since galvanic corrosion will result. When two unlike metals are con- 

 nected in an electrolyte, such as seawater, an electric current flows 

 in the metals and through the electrolyte . The least noble metal or 

 alloy (the one highest in the galvanic series) will corrode. The metal 

 which corrodes is also the one from which the current flows into the 

 electrolyte. 



The order of some metals and alloys in the galvanic series as de- 

 rived from tests in seawater are shown in Tahle XV, Appendix A; the 

 information in this table is from a report by LaQue and Cox (l9^0). 

 Revisions were made to show the current aluminum designation. The order 

 of the metals in the various groups may change depending on incidental 

 conditions of exposure. Galvanic action due to combining metals shown 

 within a group in Table XV should be relatively low (LaQue and Cox, 

 19^0). In the event it is desirable to use materials from different 

 groups in combination, the galvanic effect will ordinarily be less if 

 materials are selected from groups closest together in the table. 



Another practice which should ordinarily be followed when coupled 

 unlike metals will be in contact with seawater is to keep the area of 

 the anodic metal or alloy large in comparison to the cathodic material. 

 This practice spreads the corrosion due to the galvanic electric current 

 over a larger area. 



Section VI. CORROSION RATE OF UNPROTECTED STEEL 

 PILING IN SEAWATER 



1. General 



The corrosion rate of unprotected steel piling in marine struc- 

 tures can vary considerably depending on environmental conditions. Some 

 of the more influential environmental factors were discussed in the 

 preceding paragraphs . 



Due to the vertical extent of steel piling in marine structures, 

 there are variations in environmental exposure, and therefore, differ- 

 ences in rates of corrosion at various levels. Corrosion rates for steel 

 piling should, therefore, be stated for particular exposure zones in 

 order to be of value. 



2. Ordinary Carbon Steel 



There appears to be general agreement among corrosion special- 

 ists that bare steel structures of ordinary steel continuously submerged 

 in relatively uncontaminated seawater will corrode at a rate of approxi- 

 mately 5 mils per year. As previously stated, corrosion rates can vary 

 considerably between the various corrosion zones, and at the same zone of 

 structures at different locations, depending on conditions at the site. 

 Figures 5 through 7 show rates of loss of metal thickness at various 

 elevations along unprotected piles installed at the locations indicated. 



13 



