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Zinc phosphate plating is wrong color

2007

I sent some .2359 dia. x 1-5/8 long dowel pins ( made from 41l42 heat treated to Rc 60/62) to be zinc phosphate barrel plated (.0002/.0004 thick) They came back black instead of grey and now measure .2352 dia. Any ideas what the plater did wrong?

Ken

Kenneth Carlson
Machine shop - Worcester, Massachusetts

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I would not concede that "the plater did wrong" until I knew a little more about it, Ken, but I would guess that the dowel pins may have been stripped and re-done, losing some material in the stripping process. "Zinc phosphate plating" is a little bit sloppy terminology, Ken -- are you sure what you are/were looking for? Did you want this hardware zinc plated or did you want it zinc phosphated? It is unusual to specify an added thickness for a zinc phosphate coating, usually these are specified by weight.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007

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Ted- I originally did 4000 pieces. of these 9/06 supplying Rc 60/62 4140 Alloy dowel pins with a ground OD of .23615/.23610 and requested a Phosphate coat per DOD-P-16232 [ed. note: replaced by MIL-DTL-16232 [from DLA] ] 5 µm/15 µm thick. I received back dowels with a OD of .2375 and I finish ground them to .2363/.2367 and everything is OK. This time I supplied 2000 of the same pins with a ground OD of .2359/.2360 and received back an OD of .2353 and the finish did not appear the same. I brought them back and the floor boss said that something went wrong and he would redo them-I picked them up today and now the OD is .2343 and the coloring still looks wrong. HELP

Kenneth Carlson
- Worcester, Massachusetts
2007

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Ground pins with a fine finish are no fun to phosphate. With that loss, he either grit blasted them in bulk or he left them in the acid too long. When you affect the surface finish, the appearance of the phosphate will change.
Sounds like it is time to find a new phosphate shop. This person either lacks quality control or supervision or knowledge or?

James Watts
- Navarre, Florida
2007

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Thanks all--I do believe that it is time to find a new plater. One last question-Should I experience a material size build-up or reduction with phosphate coating per DOD-P-16232 [ed. note: replaced by MIL-DTL-16232 [from DLA] ]?

Ken

Kenneth Carlson
- Worcester, Massachusetts
2007