k. Cathodic Protection 



a. General . Cathodic protection is another method of mitigating 

 the corrosion of steel piling in seawater. This method is suitable for 

 protecting the immersed zone of the piling. Protective coatings for steel 

 are often used in combination with cathodic protection in order to reduce 

 the area requiring protection. 



b. Principle of Cathodic Protection . Corrosion of steel is 

 an electrochemical process vhich takes place in a corrosion cell. Cor- 

 rosion cells exist when a metal, or metals, which are electrically con- 

 nected, have areas differing in electrical potential and are in contact 

 with an electrolyte such as seawater. Electrodes of corrosion cells are 

 either cathodic or anodic. The electric current leaves the metal surface 

 at the anode and travels through the electrolyte to the cathode by ion 

 transfer while electrons flow through the metal from the anode to the 

 cathode. Corrosion of the metal occurs at the anode where the electric 

 current leaves the metal. Cathodic areas of metal are usually unaffected 

 by the entry of electric current, however, in some cases a protective 

 film results on the metal surface such as calcareous deposits which may 

 develop when seawater is the electrolyte. 



Cathodic protection of a metal is based on forcing a reversal in the 

 direction of electric current flow from that which normally occurs when 

 the metal is corroding. The current must have sufficient magnitude and 

 polarity to force the metal to be protected to become the cathodic elec- 

 trode. Cathodic areas do not corrode when an adequate electric current 

 flows to them. The current density required for the cathodic protection 

 of steel varies with the type of steel being protected, its condition, 

 and its environment. Field tests at the structure site and experience 

 should be utilized in estimating current density requirements. The cur- 

 rent density required for the corrosion protection of bare steel installed 

 underground or in fresh water usually ranges from 1 to 6 milliamperes per 

 square foot of surface area whereas from 3 to 10 milliamperes per square 

 foot of surface area is usually required for installation in seawater. 

 The range of current densities for coated steel sheet piling are usually 

 within the range of 0.5 to 6.0 milliamperes per square foot for the sea- 

 water side and 0.95 to 1.0 milliampere per square foot for the land side. 

 (U. S. Army Corps of Engineers, 1962 ). 



c. Types of Cathodic Protection . Two types of cathodic protec- 

 tion systems are used - the galvanic system and the electrolytic system. 

 The basic difference in these two systems is that, in the galvanic system, 

 the soxirce of the required electric current is the difference in electri- 

 cal potential between two connected unlike metals in an electrolyte. The 

 anodic metal corrodes as current flows to the cathode. The principle is 

 illustrated in Figure lU. In the electrolytic system, direct current 

 electricity of sufficient magnitude is supplied by an outside source. 



The source is usually a rectifier, which converts alternating current 

 electricity to direct current which flows from one or more anodes through 

 the electrolyte to the metal being protected. 



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