Cooling system
49
Choice of materials for pipework
Various metals may not be freely combined with each other. If “noble" and "non−noble" metals are
combined, the “non−noble" metal will corrode both of them due to bimetallic corrosion.
This process is accelerated still further in humid or even salty atmospheres.
The more non−noble a metal is, the more negative its
negatively charged. Two different metals have a
bimetallic voltage difference that wants to be balanced out when they are combined (direct contact or
conduction through water). The following lists metals according to their electric potential starting with the
"most noble" (platinum) down to the “most non−noble" (magnesium).
The further two metals are
apart in this list, the greater the problems to be expected by bimetallic corrosion.
“Nobel” Platinum
Titanium
Silver
Nickel
Cupro−nickel
Lead
Stainless steel
Tin bronze
Copper
Tin
Brass alloys
Ferronickel
Low−alloy steels
Shipbuilding steel
Aluminium alloys
Zinc
"Non−noble" Magnesium
Commissioning
Note:
Proof of an adequate sea water supply is a decrease in vacuum pressure in the inlet pipe with
increasing ship speed, ideally to overpressure.
D At standstill: 0.3 bar at set rated speed
D At maximum speed: 0.05 bar
If the vacuum pressure increases while the ship is moving, then seawater supply cannot be
ensured.
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