mixing of the solution adjacent to the anodes placed edgewise to the cathodes.
Rolled and cast bars and electrolytic copper sheets have been employed as anodes in sulfate and fluoborate baths. Anode sludge is decreased by using oxygen-free, high conductivity copper. On the other hand, the tenacity of the anode film is improved and the number of particles that become detached from anodes surface in air agitated baths is decreased by adding 0.02 to 0.04% phosphorous to cast copper. The films on phosphorized copper anodes are responsible for the slight polarization of about 0.5 V. Boiled copper anodes containing at least 0.004% phosphorous are customarily recommended by the vendors of brighteners for plating bright copper in copper sulfate solutions. Chunks of phosporized copper are frequently used in titanium anode baskets.