Editing Notes
– Replace picture ? Add Colchek diagram?
– What is the depth of corrosion monitoring?
1. Below ground corrosion of planted (rooted) lighting columns
This page discusses the monitoring of below ground corrosion of planted lighting columns.
Planted (or “rooted”) lighted columns are fixed by inserting the bottom of the column into a concrete foundation or directly into the soil.
Fig 1 Planted (Rooted) lighting column
This column is fully galvanised with additional coating protection for corrosion protection just above and below ground level.
The additional corrosion protection must extend high enough to minimise the effects of road salting and dog urine.
Fig 1.2 Planted (rooted) lighting columns prior to installation
(Courtesy mallatite.co.uk)
This photograph includes both painted and galvanised lighting columns.
2. Corrosion
Corrosion at or below ground level is often the greatest risk to planted (rooted) lighting columns.
Loss of material due to corrosion will progressively weaken the column until collapse occurs.
Corrosion can be the most severe just above and or below ground level. i.e. In areas that are subject to the longest exposure both moisture and oxygen. Corrosion is accelerated by the presence of acids (decay of organic matter and dog urine) and de-icing salt.
Below ground corrosion is of most concern because it is often not visible. The first indication of a problem can be when the lighting column collapses.
Fig 2 Severe below ground corrosion of a planted (rooted) lighting column.
Most instances of external below ground corrosion will not be visible from the inside of the lighting column.
This photograph is one of the few examples when below ground corrosion is visible by inspection. i.e. External corrosion has breached the wall of the lighting column.
Water and soil has washed into the lighting column. ***
The loss of soil from the outside of the column
a) Reduces structural support for the column.
b) Allows free access of oxygen to the outside surfaces. The rate of both consumption of the zinc coating and corrosion will both increase.
*** Comparison of this photograph with Fig 1.2 above indicates that the quantity of washed-in soil is very large. The loss of structural support for the lighting column could be very significant.
Fig 1.2 above shows that the enhanced protective coating applied to the inside of the column does not extend as far as it does on the outside.
3. Inspection and monitoring of below ground corrosion
Caspian NDT use “Eddy Current Testing” to assess loss of material due to corrosion.
Eddy current testing is undertaken by inducing an electric field in the lighting column using an external drive coil.
A detector coil is then inserted into the column to measure the strength of the electric field.
The strength of the electric field is measured at various locations. Material loss below ground is assessed by comparing the electric field from known wall thickness above ground with that below ground.
Knowing the material loss allows the remaining safe working life of the lighting column to be assessed.
Fig 3 Eddy Current testing of a column using a COLCHEK 2
A magnetic field is induced using the external coil placed around the column at ground level.
A detector is then inserted into the column to measure the magnetic field at various depths. Measurements of the magnetic are then used to calculate the wall thickness of the lighting column.
The detector is fitted to the bottom of the black plastic tube shown being inserted into the lighting column.
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