•CONVENTIONAL CURRENT FLOW (+ TO -) - 



ELECTRON FLOW 



H+ 

 OH- 



2e- + 2H + — -H2 



H + 

 OH- 



V .^. 4e-4 02 4 H2Q— - 



H+ I 

 OH- 5 



IONS 



H + 

 OH- 



Fe— »-Fe + + + 2e- 



4 0H" 



Fe ++ +20H—- Fe(OH)2 



H* 

 OH- 



CONVENTIONAL CURRENT FLOW 



Fe — — Fe + *+2e- 



FC+++20H — -Fe(OH)2 



■mm*. 



2e- + 2H+-«-H2 



N H + 

 ) OH- 



ION FLOW 



CATHODE" 



I 





STEEL ANODE 



Figure 90. Corrosion process. 



If the structure is to be immersed, the corrosivity will depend on the 

 temperature of the water, chemical composition (salt content, dissolved 

 oxygen and the presence of other chemicals), velocity of the water (movement 

 by tides, waves, gravity, etc.), and splash zone effects (a combination of 

 mechanical and corrosive effects). Corrosivity on immersed or partially 

 immersed structures generally increases with increases in temperature, 

 increases in corrosive chemical content, increases in fluid velocity and 

 in the splash zone. It should be noted that high purity water such as 

 distilled water or deionized water is a special condition and can cause 

 coating to blister or delaminate. 



319 



