Electrolysis and galvanic corrosion are frequently
confused. Galvanic corrosion is caused by two dissimilar metals
being in contact with each other, in the presence of an electrolyte,
such as seawater. Electrolysis is caused when an external current,
called a stray current, finds a path between two metals in the
presence of an electrolyte. The two metals may be of exactly the
same type or different types.
The destruction of metals aboard any yacht is a
never-ending process and requires constant vigilance. With a basic
understanding of the many and varied causes, much can be done to
arrest this on-going process.
All metals are subject to oxidisation. Oxidation
is the natural process of the metal returning to its base elements
by combining with oxygen from the atmosphere or the local environment,
for example, oxygen from the surrounding seawater. Some metals
use the oxidation process to protect themselves, while the process
destroys other metals. Rusting is the common name for the metallic
flaking of steel and iron, however it is still oxidation.
This new bronze
skin-fitting (above) is beginning to oxidise. Note the
characteristic green colour. The skin-fitting should have
been mounted on a block to provide additional strength
to the local area of the hull.
||A large rust patch
was started by water getting under the protective paint work
at the edge of the hole and has resulted in substantial damage.
The effects of oxidation on metals such as mild
steel are immediately apparent and are very destructive to the
metal component. Metals such as stainless steel, titanium and anodised
aluminium are protected from further destruction by the oxidation
process as they form a thin oxygen-impervious layer over the surface.
As long as a supply of oxygen is available from the atmosphere
or the surrounding environment, for example seawater, the thin
oxidation layer is self-healing if subjected to scratching or abrasion.
The rate of oxidation of steel is approximately
proportional to the amount of oxygen available. So if the steel
component is in an oxygen-starved environment, the rate of deterioration
will be dramatically reduced. Layers of surface rust hinder the
development of further rusting by inhibiting access to the oxygen
supply. However, constant abrasion will remove these layers and thus accelerate
the oxidation process. Metals that are destroyed by the oxidation process should
be protected by a suitable two-pot epoxy painting system or galvanised, depending
on the application of the metal component. I would always recommend consultation
with a reputable paint manufacturer before purchasing and applying any painting
system. I have always found the technical staff at reputable paint manufacturers
most willing to provide assistance.
Pitting frequently occurs in metals that use an
oxide layer to protect themselves. Metals such as stainless steel
tend to pit in seawater if the mechanism that maintains the film
breaks down for any reason. The hindering of the self-repairing
film is usually the result of changes in the environment over the
surface of the metal. Some examples include: differences in
temperature, variations in the oxygen supply over the surface,
or an uneven flow of water over the surface of the metal component.
Pitting frequently starts in a crevice. Some examples
of man-made crevices include threads, two plates overlapping each
other, or wherever dirt can be trapped. This is sometimes called
crevice corrosion. The dirt then holds moisture, which forms a
galvanic cell (see Galvanic Corrosion). The 300 and 400 series
of stainless steels are particularly susceptible to pitting, for
example 316 and 304 stainless steel.
|Left: The corrosion of this steel keel
is so severe that it has
penetrated the metal at the
top, near the leading edge.
Note the water pouring out
of the keel on the starboard
side, due to another hole.
The local area that sustains the damage becomes
the anode and the large surrounding area becomes the cathode. Once
started, the cell tends to feed on itself. Thus the pit grows ever
Pitting can be minimized by using metals that are
less susceptible to this form of attack and also by using fabrication
techniques that minimize the opportunity for the development of
galvanic cells. Table 1 indicates the typical susceptibility of
metals to pitting in the marine environment.
The correct installation and maintenance of anodes
(zinc alloy blocks) can dramatically reduce the risk of pitting.
Another practice that reduces pitting is to use zinc grease on
all threads. The grease prevents the penetration of water, thus
eliminating the presence of an electrolyte. The zinc also acts
as an anode.
Stainless steel is less susceptible to pitting as
the electrolyte becomes more alkaline. This is what happens when
cathodic protection is operating properly. An electrolyte also
becomes more alkaline as the salt content in the water decreases.
of noble metals is dramatically reduced when the water velocity
increases above two metres per second (7.2 kilometres per hour).
This is because of diminished marine fouling and also because
the surrounding water is highly likely to be aerated, providing
additional oxygen to the surface of the metal.
Cavitation occurs when the local water pressure
falls to near zero. This causes cavities or bubbles in the water.
If the bubbles collapse on the surface of a metal component, such
as a propeller blade, the implosion is so violent that the
surface is mechanically attached and some of the metal is plucked
from the surface, resulting in a pit on the surface.
Some metals have a greater resistance to attack
from cavitation than others. The more noble metals are much more
resistant to cavitation attack than metals such as mild steel (see
Table 2). As a general guide, cavitation resistance is increased
with the hardness of the metal.
Left: The small red patch at
the tip of this propeller is de-zincification. The large
pitted area close to the blade tip is caused by cavitation.
