U-Boat U-534 Engine room and workshop
Corrosion Of Metals In Sea Water
With dissimilar metals in sea water, galvanic action results and the more anodic metal corrodes.
Any material in the Table below is anodic to those below it.
For example steel is anodic to bronze in sea water and therefore it will corrode. We can say that the steel has given ‘cathodic protection’ to the bronze.
Anodic End Of Table (Corroded)
Magnesium
Aluminium
Zinc
Mild Steel
Manganese Steel (without oxide film)
Admiralty Brass
Copper
Aluminium Bronze
Gunmetal
Cupro-Nickel 70/30
Nickel
Stainless Steel (with oxide film)
Monel Metal
Graphite
Titanium
Cathodic End Of Table (Protected)
To minimise galvanic effect:
1. Choose materials close to each other in the series.
2. Make the key or main component of a more noble metal (i.e. cathodic).
Corrosion And Deposition
Most of the metal equipment in marine power plants is made up of steel or copper alloys (brass, copper-nickel,
bronze and others). All of these metals will dissolve slowly in water unless the water is properly treated. This is called corrosion.
Some of the most common kinds of corrosion damage which can occur on ships and ship equipment are:
a. Thinning of the tube metal. This is the result of corrosion that is continuous and over a fairly large area of
metal. This kind of a damage is also called general corrosion. Thinning can progress to the point at which the
metal can no longer contain the internal pressure which may cause the metal to swell and eventually burst.
b. Pitting – when only a small area of metal is corroded the result is a deep hole called a pit. If pitting corrosion
is not controlled, some pits may go all the way through the metal. This causes leaks. When there are many pits
close together, they may become connected. The effect on the metal is the same as that of general corrosion.
c. Corrosion cracking is another form of corrosion which can effect certain materials. In general, alloys, which
are mixtures of metals, are most susceptible to cracking. Stainless steel and brass such as Admiralty are
particularly susceptible to cracking under certain conditions. Cracking is a form of corrosion which occurs
along a very narrow band through the metal.
d. Some metal alloys are susceptible to exfoliation or de-alloying. Both of these types of corrosion are
associated with the selective reaction of only one of the metals in a metal alloy. Exfoliation generally occurs in
feed water heaters. Nickel is selectively oxidised from the copper-nickel alloy tubing leaving layers of copper
metal and nickel oxide. Brasses are mixtures of copper and zinc. When de-alloying occurs, zinc is removed
from the metal leaving a spongy mass of copper behind. This is commonly referred to as dezincification.
e. Embrittlement is an effect of corrosion that changes the physical properties of a metal. Some corrosion
reactions cause metals to lose their normal strength and ductility and become brittle and weak. Embrittlement
cannot be seen by inspecting a boiler tube that has not failed. However, an embrittled tube that has failed will
have a crystallized appearance at the edge of the point of failure and usually there will be no evidence of
bulging.
