6. Water Velocity 



The highest water velocities at pile structures which may signi- 

 ficantly affect their corrosion are probahly those caused by wave action. 

 These velocities depend upon wave conditions possible at the site and 

 should seldom exceed 25 feet per second. The effect of water velocity on 

 corrosion rates in seawater is quite different from the effect in fresh 

 water. It is more difficult to attain passivity of metals in seawater. 



Tests in neutral waters have shown that the corrosion rate in- 

 creases with an increase in water velocity up to a certain point. Further 

 velocity increase may then cause the corrosion rate to decrease (Copson, 

 1952). The increase in corrosion rate with increased velocity is attrib- 

 uted to the force of the water eroding the existing protective oxide 

 films thereby exposing new metal to the corrosive environment. The de- 

 crease in the corrosion rate when the water velocity exceeds a certain 

 high has been shown to be due to the increased oxygen supply pennitting 

 a film of ferric hydroxide to form (Copson, 1952). This film apparently 

 adheres to the metal at relatively high velocities. Other tests (Copson, 

 1952) are reported to have shown that it is quite difficult to reach a 

 state of corrosion passivity in seawater. Figure No. h shows the effects 

 of water velocity on the corrosion of steel in seawater. The difficulty 

 in producing corrosion passivity in seawater can probably be attributed 

 to the chloride ions. Water with zero velocity (stagnant) can usually be 

 expected to have a lower corrosion rate but a higher rate of localized 

 pitting (Copson, 1952). 



T. Marine Organisms 



Marine organisms may affect the corrosion rate of steel piling 

 in several ways, such as: 



a. Penetrating soft protective coatings and exposing base 

 metal to seawater. 



b'. Forming protective films on metal surfaces. 



c. Burrowing into protective concrete encasements. 



d. Organisms smothered by larger ones may form acids 

 which attack the metal surface. 



e. By producing metabolic byproducts, including hydrogen 

 sulfide, acids, carbon dioxide and ammonia which may take part in the 

 corrosion of metal (Snyder and Hull, I965). 



f . The formations of fouling organism may cause localized 

 stagnant water conditions which in turn create differential oxygen cells. 



8. Galvanic Effect of Unlike Metals 



Caution should be taken in connecting unlike metals or creating 

 unlike conditions in the same metal or alloy for structures in seawater 



