Metal finishing Q&As since 1989

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Can EN stretch enough for nuts to be used under sea?

2003

I am interested in knowing the ductility or strain to failure of Xylan and electroless nickel (EN).

These are used to coat threaded carbon steel fasteners: the Xylan for the bolt, and the EN for the nuts. The environment is the subsea at 40 degrees F.

Strains can be high at the radius between the head and the shank and at the roots of the threads. Therefore, the coating should be able to accommodate the peak strain.

Thanks,

Michel Dib
offshore riser engineering - Houston, Texas

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First of two simultaneous responses --

Using coated steel fasteners in a subsea application is an invitation for disaster. Damage to the coatings due to handling, assembly, etc. will undoubtedly promote premature corrosion. Duplex or superaustenitic stainless steels, titanium alloys, and nickel-based alloys are typically specified for marine applications.

Toby Padfield
Automotive module supplier - Michigan
2003

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Second of two simultaneous responses --

Electroless nickel is a barrier coating. This means it protects by encapsulating the part. Anywhere this barrier is compromised it will allow corrosion.It has been my experience that on threads the metal is so coarse that on a microscopic level the nickel will not encapsulate the part because it can not wet and therefore plate all the tiny cracks and the V at the bottom of the threads. This allows corrosion to occur on any wet or corrosive use.

Todd Osmolski
- Charlotte, North Carolina, USA
2003