Rust Never Sleeps
Story and photos by Patricia Kearns
Keeping a sharp eye out for corrosion and taking action
early are a critical part of preventive maintenance
e may not be able to stop
corrosion, but if we can identify what kind it is, we can
probably control it or at least
mitigate its effects. Below are
some of the most common types of corrosion on boats:
Atmospheric corrosion is what happens to virtually every metal exposed to the
air in my homeport in southwest Florida.
The air is laden with moisture and salt,
and it’s a contest of wills to keep rust from
Immersion corrosion: Put metal in
water, especially saltwater, and it will corrode. This is a no-brainer. Regional variations influence this process.
Galvanic corrosion occurs when you
marry dissimilar metals and they get wet.
The more noble metal will try to protect
itself by “stealing” mass from the other. The
transfer of mass occurs in the electrolyte
(water). My description is an oversimplification but the concept shouldn’t be
foreign. This is not “electrolysis.”
Stray-current corrosion happens
when electrical current is added to the
formula for galvanic corrosion. This is not
Pitting corrosion can grow a tiny pinhole into a big perforation and its potential
is often overlooked on stainless steel. This
is insidious stuff that most affects metal
surfaces that are hidden from view – for
example, the area of a stainless-steel shaft
that is totally isolated in a cutlass bearing.
Poultice corrosion, like pitting corrosion, is hard to see easily because the
corrosion takes place where the metal bears
on a wet, porous material. An aluminum
fuel tank supported directly by plywood
that has become wet might suffer poultice
Erosion corrosion is just that. The
constant velocity of moving water gradually
wears away the metal.
Cavitation corrosion damage can
occur on any metal, anywhere in or on the
boat that is exposed to the constant shock
and explosive shock waves of air or vapor
bubbles in water.
Stress corrosion cracking is commonly seen in metal fittings that have been
formed (bent or worked) – for example,
swaged fittings in standing rigging, angle
brackets, and such. The hairline cracks
look like a tree without leaves, with cracks
branching out from the main crack. One
minute, these fittings are giving good service, and the next, they fail without warning and the mast comes down. You can find
cracks if you look closely with a magnifier.
Corrosion fatigue is similar to stress
corrosion and results from a combination
of corrosive action and a recurring stress
that causes a fracture to develop.
Crevice corrosion is opportunistic.
Any flaw in a metal can be subject to this
form of corrosion when it’s contaminated
by stagnant water, specks of dirt, or other
Notice that none of these terms are
tagged “electrolysis.” Electrolysis is the
breakdown of a chemical compound, like
metal, in solution by passing an electric
current through the solution. The process
requires a source of direct current (DC),
two electrodes, such as metals, and an
The physical evidence of corrosion
activity is a symptom, and seeing the color
and textures of the process at work is a call
to action. If you see rust, you know something is corroding. To keep corrosion from
ruining your day, you need to know what
causes it and what it looks like. If you spot
corrosion and can’t identify the type or
cause, it’s time to call in an expert.
Patricia Kearns worked for more than two
decades as a marine surveyor and technical editor. This article first appeared in Mad
Mariner’s DIY Boat Owner Magazine.
From DIY Boat Owner Magazine
1 Custom-made sailboat chain-plate
assembly is being attacked by water leaking through the deck at the chain plates.
Stainless-steel chain plates are subject to
crevice corrosion, hidden from view without disassembly, when water soaks the deck
core where it bears against the chain plate.
2 Both the threaded and hose-barb sections of the pipe elbow have wasted from
erosion corrosion. Only two threads were
engaged at the thru-hull fitting and the leak
was a mystery to the boat’s owner until
a technician disassembled the fitting and
removed the hose.
3 Engine mounting hardware compromised by corrosion from leaks from engine