The Behaviour of Stray Currents with Track-Side Steel Mesh Geotechnical Solutions
Gabions, rockfall netting and other steel mesh systems have long been used adjacent to railways and metro systems.
In order to provide greater understanding about the performance of these metallic systems adjacent to railway electrical power delivery systems, a recent study was carried out by TNO Delft(1) focusing on the possible interference of Maccaferri wire mesh gabions and Terramesh products as well as rockfall protection systems, with stray currents.
The aim was to verify whether the design life of these solutions might be affected by possible corrosion caused by stray currents.
The report refers only to direct current (DC) systems since stray currents are considered more dangerous for DC than foralternating current (AC) traction power systems: the corrosion acceleration effect is associated with the charge flow direction, therefore if the flow is alternated (as in AC) the effect on steel is mitigated.
The main characteristics considered were: proximity of steel mesh products to the soil surface; the use of galvanised and / or polymeric coated steel material; effect of different soil types.
The traction power station (TPS) delivers the current into the overhead wires, from where it passes into the electric motor of the train before returning to the TPS through the rails. A small part of the current leaves the rail and flows into the soil. Before the current ‘closes the loop’ by re-entering the TPS, it must enter the rails, since the TPS itself is not grounded. Once in the soil the stray current distributes to minimise the total resistance.
If a steel structure is present in the soil, it constitutes an alternative return path. Current will enter the steel structure and flow through it before exiting. As steel is a far better electrical conductor than soil, this current has the potential to be significant. At the part of the steel surface where the current leaves the structure, corrosion can be initiated and accelerated. Corrosion is caused by the electrochemical oxidation of iron ions at the interface.
For Maccaferri solutions, the significance of stray current depends on the orientation of the structure in the soil, the isolation of the structure from soil, and soil resistivity.
The influence in transmission for horizontally oriented elements is smaller compared to vertical conductors as the current tends to pass to greater depths dissipating through the soil resistivity.
The current conductivity is different for soil and steel structures. In soils, transportation of current is through ions, whereas in steel it is through electrons; iron ions in aqueous solution leave the steel structure and contribute to the current transport within the soil.
The iron ions departing the steel leave residual electrons in the steel causing corrosion. Polymer-coated steel prevents this aqueous phase developing thereby stopping the electrochemical reactions and, because of the high electrical resistivity, does not allow the current to travel; there are no free electrons to cause corrosion.
Therefore, soil resistivity and moisture content are important parameters in stray current phenomenon: in dry conditions the corrosive effect for steel structures is less relevant as the aqueous solutions of iron ions cannot form.