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-----Stains on Sandblasted areas of plated parts
We plate precious metals on decorative articles which are partly polished and partly sandblasted. The articles are cleaned in an ultrasonic cleaner prior to degreasing and plating to remove polish paste. The sandblasted areas get "stained" after plating. When the ultrasonic efficiency (power ) is reduced to a minimum the stains are reduced. We sandblast with a mixture of Silica beads and Aluminium powder. Has anyone had any similar experiences? Mark
Mark Camilleri- Malta
1999
It may be porosity in the base metal exposed by the sandblast, and not covered over by the thickness of electroplate. I would try several hot and cold cycles in the rinse stage after the pickle tank, or reduce the acid pickle to a minimum time and concentration. If you plate a base metal such as nickel before gold, then increase the thickness to 0.0002" (but that would level the sandblast effect, wouldn't it?) Then hot and cold rinses after the final electroplate.
Tom Pullizzi
Falls Township, Pennsylvania
1999
Thanks Tom
It was noticed that the stains are more prominent around the area where there are drilled holes or near the edge of the piece.. where the metal was cut. From your suggestion.. it makes sense that the metal was deformed mostly in this region and most likely to expose pores. But why do the stains show only under ultrasonic effect. When the same parts are dipped in the U.S. bath but the U.S generator is not ON.. the stains don't show up! ..
and yes.. a thick nickel layer will level the sandblast as well .
Mark Camilleri- Malta
1999
Hey Mark,
The ultrasonic's could be cleaning the Aluminium out of the pores of the base metal and creating a coverage problem (throwing into a deeper hole?). The high energy of ultrasonic (cavitation) might be causing a problem at the bottom of the hole and you might need less energy (megasonic). A surface profiler might help to control the depth of the sandblast. (what, manually controlled - time and pressure) Larger or smaller grit could give you the "look" faster and with less damage to the base metal.
Regards,
Fred Mueller, CEF- Royersford, Pennsylvania
1999
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