DISCUSSION 



The cathodic protection system with the specially cast zinc anodes 

 and steel cables woven through the ground tackle has provided complete 

 protection from corrosion to the underwater portion of the buoy and to all 

 its mooring chains for over 3-1/2 years. The appearance and present size of 

 the anodes indicate that such protection should continue for at least twice 

 this time. In an earlier work,'''^ a single 142-pound zinc anode protected the 

 underwater portion of a Mark 1 1 peg-top buoy and a portion of the riser-chain 

 for 3-1/2 years. The proof of complete protection of steel from corrosion 

 includes: ( 1 ) periodic potential profiles of the complete mooring, (2) appear- 

 ance of the exposed steel, (3) condition of the protective coating on the 

 chain, and (4) constancy of chain diameter measurements. 



The electrical potential profiles changed slightly with each measurement 

 but continued to remain well above the desired minimum level of -850 mv at 

 all times. These changes are attributed to tightening and slacking of the chain 

 by daily tides and by moored ships. The tight portions of the chain near the 

 buoy and ground ring had relatively good electrical continuity, but the portions 

 of the ground legs with relatively little motion needed the steel cables that were 

 woven through them to obtain the electrical continuity required for distribution 

 of the cathodic protection. Even the steel anchors received periodic cathodic 

 protection despite an attempt to prevent this. (Anchor weight losses by corrosion 

 are relatively small and have little effect on holding power.) The cathodic pro- 

 tection system performed well both on a rocky, sandy bottom and on a muddy 

 bottom. The anaerobic microorganisms in the bottom mud did not cause 

 passivation of the zinc anodes. 



The square of bare steel exposed on the underwater portion of the buoy 

 had only the passive film of iron oxide associated with cathodic protection. The 

 mooring crew remarked that the ground ring assembly looked as good as when it 

 was first placed into service. This was surprising to them, since the usual losses 

 of metal and resulting reductions in diameter had been attributed by them to 

 wear. 



The soft coal tar coating on the chains was in good condition, although 

 there was barnacle penetration on the upper riser-chain. This coating is rapidly 

 undercut by rust on unprotected chain and is in poor condition within 1 year. 

 The good condition of the coating on the protected mooring, in turn, greatly 

 reduced the electrical current needed for cathodic protection. 



The control mooring installed at the beginning of the test was removed 

 after 31 months of service. It required extensive and costly rehabilitation before 

 it could be reused by the Fleet. 



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