Thursday, January 3, 2008
Plating Small Parts
Small parts such as screws, nuts, bolts, and rivets can be chromium plated in wire mesh baskets or by stringing them on wires. Stringing on wire or racking is convenient for particles of moderate size, perhaps 25 mm long or longer. For basket plating, horizontal copper wire mesh trays are generally used with a rim about 13 mm high, soldered to a frame for suspension from the cathode rod. The small parts are spread in a thin layer on the tray so that they do not cover each other, and the whole plate, at as high a current density as possible without burning, for 5 to 10 min. Generally the entire basket is shaken or jarred a little a few times during plating, so cause the parts to shift position and avoid contact marks.
Plating time are vary for every purpose, for decorative purposes is about 5 to 10 min, but heavy hard chromium plates can also be produced in the barrel by using longer plating times.
The large volume of hydrogen and oxygen evolve during chromium plating cause a chromic acid spray to be given off. The chromic acid is sharply irritating and corrosive to mucous membranes of the nose and throat.
The concentration of chromic acid should not be permitted to rise above 0.1 mg/m3 of air. This requirement have been adopted by the American National Standard Institute. Many measures have been proposed to replace or supplement the necessary exhaust hoods, and to prevent some of the chromic acid and heat losses which these entail.