Our experiment focused on showing the effects of metals being underwater for a long period of time, especially in salt water, and on showing the rusting of iron in particular in different conditions. Our results proved that rusting and corrosion are serious problems that have to be dealt with in the engineering and use of submarines, and that corrosion protection is effective.
Corrosion protection
Many modern submarines use corrosion protection in open water, where salt content is on average 33 to 39 %. This can vary in fresh and tropical water, but nonetheless corrosion protection is a top priority. In particular, the submarine power cables are at risk, especially the armouring wires, in most cases made from zinc-coated steel wires. This zinc layer, i.e. galvinisation, is the primary protection for these armouring wires. The secondary protection is flushing the armouring with hot bitumen during manufacture. If the bitumen layer is damaged through erosion by sand, the zinc layer takes over. The corrosion rate of the zinc depends on factors such as salinity, temperature and water exchange around the cable. As well as zinc used for galvanisation, it is used in sacrificial protection, which only works if the sacrificial zinc anodes have sufficient galvanic contact to the protected steel wires. This is not considered to be a very successful method. Thirdly, the armouring wires can also ve corrosion-protected by a polymeric sheath. As long as the sheath isn't washed away by sand abrasion, it stops direct contact to salty seawater. This avoids using the troublesome bitument layer.
problems with corrosion protection in submarines
When submarines use corrosion protection systems, they create underwater electrical potential (UEP) signatures, detectable by enemy vessels. This is a downside to corrosion protection, a side effect which is being worked on by Germany’s Technical Center for Ships and Naval Weapons and the Laboratory for General and Theoretical Electrical Engineering at the University Duisburg-Essen. These organisations are trying to find a way to reduce them. The team used simulations of electra potential distribution on the hull of a submarine to see whether or not the material is protected from corrosion. The higher the electric potential, the more the hull is protected. They then modelled the signature left by different electrical potentials. Their findings was that a higher amount of corrosion leads to a more detectable signal, and that the signature is least detectable between zero corrosion protection and standard operating conditions for full corrosion protection.