As the costs of these components increase rapidly with pipe diameter, then pumping costs and component costs have to be optimized. However, velocity also influences the corrosion behavior of the materials, and the design value chosen is often controlled by corrosion considerations. This is the case for carbon steel and copper-base alloys. However, for stainless steel systems, corrosion under flow conditions is not a problem. Corrosion of carbon steel in seawater is controlled by the availability of oxygen to the metal surface.
Pitting also occurs.
Galvanizing confers only limited benefit under flow conditions, as corrosion of zinc also increases with velocity. For the thicknesses normally used in seawater piping, it will extend the life of the pipe for about six months. Thus, these alloys, if they are to exhibit high corrosion resistance must be used at design velocities below this limiting value. Stainless steels are not subject to impingement attack, but are prone to pitting and crevice corrosion under low velocity conditions and this must be taken into consideration when these alloys are used in seawater. In recent years grades of stainless steel with high resistance to pitting and crevice corrosion have been developed.