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GALVANIC CATHODIC PROTECTION Galvanic cathodic protection is a method used to protect metal structures from the action of corrosion. As explained before, galvanic cell corrosion is the major contributing factor to the deterioration of metal by electrochemical reaction. The area of a structure that corrodes is the anode or positive side of the cell. Corrosion occurs when the positive electric current leaves the metal and enters the electrolyte. Galvanic cathodic protection is designed to stop this positive current flow. When the current is stopped, the corrosive action stops and the anodes disappear. This type of protection depends upon the neutralization of the corroding current and the polarization of the cathode metal areas. METHODS OF GALVANIC CATHODIC PROTECTION Galvanic cathodic protection is a means of reducing or preventing the corrosion of a metal surface by the use of sacrificial anodes or impressed currents. When sacrificial anodes are used, it is known as the galvanic anode method. If impressed currents are used, it is known as the impressed current method. These two methods can be used separately or with each other, depending upon the corrosive characteristics of the electrolyte surrounding the structure. Galvanic Anode Method The galvanic anode method of cathodic protection uses an electrode referred to as a sacrificial anode that corrodes to protect a structure. This sacrificial anode is electrically connected to and placed in the same electrolytic area of the structure. The anode used to protect iron or steel structures should be made of magnesium or zinc so it will produce a sufficient potential difference to cause the structure to become a cathode. The action of this type of galvanic protection causes the electric current to flow from the sacrificial anode through the electrolyte to the structure to be protected. The electrical connection between the two metals completes the circuit and allows the current to return to the corroding metal. The sacrificial anode becomes the anode of the established dissimilar metal galvanic cell, and the structure to be protected becomes the cathode. The current from the sacrificial anode is intense enough to oppose or prevent the positive current from leaving the anodes in the structure to be protected. These structure anodes are then suppressed, and the metal in the structure becomes a cathode. The prevention of these positive currents from the anodic areas in the structure reduces the corrosion rate to almost zero. Galvanic cathodic protection is used in areas where the corrosion rate is low and electric power is not readily available. A typical example of galvanic cathodic protection is shown in figure 7-14. Impressed Current Method The impressed current method of cathodic protection is designed to protect large metal structures located in corrosive areas. With this method of protection a source of alternating current is required. Also, a rectifier is needed to obtain the required direct current potential. The basic principle of the impressed current method is merely the application of the galvanic cell reaction. The component parts of this method are the cathode (the metal structure to be protected), the anode (made of suitable anodic material), the electrolyte or ground (the ionized corrosive material), and the rectifier and various connections that serve to complete the electrical circuit. The operation of this method depends on the rectifier forcing direct current from the anode through the electrolyte (ground) to the metal structure to be protected. This method causes the metal structure to be the cathode, suppresses the anodic currents from it, and, in turn, prevents corrosion of the structure. An impressed current method of cathodic protection is shown in figure 7-15. |
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