why not use ZP bolts?

might be that corrosion in a zinc plated bolt loses the clamp load, while corrosion in carbon steel does not?

Hmm. good point.

SS

Zinc and steel are different enough in potential to set up an electrolytic (galvanic) coupling. Although the coupling isn't strong, it's still a potential CAUSE of corrosion, especially in the presence of an electrolyte like chlorine. You'll quite possibly get MORE corrosion than with carbon steel to carbon steel.

Although it may be obvious enough, zinc plating on a carbon steel bolt does not cause internal corrosion because in the bond there is no room for oxidizing or reducing agents. Only on the outside of the bond is there room for such agents. Once a flake comes off, though, things change dramatically.

Mark 'Sporky' Stapleton Watermark Design, LLC

Nobody mentioned the flange temperature.

ISTR that zinc and/or cadmium causes or catalyzes cracking of carbon steel fasteners at elevated temperatures. You never find plated fasteners on exhaust manifolds or clutches.

-Mike-

SS:

Zinc plated bolts have both advantages and disadvantages relative to carbon steel.

At or near room temperature the zinc plating will be anodic to steel. The result will be galvanic or bimetallic corrosion. The zinc will be sacrificed to protect the adjacent steel. How rapidly corrosion occurs will depend on the exposed area of the cathode, the carbon steel flange. If it is painted and the exposed steel area in contact with the zinc is relatively small, then the zinc plating may well be useful. If the whole flange is bare, then the zinc may disappear too fast to be worth the additional expense.

Between about 60 C and 80C the potentials will reverse. The zinc plating now will be the cathode. If the flange is painted and the exposed steel area in contact with the zinc is relatively small, then the bare spots on the flange will corrode rapidly(so adding zinc will make things worse!).

You can down load a "Basics of Bimetallic Corrosion" document from the National Physical Laboratory web site at

also can download a more detailed "Bimetallic Guide" from another NPL page at At high temperatures such as steam service or exhaust systems the zinc will melt. Molten zinc plus tensile stress equals liquid metal embrittlement (LME) cracking. The liquid wets the grain boundaries and results in very brittle fracture. (This already was mentioned in a previous reply by Mike Halloran).

Pittsburgh Pete

------------------------------------------------------------------------------------ DISCLAIMER

We don't believe what we write, and neither should you. Information furnished to you is for topical (external) use only. This information may not be worth any more than either a groundhog turd, or what you paid for it (nothing). The author may not even have been either sane or sober when he wrote it down. Don't worry, be happy.