53107

Need corrosion-resistant treatment for carbon plugs (bolts) for saltwater environment [Oklahoma] 

September 15, 2009

My company is a manufacturer of box header pressure vessels for use in the oil and gas industry. When building a metalized pressure vessel, we currently use SA105 zinc-plated plugs. We are seeing rusting on the plugs in off-shore environments. I'm looking for an alternative treatment for carbon steel plugs that will provide high corrosion resistance; however, it must withstand the abuse of torque wrenches when the plugs are tightened in the field. (A similar issue was addressed in Letter 51980.)

Stainless: concerns with carbon contamination on the surface would result in poor appearance

Teflon: would the coating flake off with the use of torque wrenches?

Hot Galvanized: I've seen threads that address the concerns of thickness build-up on the threads; what about flaking off when the plugs are tightened?

Cadmium plated: environmental concerns

Zinc-Nickel plate: I'm not familiar with this process; would it hold up against the torque wrenches?

Shannon Faust
Engineering Supervisor - Tulsa, OK, USA

For this application you probably want a plating that offers sacrificial protection, like zinc does, so that a tiny scratch doesn't cause the plug to corrode to protect the remaining plating.

Zinc-nickel (and other zinc alloys platings like zinc-cobalt, zinc-iron, and tin-zinc) function similarly to zinc in this regard, but by having more impervious corrosion products and/or engineering the electrochemical potential to be closer to steel through alloying, they provide superior corrosion resistance. This is the simplest approach because it can solve the problem while introducing no new problems. Electroplated finishes adhere very well to steel, and there should be no issues of anyone being able to chip the plating off even if they tried. Good luck.

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