SubscribeCorrosion of a ship or submarine hull is accompanied by electric currents flowing in the surrounding seawater and in the metal of the hull itself.
Corrosion and Stealth - early decisions can save money
These currents result in corrosion-related signatures (Underwater Electric Potential and Corrosion-Related Magnetic) that can be detected by mines, by underwater sensors, or from the air.
A good coating will of course reduce the corrosion currents, but when the coating becomes damaged or degraded, corrosion will increase and so will the corrosion-related signatures.
For this reason cathodic protection systems are usually fitted; these provide electrical currents themselves, which oppose the effect of the corrosion currents and so reduce corrosion to an insignificant level.
Cathodic protection systems can be (a) sacrificial, where zinc or aluminium anodes form a cell with the seawater and hull to provide protection current, and in the process sacrifice themselves by losing material, or (b) impressed current, where the protection current is supplied from power supplies within the vessel, via inert anodes.
In either case, however, electrical currents still flow around the vessel and produce detectable signatures in the same manner as currents from an unprotected vessel.
Over the last two decades, much progress has been made in predicting corrosion-related signatures using computer modelling or physical scale modelling.
Predictions can be made during a platform's design phase for both sacrificial and impressed current cathodic protection systems.
Only with impressed current systems, however, can signatures be managed once a vessel is at sea, and changes in the hull coating and seawater environment cause changes to the protection currents.
In the same timescale, military impressed current systems have taken a quantum leap in computer-controlled sophistication in order to manage signatures as well as maintain corrosion protection. Traditionally, hull coatings and cathodic protection have taken a much lower priority than platform design and weapons systems, and so have been considered late in a platform's design process.
This can leave little room for manoeuvre when the design of a hull and its appendages results in a corrosion-protection system that produces a detectable signature.
This can result in unexpected expenditure to re-site hull components (eg ground planes, arrays, domes), or in a platform with a less than optimal stealth performance.
Today's ships and submarines can have many hull components below the waterline made of very different materials, and these will have a significant effect on the static signatures produced by corrosion or impressed currents.
If these effects are examined early in the platform design, re-siting of components in order to reduce signatures to a minimum can be considered long before metal is cut.
The result is a platform design with enhanced stealth characteristics, coupled with the inherent and significant through-life cost savings that impressed current cathodic protection brings.
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