2. The Corrosion Process 



Steel in contact with an electrolyte inherently has areas of 

 differing electrical potential. The difference in potential causes 

 electric currents to flow in the steel and through the electrolyte. 

 The current flow in the electrolyte is in the form of ion transfers. 

 Positive Fe+''" ions are released into the solution (electrolyte) from 

 the anodic surfaces of the steel. The positive ions in the electrolyte 

 are attracted to the cathode since there is a reduction process present 

 which produces OH ions. These ions combine to form Fe ( OH )2( ferrous 

 hydroxide). The OH ions are formed by the dissociation of water (H2^) 

 producing hydrogen atoms (H"*") and the OH (Hydroxide ion). When an Fe''"'' 

 ion is released into an electrolyte, it gives up two electrons (2e ). 

 These electrons are given up at the anode and flow through the metal to 

 a cathodic area. Two hydrogen atoms (2H''') from dissociated water may 

 combine with two electrons (2e ) at the cathode to form hydrogen mole- 

 cules (H2) which will either cling to the cathodic surfaces, bubble off 

 as gas or combine with oxygen to form water. In addition to the re- 

 action described above in the steel corrosion process, there can be 

 other reactions such as the conversion of ferrous iron to ferric iron. 



Section V. FACTORS AFFECTING THE CORROSION OF 

 STEEL PILING IN SEAWATER 



1. General 



A number of factors may influence the rate at which the corrosion 

 process of steel piling proceeds in seawater. Some are: 



Water temperature 



Concentration of oxygen in electrolyte (seawater) 



pH value of the seawater 

 d. Marine fouling on piling 



Salinity of the seawater 

 f. Velocity of the water relative to the structure 



Galvanic effect of unlike metals 



Details concerning the effects of these factors are given in the para- 

 graphs to follow. 



2. Temperature 



Temperature affects the corrosion of steel in seawater in several 

 ways. One is that chemical reactions of the corrosion process are ac- 

 celerated in warmer water. Another is that the marine fouling organisms 

 which may affect the corrosion rate are more numerous in warmer waters. 

 Temperature also affects the capacity of the water to dissolve oxygen. 



Data compiled by LaQue (Uhlig, 19^8) indicates that the tendency 

 for the chemical process of corrosion to proceed faster in warmer water 

 is often counteracted by the metal surface having a heavier protective 



