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 the structure.

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.

Measuring and Monitoring

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

What you need:

9999 Reference Electrode,       5 Meter Cable,           QP451 Meter

            


CATHODIC DISBONDMENT TESTING

The Cathodic Disbondment test, regardless of the International Standard being followed requires the use of a Laboratory quality Reference Electrode.

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CATHODIC OVER PROTECTION RISKS

What happens when you over protect the cathode?

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HOW TO MEASURE HULL POTENTIAL

A practical guide to check if your boats hull is protected.

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Hull Potential & Corrosion.

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METAL CORROSION IN BOATS

An explanation of why boats rot in water and how to manage this.

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SHOULD I MOOR NEXT TO A RUSTY BOAT?

Find out where you should moor your boat.

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SILVER SPOONS AND HULL POTENTIAL

Why you should not use a Silver Spoon to measure Hull Potential?

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