99. How are ships protected from corrosion and fouling? 



In the days of wooden ships, copper sheathing was used for protec- 

 tion against fouling organisnns because of its toxic properties. It served 

 the additional purpose of protecting the hull against borers. By 1783 

 all English vessels were copper sheathed, and by the early 1800's the 

 French and Spanish had followed suit. Copper sheathing has now been 

 replaced by coatings and paints, many of which contain copper. Be- 

 cause the toxic material must dissolve fast enough to prevent attach- 

 ment, these coatings must be renewed periodically. 



Before World War II, development of antifouling coatings was on a 

 trial-and-error basis. During the war, oceanographers of the Woods Hole 

 Oceanographic Institution worked with the U. S. Navy to learn how 

 marine paint actually works and which compounds are most effective at 

 the least cost. Their research saved millions of dollars by cutting the 

 cost of paints, lengthening the stay out of dry dock, and saving fuel. The 

 Navy attributed a 10-percent fuel bill reduction to the improved anti- 

 fouling paints. 



Corrosion of ships' hulls is prevented by organic coatings or cathodic 

 protection. Sea water is very corrosive, and the copper and mercury 

 compounds used in antifouling paints, if not isolated from the hull, may 

 accelerate corrosion. The most widely used anticorrosive compounds 

 are vinyls, epoxies, and combinations of epoxy and coal tar. Fiberglass 

 coatings are being tested. 



When a metal corrodes, metal ions enter the electrolyte (sea water) 

 at the anode, leaving behind electrons which flow to the cathode through 

 the metal. In cathodic protection, the corrosion potential of the hull 

 is made more electronegative and the direction of flow is reversed at the 

 sacrifice of the cathodic metal. 



Nowacki, Louis J., and Walter K. Boyd 



Metals Protection in the Marine Environment, Battelle Technical 



Review, June 1964. 

 Turner, Harry J., Jr. 



"A Practical Approach to Marine Fouling," Geo-Marine Technology, 



Vol. 3, No. 3, March 1967. 



119 



