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Trivalent coating and surface finish

(-----) 2006

I am currently working on a project that requires us to plate a metal plate made up of 1010 steel using a trivalent process. The surface finish of the part prior to plating has an Ra of .2-.3 micrometers. After plating, the surface finish almost doubles in value exceeding our upper spec limit. The process has been walked through and we know of these steps that occur at the plater:
1)Hang parts
2)Detergent bath wash
3)2 Water bath rinses
4)Electric charged bath to remove surface impurities
5)Hydrochloric acid bath - 15% - 20% HCl solution
6)Electroplate Zn
7)Electroplate Tri-chrome
8)Silicate Sealant bath

After plating, the parts appear very streaky. Is this typical of this type of plating? I am relatively new to this process so any input would be appreciated.

Tom Roth
Mfg. Eng - Deerfield, Wisconsin


What does your plater say? Ask him to run a Hull cell test for you. This will tell if there is something wrong in his process. If the test coupon comes out right, he's fine. Since you are new it might be good for you to read about the subject in this site or better to get a copy of the Metal Finishing Directory and Guidebook.

Guillermo Marrufo
Monterrey, NL, Mexico


Why are you specifying plating zinc then chromium? Are you sure what you are getting for is not zinc plate and trivalent chromate (a dip)? It would seem that you are getting zinc and trivalent chromate based on the fact that you are also getting the parts sealed. There is no reason for the parts to t be hazy or murky. A quality plater should be able to produce parts that are either clear (silver), clear (blue) or yellow without haze or streaks. Talk to your plater or find another one.

Gene Packman
process supplier - Great Neck, New York

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