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Subject: Re: [worldcruising] Sea Cocks
From: Eric Lamoray (elcruisr@XXX.XXX)
Date: Thu Jan 11 2001 - 21:53:13 EST
Jim Manzari wrote:
Stainless steel sea-cocks are common on all types of steel boats built in
Holland and Germany. For instance, our steel boat uses stainless steel for
> sea-cocks and some parts of the rudder fittings. As I understand it, there
> are two basic types of stainless steel, active and passive. Passive
> stainless (304 passive and 316 passive) is very close on the galvanic
> scale to Monel and can be used anywhere Monel can be used. Naturally, both
> Monel and passive stainless need to be protected by zinc(s) of the correct
> size and placement.
>
> I've been told, but have not yet experienced this, the failure of sea-cocks
> on many steel boats usually occurs (if at all) when the mild steel
> stand-pipe corrodes -- not at the stainless or Monel seacock itself.
> Modern coating systems have pretty much made this a non-issue, provided
> routine annual maintenance is carried out.
>
I'll stand by my reccomendation after repairing boats for a living over the
last 15 years. I also currently make my living as a marine surveyor. I will
agree that a Monel valve is a wonderful choice if you have pockets deep enough
to afford one. However Monel and any common stainless marine alloy act very
differently in a corrosive environment like seawater. I have personally had
numerous stainless fittings crumble in my hands while trying to service them
that had seen submerged service. The types of corrosion they are subject to are
many more that just galvanic. There's stress, pitting, oxygen starvation,
chemical, crevice and more. One of the great faults of stainless used anywhere
is that it gives VERY LITTLE if ANY warning of imminent failure unlike bronze
or monel. (And remember monel is not a stainless steel) Which is why surveyors
like to use advanced testing systems for cracking on stainless hardware. You'd
be suprised the amount of standing rigging we fail that passes visual
inspection. The corrosion problems worsen as the water temp. increases making
the troubles worse in tropical waters. Yes, many boat manufacturers use alot of
stainless hardware underwater but we in the service industry sure have to
replace alot of it every year! I guess it's good job security. One more
comment, a friend and client of mine is an oil field metalurgist and all the
fittings on his boat are converyed to bronze!
Eric
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