Below: This zinc block has been seriously eaten away. It
should be replaced immediately
Cavitation can be a very serious problem with propellers,
as they are frequently made to work in less than optimal conditions.
In addition to the characteristic pitting which results from cavitation,
other side-effects include additional noise, loss of speed and
in extreme cases, intermittent engine racing.
|This skin-fitting is suffering from well-advanced
de-zincification and should be replaced immediately. The
owner did not replace it when advised and it nearly caused
the sinking of the vessel.
Propeller cavitation usually starts around the blade
tips or near the root of the blade.
Cavitation issues should be addressed by a naval architect or cavitation specialist
as this is a complex subject.
Metals in fast flowing water:
While the more noble metals are protected in fast
flowing water environments (greater than two metres per second
or 7.2km/h) the copper-based alloys start to lose their protective
coatings and corrode rapidly. The rate of corrosion of metals such
as aluminium, lead, steel and zinc also increase in faster flowing
water. Fast flowing water can be found in onboard systems such as, pump housings,
exhaust systems, engine cooling jackets and piping, to name just a few. Ship
board piping systems typically have water velocities from 1.2 to
3.6 metres per second. Pump housings typically have water flow
velocities from 9.1 to 24.4 metres per second.
Copper piping should be avoided when the water velocity
exceeds approximately 1.2 metres per second. Piping made of a more
resilient metal should be selected or a larger diameter should
be used to reduce water velocity and subsequent damage.
Galvanic corrosion is caused by two dissimilar metals
that are in contact and in the presence of an electrolyte.
||Diagram 1: Galvanic corrosion
is caused by two dissimilar metals that are in contact and
in the presence of an electrolyte.
1 illustrates an example. This generates an electrical potential,
that is, an electrical current. The less noble metal, called
the anode, is eaten away. The other metal, the more noble metal,
is called the cathode and is protected. Thus the cathode is protected
by the anode. In the marine environment, the electrolyte is usually
seawater, although rainwater can also act as an electrolyte.
Very pure water will not operate as an electrolyte as it cannot
conduct electricity. Seawater has approximately 2.5 per cent
salt content, which facilitates the conductance of an electric
current through the water. Water will generally act as an electrolyte
as it contains some form of impurity or pollutants.
Table 3 details the typical galvanic series for
metals used in the marine environment. The metals at the top of
the table are said to be more noble while the metals at the bottom
of the table are less noble.
As a general rule, the further apart the two metals
are in the galvanic series the more they will react with each other
and the faster anode will be destroyed. However, this is not a
hard and fast rule as there are other issues to be considered.
These issues can become convoluted and are outside the bounds of
Aluminium, zinc and magnesium are at the bottom
of the table. This is why they are used in an alloy to make sacrificial
anodes and thus protect all other metals. Because aluminium is
very close to the bottom of the table, it is imperative that all
aluminium vessels be carefully protected with anodes at all times.
Aluminium hulls can be subject to vigorous attack from galvanic
The risk of galvanic corrosion can be reduced by
minimizing the number of different metals used. Galvanic attack
can also be dramatically reduced by ensuring sufficient sacrificial
anodes are correctly installed and maintained. I would always recommend
replacing all of the anodes at every annual haul-out, unless they are in near
new condition. The efficiency of the anode is significantly reduced as the
zinc alloy block is eaten away. Annual replacement of the anode
is very cheap insurance against galvanic corrosion and a sound
preventative maintenance procedure.
Anti-fouling paints containing copper-based biocides
can cause extensive galvanic corrosion to aluminium hulls, outboard
legs and any other aluminium alloy components, such as below hull
instrumentation sensors. Special barrier paints can be applied
to insulate the aluminium component from the copper-based anti-fouling
paint. There are also
anti-fouling paints specially developed for aluminium hulls. I would always
strongly recommend contacting a reputable paint manufacturer
Electrolysis and galvanic corrosion are frequently
confused. Electrolysis is caused when an external current, called
a stray current finds a path between two metals in the presence
of an electrolyte. The two metals may be of exactly the same type
or different types.
|Damage or corrosion to propeller blades can sometimes be
repaired, but it requires the skill of an expert. Do not
try to do this yourself
If two dissimilar metals are subject to a stray
current, the more noble metal may not necessarily be protected,
as with galvanic corrosion. If the stray current is sufficient
to overcome the natural galvanic current then the more noble metal
may well become the anode and is thus destroyed. The rate of electrolysis attack
is dependent on the amount of current present. A stray current caused by a
short circuit can ‘eat’ metal components away in a
very short period of time.
The risk of electrolysis attack can be minimized
by always ensuring all electrical systems, including wiring, switches
and all electrical equipment are installed correctly and are maintained
in good working order. Another good preventative maintenance practice
is to always isolate all batteries before leaving the vessel. Isolating
the batteries can also assist in optimizing the life span of the
batteries. Batteries should be kept clean and dry and this includes
the battery posts, casing and the battery box as dirt can act
as a pathway to facilitate a
current leakage, which
may result in a stray