Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 193 of 707

Current Density, Asi (ASD) 1.0 (15.50) 1.5 (23.25) 2.0 (31.00) 3.0 (46.66) 4.0 (62.21) 5.0 (77.76) 6.0 (93.31) Ordinary Bath 130°F 54°C 10.9 12.4 14.0 16.3 18.1 19.4 20.7 140°F 60°C Mixed Fluoride Bath 130°F 54°C 140°F 60°C 10.8 15.0 12.0 18.5 13.6 21.4 14.9 24.0 17.0 26.0 18.2 26.8 19.3 27.5 Table 2, Temperature versus Cathode Efficiency allows smoother deposits with less burning or nodulation. As we view the tem- perature current relationship, it can be seen that efficiency does increase as cur- rent density increases. The bath formulations also play a role in this as well. In the ordinary baths we would seldom see the high current density without special con- ditions. Mechanical conditions, (tank material, fixtures), will prevent obtaining these higher temperatures. The mixed catalysts baths using fluoride will typically make control of the fluoride catalyst very difficult to control. The non-fluoride mixed catalyst, however, will allow operating at much higher temperatures and thus higher current densities. To take advantage of this, special materials of con- struction would be required that would resist attack of hot chromic acid solutions. As temperature rises the physical characteristic of the deposit decreases. Hardness of the deposit is affected and the appearance of the deposit becomes frosty or dull. The microcrack density is also reduced at the high temperature. These effects can be overcome in most cases with higher current densities. Chromic acid concentration obviously is important since in plating chrome the chrome metal comes from the reduction of soluble chromic acid to the metal state. The chromic acid also is the conductive media that allows current to flow between the anode and cathode. The chromic acid bath is different in this respect to other plating chemistry whereby the metal reduced from solution is replaced by metal being dissolved at the anode. As the source of chrome metal and the conducting media, the concentration of chromic acid is important. Higher chromic acid concentrations result in better conductivity of the bath. Most of the chemistries in commercial operation start off running at 250 g/l. The higher con- centration of chromic acid also reduces the energy requirements. Chromic acid baths also differ from other types of plating baths in the effect that impurities have on the bath. Trivalent chrome is formed during the normal deposition process and is in effect a "self-contaminating" reaction. The forma- tion of trivalent is, to a degree, mitigated by the anode reaction that oxidizes it back to the hexavalent state. The effect of trivalent chrome as well as other metallic impurities is that they lower the conductivity of the bath, requiring more energy to be consumed. Chart 1 shows the almost linear decrease in ohms as the impurities increase. To a limited extent lowered conductivity by impurities can be overcome with an increase in chromic acid. As a rule of thumb, 7.5 g/l of impurities will be over- come by 60 g/l of chromic acid. So we have now increased the chromic acid from 192 Mixed Non-Fluoride 130°F 54°C 150°F 66°C 14.2 15.0 17.9 18.5 20.6 21.4 23.4 24.0 25.3 26.0 26.2 26.8 27.0 27.5 14.2 17.9 20.6 23.4 25.3 26.2 27.0

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