Letter 31001

Substituting Zinc Electroplated for Mechanical Plated Screws  

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My question is next: I have 2 screws made from the same material, the only difference is that one of them has a zinc plate finish and the other has a zinc mechanical plate finish. Is there a difference between these both in terms of applications in the industry? If my spec. asks for a zinc mechanical plate and I applied a zinc plate, is going to cause an effect in application of the screw in field?

Thank you

Sarai Sanchez
Assembly shop - Apodaca, Nuevo Leon, Mexico

You probably already know that they will look different because the electroplating will be bright and the mechanical plating will be matte. Usually the mechanical plating will be thicker and therefore longer lasting, but not necessarily. However . . .

It is very possible that mechanical plating was deliberately specified either because of concern over hydrogen embrittlement or the need for the greater corrosion resistance of the thicker coating.

If the customer agrees that this is a non-critical application, it may be okay; after all, you can make a cake with egg-beaters instead of eggs and margarine instead of butter. But it quickly becomes a bad habit to start seeing specs as a nuisance when they are actually clear instructions. You usually don't know the full background, and it is the engineer's job to decide the specs, not the job of the purchasing agent or the plating shop or assembly shop.

If I were you I wouldn't substitute electroplated screws unless the buyer agrees; and if I were the buyer, I wouldn't agree unless the engineer agreed. And if I were the engineer, 75 percent of the time I'd say "fine", or at least "duh?"; but some percentage of the time I'd say no substitution is permissable.

Ted Mooney, P.E.  finishing.com Brick, New Jersey

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