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Propeller
Corrosion |
Corrosion
Electrolysis
is a form of corrosion
This is not to be confused with the breakdown of metal by mechanical
means such as wear, galling, erosion, or cavitation. Electrolysis
is the alteration, decomposition or breakdown of metals or alloys
from persistent electrochemical reactions (or by direct chemical attack).
Simplistically Corrosion can be broken down to two possible causes:
Galvanic Action or Electrolysis. It is impossible to tell which was
responsible after the fact.
Galvanic Action
This is the most common cause of electrolysis in the marine
environment. Galvanic corrosion is the interaction of two dissimilar
metals connected in an electrolyte (saltwater). In this situation
the least noble becomes the anode and corrodes. The more noble is
the cathode which in extreme cases can actually be coated with the
anode metal. Metals can be connected through a bonding system or directly
such as Bronze prop to Stainless shaft. Thus by adding a shaft zinc
the zinc deteriorates before the prop.
Electrolysis
Corrosion caused by stray current. Generally this is much more aggressive
causing a lot of damage in a relatively short amount of time.
Typically when Nickel-Aluminum-Bronze (Nibral) corrodes the aluminum
leaches out of the alloy leaving irregular shallow pitting in the
blade surface. In advanced cases the blade edges become scalloped.
Manganese Bronze
Turns copper red as the zinc leaches out of the alloy. In advanced
cases the blade edges actually begin to peal apart much like the leaves
of a book. Prior to this the propeller will cease to ring and only
thud when tapped with a hammer.
In copper based alloys once corrosion has set in it becomes almost
impossible to weld the propeller. The metal has changed sufficiently
that attempting to weld the affected areas only results in creating
a larger hole.
Corrosion can be aggravated
by cavitation
Cavitation is the mechanical breakdown of the propeller material through
the implosion of small air bubbles on the surface of the blades as
a result of the water “boiling” from low pressure. If
the structure of the metal is compromised by corrosion than cavitation
erosion occurs much more readily.
The chromium, nickel, and molybdenum content of stainless steel are
major contributors to it’s corrosion resistance. Stainless steel
is interesting because it is one of the metals that, under certain
conditions becomes more noble. The oxygen in the atmosphere and in
moving water is sufficient to allow stainless steel to form or repair
a tough transparent film of chromium-oxide that renders the metal
non-corrodible. When this film is damaged under conditions where there
is insufficient oxygen to repair it stainless becomes active and corrodes
freely. Typically this corrosion is localized but can be very aggressive.
This type of corrosion is often referred to as crevice corrosion.
Reference
Information |
| Marine Metals Manual by Roger Pretzer, International
Marine Publishing Company. The Boatowner’s Guide to
Corrosion by Everett Collier, American Boat & Yacht Council
(ABYC) |
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Resources
On the Web |
www.mcallyinstitute.com
An excellent resource with enormous amount of info with an
alphabetical listing of free Technical
Papers. |
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Contact WildCat Props!
Custom propeller configurations are also available on request. Should you
require assistance in selection of a new propeller suited to your vessel,
restoration-repair of your existing propellers or prop tuning-maintenance
please contact WildCat Props. |
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Read How WildCat Props Exceeds the Expectations
of Our Customers!
We want to again thank you for your extremely knowledgeable and professional
response and work on our Viking 45 propellers.
As you know, Pam owned a propeller shop with her Father several years ago
and is very knowledgeable. After talking to you, it became apparent we should
send the propellers to you for rework and to put your modification on to
minimize cavitation. After receiving the propellers back in a short period
of time they were installed. We immediately found reduced vibration and
were pleasantly surprised at an increase in performance of 2 mph at cruise.
Thanks again for your quick and professional response. We would not hesitate
to recommend your shop to anyone looking for quick and quality work.
Dave Hopton
Irvington, VA
Read more customer testimonials here |
Also read customer propeller case studies |
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