Letter 138

Rack plating with acid zinc

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From a conventional potassium chloride, boric acid system what affect does high zinc (ex. 5.5 oz/gal) or low zinc -2.5 oz/gal) have on:

A. Plate thickness distribution from high to low current areas?
B. Overall rate of plate thickness?

1. What affect does replacing boric acid with ammonium chloride have on A & B above?
2. What affect does high temp.( 110 deg vs. 85 deg.) have on A & B.
3. What is recognized as Max. voltage level in chloride zinc rack system?
4. Is a zinc plate rate of 1/10 Zn in 10 min. at 10 amps/sq.ft. considered avg., below, above ?

I realize this is a lengthy question, however, any input would be greatly appreciated.

Erik Esarlano

A. The theoretical effect of higher metal concentration is an increased tendency for plating thickness to track the geometry of the current flux--that is worse thickness distribution.
B. But faster plating.

1. I think the function of the boric acid is the same as in a nickel bath--as a pH buffer to offset the tendency of the bath to rise in pH as water hydrolizes into hydrogen bubbles and hydroxyl ion (alkali). I don't know how effective ammonium chloride is in this regard or if it serves a different function.
2. In theory, raising the temperature has the same effects described above under raising the metal concentration. But in actuality, brighteners degrade faster at higher temperatures; I believe that proprietary brighteners for ammonium chloride baths can take 110 or more, while those for potassium chloride baths are much more limited.
3. Voltage depends on anode to cathode distance, but 6 to 12 volts is pretty common.
4. Sounds right on target, but the math can be checked on page 791 of the Metal Finishing Guidebook.

 
Ted Mooney, P.E.
Finishing.com Inc. - Brick, NJ

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