Metal Finishing Guide Book

2013

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Examples of these additives are certain organic compounds, which usually contain sulfur in their molecule, and complexed forms of a group V or VI element such as selenium, bismuth, or antimony. Deposits become harder as brightness increases; for fully bright deposits the usual hardness range will be between 100 and 200 Knoop. Antimony and selenium will produce harder deposits than most organic compounds, although the latter generally have better electrical properties. Potassium carbonate is added to increase the solution conductivity and, since carbonate is an oxidation product of cyanide, additions are not needed after the initial solution makeup. This oxidation process occurs slowly even when the solution is not in use. When the potassium carbonate concentration has reached 120 g/L (16 oz/gal) it can cause deposits to become dull or rough. Removal of carbonate can be accomplished by crystallizing at low temperatures (known as "freezing-out") or precipitation with calcium or barium salts followed by careful and thorough filtration. Anode purity is of paramount importance when using soluble silver anodes since typical impurities, such as copper, iron, bismuth, lead, antimony, sulfur, selenium, tellurium, and platinum-group metal, will cause solution contamination and may lead to anode filming, which inhibits proper dissolution of the silver. Silver anodes are produced by rolling, casting, or extruding the metal. Care should be taken to ensure adequate annealing has taken place after fabrication. The object of annealing is to obtain correct grain size so that the anodes do not shed during dissolution. (Shedding means that small particles break away from the anode and these can cause roughness in the silver deposit.) Improper anode dissolution or shedding can also occur if the free cyanide concentration is too low or if there is insufficient anode area in use. Free cyanide concentration should be analyzed regularly, even when the bath is idle, and additions of potassium cyanide made as needed. Typical analysis frequency might be once daily for heavily used solutions. Optimum anodeto-cathode area ratio is 2:1; a maximum anode current density of 1.25 A/dm2 (13.5 A/ft2) is recommended. Silver anodes should be removed from the solution if the process is to be idle for any extended period of time. Silver will continue to dissolve chemically into a cyanide solution and excessively high silver concentrations may result. Should this occur, mild steel anodes may be substituted for some of the silver anodes for a short period of operation until the silver concentration is brought back into its normal range. Silver Strike Silver is a relatively noble metal and as such will form immersion deposits on the surfaces of less noble metals that are immersed in its solution. This tends to happen even when the base metal enters the silver solution "hot" or "live," that is, with a voltage already applied. The inevitable result of this phenomenon is poor adhesion of subsequent deposits. To minimize this effect, it is essential to employ a silver-strike coating prior to plating the main deposit. A typical silver strike would be as follows: Silver as KAg(CN)2 Potassium cyanide (free) Potassium carbonate (min) Temperature Current density 3.5-5 g/L 80-100 g/L 15 g/L 15-26ºC 0.5-1.0 A/dm2 (0.5-0.7 oz/gal) (10-13 oz/gal) (2 oz/gal) (60-80ºF) (5-10 A/ft2) 311

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