Corrosion Of Metals
Corrosion of Metal
Structures in Water
Metal structures start corroding the minute they
get placed in water. These could range from Galvanised steel pontoon supports
in a Marina to Steel wind turbines at sea.
One thing they do have in common is that some measures will
have been taken to protect them from corrosion. These measures slow down the
Oxidation to a manageable level but cannot entirely prevent the degradation of
In the two examples above the first is protected by Zinc on
the outside of the steel joist. When water contacts the structure Zinc will
readily give up electrons (become oxidised) in preference to steel. This way the steel retains its integrity.
Without the Zinc, the electrons are taken from the Steel which then is corroded (oxidised).
Wind turbines are impressive structures and are coated to
protect them from the elements. However this is not enough as they are in contact
with the sea via steel embedded in the sea bed. To protect the steel, the
structure is connected to a large Aluminium block which serves as a Cathodic
protector. The electrons are preferentially taken from the Aluminium thereby
protecting the Turbine.
There are other means of protection such as Impressed
Current systems for Large ships and pipelines but they all need to be monitored
to ensure the protection is still effective.
Connecting the metal structure above the waterline to a mV
meter and then placing a reference electrode into the water will give a
millivolt reading which is the potential of the structure. This will be a
highly negative number and represents the number of electrons available in the
structure. As the structure corrodes this result becomes less negative. It is
therefore easy to measure the progress of corrosion.
For example a steel Joist unprotected will start with a
reading of -600mv. When it is corroded this will reduce to around -300mV. A Galvanised joist will have a reading of
around -900mV and will reduce much more slowly.
For a few hundred pounds it is easy to measure the potential and keep track of the effectiveness of the protective measures
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