**Classic Galvanic Corrosion**

![](media/image1.jpeg){width="4.111111111111111in"
height="6.666666666666667in"}

.

**What is it?** The photo shows the remains of an aluminum hose barb and
the bronze sea strainer on which it was installed, in this case in line
with the raw water intake hose on NOAA / NRT6s SeaArk Survey Launch
S3003, during fabrication.

**What\'s going on?** This is what happens when dissimilar metals are
placed in contact with each other, in the presence of an electrolyte, in
this case sea water. Aluminum is near the bottom of the galvanic (noble)
scale (or near the top, depending on which scale you are using) and
therefore becomes the anode, while bronze is closer to the top (or
bottom) of the galvanic (noble) scale and becomes the cathode. This is
essentially like a battery. In simple terms, the less noble aluminum
gives up its electrons to the more noble bronze. In the process the
aluminum breaks down (corrodes) to aluminum oxide plus free electrons
flowing through the electrolyte (sea water) to the bronze. Another good
example of this is when we install sacrificial anodes or \"zincs\" to
prevent an aluminum hull (or other metal parts) from corroding. The
zincs (less noble) \"sacrifice\" themselves by giving up their electrons
to the aluminum (more noble).

**To quote Wikepedia: \"**Galvanic corrosion is an
[electrochemical](http://en.wikipedia.org/wiki/Electrochemical) process
in which one [metal](http://en.wikipedia.org/wiki/Metal)
[corrodes](http://en.wikipedia.org/wiki/Corrode) preferentially when in
electrical contact with a different type of metal and both metals are
immersed in an [electrolyte](http://en.wikipedia.org/wiki/Electrolyte).
Conversely, a galvanic reaction is exploited in [primary
batteries](http://en.wikipedia.org/wiki/Primary_batteries) to generate a
voltage. A common example is the
[carbon-zinc](http://en.wikipedia.org/wiki/Carbon-zinc) cell where the
zinc corrodes preferentially to produce a current. The [lemon
battery](http://en.wikipedia.org/wiki/Lemon_battery) is another simple
example of how dissimilar metals react to produce an electric current.
When two or more different sorts of metal come into contact in the
presence of an electrolyte a [galvanic
couple](http://en.wikipedia.org/wiki/Galvanic_cell) is set up as
different metals have different [electrode
potentials](http://en.wikipedia.org/wiki/Electrode_potential). The
electrolyte provides a means for [ion](http://en.wikipedia.org/wiki/Ion)
[migration](http://en.wikipedia.org/wiki/Electromigration) whereby
metallic ions can move from the
[anode](http://en.wikipedia.org/wiki/Anode) to the
[cathode](http://en.wikipedia.org/wiki/Cathode). This leads to the
anodic metal corroding more quickly than it otherwise would; the
corrosion of the cathodic metal is retarded even to the point of
stopping. The presence of electrolyte and a conducting path between the
metals may cause corrosion where otherwise neither metal alone would
have corroded.**\"**

**How come?** The aluminum hose barb should never have been installed
into the bronze sea strainer. The hose barb should have been made of
bronze like the sea strainer or at least stainless steel which is closer
to bronze on the galvanic (noble) scale. Because aluminum and stainless
steel are quite similar in appearance, most likely this was intended to
be a stainless steel barb, but somehow an aluminum barb was installed by
an inexperienced or \"in a hurry\" technician, not realizing the
difference. This could easily happen because in building aluminum boats
one would usually use aluminum barbs on aluminum fixtures, except in the
case where it is used in a dissimilar metal fixture like our bronze sea
strainer or stainless steel valve, for instance.

**So what?** In our case this went unnoticed for about two to three
years after taking delivery of the vessel. The first clue was when we
spotted a green (bronze oxide) drip in the bottom of the bilge under the
sea strainer. (This would have gone undetected, had there been water in
the bilge). I remember pointing it out to my co-worker as something to
keep notice of, as a possible sign of future trouble. I thought at the
time that the fitting was perhaps just leaking slightly and intended to
\"get back to it later\". Wrong! Unfortunately we got busy and forgot
about it as we moved on to other things. Just by luck, a few weeks
later, we had the boat on the trailer for maintenance on the generator
when I accidentally bumped the hose attached to this barb and it
literally fell apart. It had completely corroded through due to galvanic
corrosion. Had this not happened while on the trailer, it would most
likely would have happened while under way.

**Typical scenario:** You\'re cruising along in somewhat rough seas. The
pounding action of the boat would have caused the hose barb to fall off,
causing the generator compartment to flood rapidly. The generator would
soon shut down due to over-heating, due to lack of cooling (raw) water.
Would the bilge pump be capable of removing the water at the same rate
as the inflow? Is your bilge pump working properly?

Of course this could also happen with the vessel tied to the dock. The
bilge pump, if working properly, would attempt to pump out the inrush of
water until the battery dies, then the boat would sink, at least
partially, at the dock. If the bulkhead through-valves are open, the
other compartments would also flood and sink the boat, possibly
completely.

**How to prevent this:** With the boat on its trailer or at least with
appropriate valves shut off to prevent flooding, inspect any suspect
fittings, like the hose barbs that are attached to any bronze or
stainless steel fittings, such as sea strainers, valves etc. Look for
leaks and/or signs of corrosion between the parts. Grasp the hose
attached to the barb and give it a good shake or tap with a hammer or
both. If the corrosion is severe the barb might break, like the one in
the photo.

If this does not show results, as in a newer vessel, verify the
composition (type of metal) of any suspect fittings. If the fitting is
bronze it will be a dull golden/brown color. Aluminum is usually duller
than stainless steel and has a more \"whitish\" appearance, while
stainless steel has a more silvery color and shine. Aluminum also has
less of a \"ring\" to it when tapped with a metal object such as a
wrench, but this may not work if attached to other fixtures. Aluminum is
much softer than stainless steel and will scratch much more readily with
a knife or other sharp object. Try to find other fittings for
comparison. If you\'re still not sure, have it checked next time the
vessel goes into the shop or ask someone with more experience. If you
are about to have a vessel inspection done, that might be a good option.
Ultimately, the best option is for the vessel operator to know his/her
vessel.

Any fittings of dissimilar metals that are attached to each other should
be suspect especially in critical applications, as when in contact with
seawater. Have these fittings replaced as soon as possible.

Ed Wernicke / NRT6

<edmund.wernicke@noaa.gov>

For further reference:

[http://www.ocean.udel.edu/seagrant/publications/corrosion.html]{.underline}