Al Thomason wrote:
Here is problem that has me puzzled, and looking for any ideas.

Today we hauled our boat, and I noted the prop appears to have some
electrolysis or other corrosion stress to it. The prop is 2 years
old, Bronze on a SS shaft. When hauled, its color is ink black and it
has these small 'corrosion worms' all over it. Each 'worm' appears to
be corrosion, is about 5/8" long and about 1/32" wide. There is one
about every 2-3 square inches.

I do not know the cause of this, and will have to investigate if it is
something on my boat leaking, or something from a neighboring boat.

Now, here is what puzzles me. The shaft zinc does not show any signs
of undue wear. I would think that if I had some electrolysis issue,
the shaft zink would be one of the 1st things to go...



Hello Al,

Offhand, this doesn't sound like
electrolytic corrosion.

That the zinc shows no sign of wear is
not necessarily a good thing, BTW. You
want to make sure the zinc is making
good electrical contact with the shaft.

It may be a form of self-corrosion,
perhaps caused by biofouling. While
copper alloys (bronzes) are generally
good at discouraging fouling, a certain
amount of galvanic corrosion seems to be
required for this process to occur. Was
there a lot of growth on the prop? Had
it been coated with anything? Fresh
water or salt? Prop in the mud and muck?
Is it a high-speed cigarette boat or a
clunky sailboat? For how long has the
prop been idle? Etc.

Sulfide-polluted seawater can produce a
black film on the prop.

Your best strategy is to contact the EPA
immediately and cordon off the marina
..... just kidding. You might contact the
manufacturer of the prop and see if they
can provide any further insights.

Sorry I can't give you a better answer.

Chuck

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