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MIL SPEC Clarification for Zinc Plating with Chromate Conversion

I just finished a project where we had to have 2900 lbs of hex head bolts zinc plated to Finish 1.9.2.3 of MIL SPEC 171, Supplementary Chromate Treatment to be Olive Drab. I had a local plater do the work and provide me with Certs. I am running into some issues though as olive drab color on the bolt heads is inconsistent on the corners of the hex heads and on the stamping on the heads. The bolts were barrel plated and the zinc tested out to .0052" in excess of MIL SPEC. However, at issue is the consistency of the chromate plating. Is there a mil spec that addresses the thickness and consistency of the chromate conversion?

Eric Forrester
manufacturer - Chambersburg, Pennsylvania, USA
April 28, 2009

Unfortunately, the high current density areas of a bolt take the chromate differently than the low current density areas. This would be aggravated even more if the high density areas were close to burning. (a non technical term that is commonly used by platers when an area, normally an edge is plated at too high a current density)

James Watts
- Navarre, Florida
April 30, 2009

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Olive drab chromates w ill look different depending on the current density of the plated zinc. Also the color will vary with the amount of brightener used in the zinc plating solutions. There is no loss of protection due to the color differences.

Don Baudrand
Consultant - Poulsbo, Washington
(Don is co-author of "Plating on Plastics" [on Amazon or AbeBooks affil links]
           and "Plating ABS Plastics" [on Amazon or eBay or AbeBooks affil links])
May 4, 2009

What about the zinc showing through on the bolt head edges and stamping. These are barrel plated, so some "polishing" is expected. But what is acceptable under standard industry practices?

Eric Forrester
- Chambersburg, Pennsylvania, USA
May 8, 2009