Marelon valve - crevice corrosion?

[Namoian]

Crevice corrosion and plastic seacocks don't go together, but perhaps someone else has had this experience?

I am posting this in the hope that it will not reanimate any discussion of Marelon vs bronze as a seacock material. The Marelon valves on the boat have worked well except for an occasional pulse of water at the handle when operated.

While trying to fix a slight drip from one of these valves by gently tightening the four SS through bolts that hold the valve together, one of the bolts broke off cleanly at the head. Since it broke before I had a chance to apply any torque to it at all, I suspect crevice corrosion at work. The bolt is marked F593C which is 304 stainless steel.

The eight Marelon OEM 93 series valves are 12 years old. Forespar advised that the valve needed to be replaced and shipped me a replacement which I will install shortly. I am going to check the bolts on all other valves as well.

I am wondering if anyone else had this experience or whether it is particular to my application?

[billknny]

Crevice corrosion after the fact is easy to diagnose: Look closely at the broken surface, with a magnifier if need be. If it is a simple failure under torsion load, the break will be shiny, with a "crystalline" texture all the way across. If it has a rusty colored surface across most or all of the surface with a grainy texture, that's crevice corrosion.

It would be a bit unusual in this application, usually a load is required to propagate the crack, but threads certainly make a great starting point. Certainly 304 is susceptible in a chloride rich environment.

[Namoian]

Thanks billknny. Your information further confirms a corrosion issue - the break has a dull and grainy texture.

I am buying some long 316-grade stainless steel bolts in case I find any more Marelon valves/seacocks with this problem.