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Poor coverage in barrel zinc plating

1995

1). parts are app 1/2" long, hollow tube, dia. = major app .280, minor app.160 1/2 and 1/2 of the tube. material looks to be bronze, parts have little to no coverage on tip of minor dia. only, id and balance look well plated. parts do nest, but fall apart easily, plate is zinc with clear plate looks to be for luster only. advice on coverage problem?

2). parts are a knurled ferrule, 1/2 dia. x 3/4 long, material bronze id is threaded. poor id plate, knurl plate is fair at best 1/16th area on ends not knurled no coverage, looks almost like masked (but not). ends of piece, no coverage at all, finish zinc with clear

note: parts almost feel slippery where not coated, acid zinc line

3). zinc die cast, 1 1/2" long, 1" dia., one end protrusion, 3/8 dia. x 1/2 long plate exhibits blistering and poor coverage on protrusion very end luster looks good, blue white coverage of part looks good, even in deep recesses on id finish - zinc with clear these are all done on the same line, are we exhibiting a line problem?, or a parts problem. these are the only reported problem parts for this zinc bbl line are these all symptoms of the same problem?, customer says solution and anodes are up to snuff but maybe he's not looking in the right place?

Ron Landrette
plating equipment supplier - Bristol, Connecticut

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#1 - From the phrasing of the question, I think your customer suspects what the problem is, and he may be right. Sounds like the small I.D. is inserting itself into the larger I.D. during tumbling. If so, there is no easy answer except that a different style barrel, perhaps with breaker bars, may help.
#2 - The part may be leaded brass, with the pretreatment cycle attacking the zinc and leaving passive lead on the surface? Leaded brass is tricky; you may want to have samples plated by a supplier's lab.
#3 - Is this zinc die-cast part polished? The compound may not come off as easy as other soils.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
1995