Stray Current Corrosion occurs when the normal balance between anodic and cathodic parts of the corrosion reaction has been disturbed by an external voltage or current, in such a manner that the rate of metal dissolution has been significantly increased.
This process is sometimes called electrolysis in the marine industry. Stray current corrosion is also known as induced field corrosion or Faradaic Rectification.
Typically, stray current corrosion causes an acceleration of the rate of metal loss by factors between 10 and 1000. The amount of corrosion acceleration will depend on the conductivity of the immediate environment.
For example, low conductivity soils are more resistant to stray current corrosion. High conductivity soils will enable the stray currents to increase the corrosion reaction to a greater extent.
This situation is also true in aqueous (water based) environments, where fresh water is at one end of the corrosion scale and sea water is at the other.
Stray currents can be either d.c. (direct current) or a.c. (alternating current). The source of the stray currents can be a nearby voltage source or they can be electromagnetically induced currents as a result of rectification of high intensity magnetic fields.
What Causes Stray Current Corrosion?
Stray current corrosion can commonly be induced in a structure, by high power electric motors; inverters used to generate a.c. from d.c.; power sources used to control the speed of electric motors, etc.
Often, these power sources rely on thyristor control. Faults in the filters can give rise to spikes and harmonic distortion of the waveform.
Imbalances in 3 phase systems can also cause stray current corrosion.
ICCP (Impressed Current Cathodic Protection) and ICAF (Impressed Current Anti Fouling) systems are also capable of generating stray currents.
Stray Currents in Marine Environments
Stray current corrosion is common in marine environments, because of the high conductivity of sea water. Often, on ships, jack up rigs, vessels, etc there are sources of ungrounded electrical power.
Ships with diesel – electric propulsion systems, such as dredgers, tugs, rig and platform supply vessels, cable laying ships and vessels.
Those ships and vessels with dynamic positioning systems, azipods and bow thrusters are particularly susceptible to stray currents in marine environments.
Stray currents are commonly induced where propeller shafts are not properly grounded. Severe corrosion can take place on both the prop shafts and in the shaft tunnels.
Stray Currents in Super Yachts, Small Ships, Yachts and Other Vessels
Yachts and small vessels can produce numerous sources of stray current internally to the hull. These are typically from thyristor controllers on air conditioning units and from inverters used to control equipment such as bow thrusters.
Stray currents can also originate from the dockside from poorly installed or earthed shore power.
Stray currents can also be induced from other vessels moored nearby.
Stray Currents From Welding
Severe damage can occur to both coated (painted) and un-coated installations from improperly or poorly grounded welding sets and welding processes. The ground return current can cause damage to components on the hulls.
The damage caused is easily recognisable from the morphology of the metal failure (corrosion and rust) and the characteristic patterns produced in damaged coatings and paint.
Stray Currents and Cathodic Protection
Stray currents can adversely affect both sacrificial / galvanic anodes and impressed current cathodic protection (ICCP) systems.
Typically, very rapid loss of metal occurs from sacrificial or galvanic anodes. The control systems of impressed current (ICCP) systems can be made unstable by stray currents.
Prevention and Management of Stray Current Corrosion
By far the best method of managing stray current corrosion is to identify the source and eliminate it. This may require electrical modification to systems and to nearby power sources.
If the source of the current cannot be eliminated or replaced, the effects can often be reduced by improved grounding and, where feasible, insulation.
In some instances, a cathodic protection system can be used to reduce the effect of stray current corrosion, but this would generally be a last resort option.
Detection of Stray Currents
The detection and identification of stray currents that are causing corrosion can be extremely difficult, as stray currents are rarely uniformly distributed and corrosion can often occur at some distance from the power source that is causing the corrosion.
Typically, the mapping of stray currents requires a combination of physical and electrochemical techniques. Physical techniques consist of the measurement of magnetic field strength (in micro Tesla) using specialised search coils, oscilloscopes and field strength meters.
More usually, electrochemical test cells are used in combination with specialised reference electrodes.
Amtec Consultants have many years’ experience of identifying stray current corrosion across a range of environments and industries.
We are happy to discuss any corrosion and stray current problems you might have or suspect that you have. Please contact us for information.