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Westlawn alumni are invited to submit drawings and renderings of designs for publication in
The Masthead
that are “On The Drawing Board.” Designs & commentary should be emailed to
nnudelman@westlawn.edu
On Seacock Corrosion
By Dave Gerr, © 2010 Dave Gerr
(This Issue’s Question, Page 27)
The Know It All questions and correct answers are important design tips for students as well
as other marine professionals. We suggest that you file them away for future reference.
The Question Was:
Hodge Podge
is a strongly built 52-foot fiberglass motorsailer. A well-designed ocean voyager,
Podge
is fully fitted out w
extensive systems, including hydraulic bow thruster, watermaker, A/C, even a small Jacuzzi. During a recent haul-out th
owner noticed that one of the bronze seacocks for engine intake was severely corroded and discolored and in danger of
failing. With easy access inside the engine compartment, changing the seacock out isn’t a big job, but the owner rightly
wants to know if this is likely to happen again. What is the probable cause of
Hodge Podge
’s seacock corrosion proble
The Winners are:
This time, Westlawn student and engineer Jay Jeffries and marine surveyor Kevin Ennis both demonstrated that they ar
probably too smart for their own good as they each submitted the correct answer to the September 2010 question. Nat
rally, Know It All certificates, Westlawn tee shirts and caps are winging their way to each of them. Our winners should
henceforth be addressed with the honorific of “Know It All,” with all the rights and privileges pertaining thereto. The full
answer and explanation to the question is:
And the Solution Is:
A positive wire from the DC system, likely from a bilge pump or similar equipment, is almost certainly making contact
the bronze seacock driving it negative/anodic on the galvanic scale (confusing—appears backwards) and so making the
seacock into an anode. The bronze seacock will thus corrode badly. This wire doesn't even have to make direct contact.
can be sitting in bilgewater, which then makes contact with bronze fitting. The indirect electrical contact through bilgew
ter will be less effective at conducting damaging current than direct contact with the wire, but will still lead to much fas
and more serious problems than natural galvanic corrosion.
This is stray-current corrosion (electrolytic corrosion), which is caused by external electric current artificially making diff
ent metals into an anode and a cathode. Because the current is external, it can reverse the usual polarity (potential or d
rection of flow). You could, in some instances, have a zinc driven to be cathodic to a bronze fitting. Worse, the external c
rent can be much, much larger than natural galvanic corrosion. This is because the potential between naturally occurrin
anodes and cathodes is measured in millivolts (mV). By contrast, the potential difference imposed on two different met
parts (they can even be of the same alloy) might be measured in volts (100 times more on average for 12-volt system t
natural galvanic corrosion). The result is both very serious corrosion and corrosion that can do major damage in days
rather than over weeks, months, or years. None of this stray current corrosion has anything specifically to do with the al
involved (zinc or otherwise).
Stray current (electrolytic corrosion) is not galvanic corrosion. Stray current is not dezincification. Dezincification is plain
standard galvanic corrosion—nothing more. In this case, you have copper and zinc mixed together in a single alloy (bras
In an electrolyte (seawater for boats) the anode (the zinc) is eaten away as it gives off excess electrons to the more nobl
copper. Galvanic corrosions doesn't have to involve zinc. A bronze fitting can cause corrosion in an aluminum or steel h
serious corrosion (see page 16).
The corrosion for this problem wouldn’t have been caused by an AC (alternating-current) leak. The fact is that AC virtuall
never produces corrosion in metals on boats. There is a bit of controversy about this as a few recent experiments have
shown some corrosion from high AC voltage in laboratory conditions. In 99% of situations on boats in the real world this
won't be the case. Indeed, unless the AC current density is very high (say over 80 amps/sq.meter—dangerous! — but an
AC leak is dangerous), nothing much happens with regard to corrosion. Even at these high current densities, corrosion f
AC virtually only occurs in aluminum, not in bronze, brass, steel, or stainless. Could there be rare (very rare) exceptions?
Possibly, but we haven't seen it at Westlawn or ABYC.
See the New December 2010 Question on Pag