Corrosion
and Stealth – early
decisions can save money
Corrosion of a ship or submarine hull is accompanied by electric
currents flowing in the surrounding seawater and in the metal
of the hull itself. 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.
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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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