Metal parts on boats are subject to two types of corrosion. Galvanic corrosion is a chemical reaction which
Any water which is not perfectly pure can act as a conductor. The more mineral content in the water, the more conductive it is. Conductivity also rises with water temperature, which is why metal parts on boats in Florida corrode faster than those in Alaska.
Anodes are made from a metal alloy with a more "active" voltage (more negative reduction potential / more positive electrochemical potential)
than the metal they are attached to. The difference in potential between
the two metals means that the galvanic anode corrodes, so that the anode
material is consumed in preference to the structure.
The loss (or sacrifice) of the anode material gives rise to the alternative name of
sacrificial anode.(Read More)
Anode Materials
There are three main metals used as galvanic anodes, magnesium,
aluminium and zinc. They are all available as blocks, rods, plates or
extruded ribbon. Each material has advantages and disadvantages.
Magnesium has the most negative electropotential of the three (see galvanic series)
and is more suitable for areas where the electrolyte (soil or water)
resistivity is higher. This is usually on-shore pipelines and other
buried structures, although it is also used on boats in fresh water and
in water heaters. In some cases, the negative potential of magnesium can
be a disadvantage: if the potential of the protected metal becomes too
negative, hydrogen ions may be evolved on the cathode surface leading to
hydrogen embrittlement or to disbonding of the coating. Where this is a possibility, zinc anodes may be used.
Zinc and aluminum are generally used in salt water, where the
resistivity is generally lower. Typical uses are for the hulls of ships
and boats, offshore pipelines and production platforms, in
salt-water-cooled marine engines, on small boat propellers and rudders,
and for the internal surface of storage tanks.
Zinc is considered a reliable material, but is not suitable for use
at higher temperatures, as it tends to passivate (becomes less
negative); if this happens, current may cease to flow and the anode
stops working.
Zinc has a relatively low driving voltage, which means in
higher-resistivity soils or water it may not be able to provide
sufficient current. However, in some circumstances — where there is a
risk of hydrogen embrittlement, for example — this lower voltage is advantageous, as over-protection is avoided.
Aluminum anodes have several advantages, such as a lighter weight,
and much higher capacity than zinc. However, their electrochemical
behavior is not considered as reliable as zinc, and greater care must be
taken in how they are used. Aluminum anodes will passivate where
chloride concentration is below 1,446 parts per million.
One disadvantage of aluminium is that if it strikes a rusty surface, a large thermite
spark may be generated, therefore its use is restricted in tanks where
there may be explosive atmospheres and there is a risk of the anode
falling.
Since the operation of a galvanic anode relies on the difference in
electropotential between the anode and the cathode, practically any
metal can be used to protect some other, providing there is a sufficient
difference in potential. For example, iron anodes can be used to
protect copper.
Design considerations
The design of a galvanic anode CP system should consider many
factors, including the type of structure, the resistivity of the
electrolyte (soil or water) it will operate in, the type of coating and
the service life.
The primary calculation is how much anode material will be required
to protect the structure for the required time. Too little material may
provide protection for a while, but need to be replaced regularly. Too
much material would provide protection at an unnecessary cost. The mass
in kg is given by equation
Mass =(Current Required x Design Life x 8760) ÷ (Utilisation Factor x Anode Capacity)
The design life is in years (1 year = 8760 hours).
The utilisation factor (UF) of the anode is a constant value,
depending on the shape of the anode and how it is attached, which
signifies how much of the anode can be consumed before it ceases to be
effective. A value of 0.8 indicates that 80% of the anode can be
consumed, before it should be replaced. A long slender stand off anode
(installed on legs to keep the anode away from the structure) has a UF
value of 0.9, whereas the UF of a short, flush mounted anode is 0.8.
Anode capacity is an indication of how much material is consumed as
current flows over time. The value for zinc in seawater is 780 Ah/kg but
aluminium is 2000 Ah/kg,
which means that, in theory, aluminium can produce much more current
than zinc before being depleted and this is one of the factors to
consider when choosing a particular material.
The amount of current required corresponds directly to the surface
area of the metal exposed to the soil or water, so the application of a
coating drastically reduces the mass of anode material required. The
better the coating, the less anode material is needed.
Once the mass of material is known, the particular type of anode is
chosen. Differently shaped anodes will have a different resistance to
earth, which governs how much current can be produced, so the resistance
of the anode is calculated to ensure that sufficient current will be
available. If the resistance of the anode is too high, either a
differently shaped or sized anode is chosen, or a greater quantity of
anodes must be used.
The arrangement of the anodes is then planned so as to provide an
even distribution of current over the whole structure. For example, if a
particular design shows that a pipeline 10 kilometers (6.2 mi) long
needs 10 anodes, then approximately one anode per kilometer would be
more effective than putting all 10 anodes at one end or in the center.
Advantages and disadvantages of galvanic cathode protection
Advantages
No external power sources required.
Relatively easy to install.
Lower voltages and current mean that risk of causing stray current interference on other structures is low.
Require less frequent monitoring than impressed current CP systems.
Relatively low risk of over-protection.
Once installed, testing the system components is relatively simple for trained personnel.
Disadvantages
Current capacity limited by anode mass and self consumption at low current density.
Lower driving voltage means the anodes may not work in high-resistivity environments.
Often require that structure be electrically isolated from other structures and ground.
Anodes are heavy and will increase water resistance on moving structures or pipe interiors.
Where D.C. power is available, electrical energy can be obtained more cheaply than by galvanic anodes.
Where large arrays are used wiring is needed due to high current flow and need to keep resistance losses low.
Anodes must be carefully placed to avoid in interfering with water flow into the propeller.
Cost effectiveness
As the anode materials used are generally more costly than iron,
using this method to protect ferrous metal structures may not appear to
be particularly cost effective. However, consideration should also be
given to the costs incurred by removing a ship from the water, for
example, to repair a corroded hull or to replace a steel pipeline or
tank because their structural integrity has been compromised by
corrosion.
However there is a limit to the cost effectiveness of a galvanic
system. On larger structures, or long pipelines, so many anodes may be
needed that it would be more cost-effective to install impressed current cathodic protection.
