Nodule Problem in Cadmium Sulphate Plating
October 27, 2017
Q. We run a sulfuric-acid cadmium process on steel substrates in two separate identical tanks (two plating stations each). We recently dumped one of these baths, suspecting it had become contaminated. Since we made this new bath, we are getting extreme roughness in the form of tiny nodules, which form everywhere except the underside surfaces, as the parts are positioned on the rack. Under magnification, these nodules look like tiny brown warts. If you scrub the area with a scouring pad, the nodules shine up like the surrounding cad plate surface.
We have ruled out cleaning. Parts from the other tank do not have this formation. Bath parameters for temperature, acid and metal content, etc., are in range and comparable to the other tank. No leaks identified. We filter these baths with .5-micron cartridges and have no detectable particulate in solution. Is this some sort of stress formation? So far the only difference we have detected between the two tanks is rectifier ripple: the roughness tank has 8%, while the other is only 1%. Even so, 8% doesn't strike us as alarming, but we are working to bring that down.
We use iridium-oxide inert anodes (instead of graphite), and our cadmium anode baskets are bagged.
Any ideas? Our chemical reps have all but given up.Bruce Brady
- Lincoln, Nebraska, USA
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