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Cathodic Protection Overview
 
Metal that has been extracted from its primary ore (metal oxides or other free radicals) has a natural tendency to revert to that state under the action of oxygen and water. This action is called corrosion and the most common example is the rusting of steel.
Corrosion is an electro-chemical process that involves the passage of electrical currents on a micro or macro scale. The change from the metallic to the combined form occurs by an “anodic” reaction:
M
M+
+ e-
(metal)
(soluble salt)
(electron)
A common example is:
Fe Fe++ + 2e-
This reaction produces free electrons, which pass within the metal to another site on the metal surface (the cathode), where it is consumed by the cathodic reaction. In acid solutions the cathodic reaction is:
2H+ +
2e-
H2
(hydrogen ions
 in solution)
 
(gas)
In neutral solutions the cathodic reaction involves the consumption of oxygen dissolved in the solution:
O2 + 2H2O + 4e-
4OH-
 
(alkali)
Corrosion thus occurs at the anode but not at the cathode (unless the metal of the cathode is attacked by alkali).

Figure 1. Corrosion cell / Bimetallic corrosion
The anode and cathode in a corrosion process may be on two different metals connected together forming a bimetallic couple, or, as with rusting of steel, they may be close together on the same metal surface.
This corrosion process is initially caused by:
Dereference in natural potential in galvanic (bimetallic) couples. Metallurgical variations in the state of the metal at different points on the surface.
Local differences in the environment, such as variations in the supply of oxygen at the surface (oxygen rich areas become the cathode and oxygen depleted areas become the anode).
The principle of Cathodic protection is in connecting an external anode to the metal to be protected and the passing of an electrical dc current so that all areas of the metal surface become Cathodic and therefore do not corrode. The external anode may be a galvanic anode, where the current is a result of the potential difference between the two metals, or it may be an impressed current anode, where the current is impressed from an external dc power source. In electro-chemical terms, the electrical potential between the metal and the electrolyte solution with which it is in contact is made more negative, by the supply of negative charged electrons, to a value at which the corroding (anodic) reactions are stifled and only Cathodic reactions can take place. In the discussion that follows it is assumed that the metal to be protected is carbon steel, which is the most common material used in construction. The Cathodic protection of reinforcing carbon steel in reinforced concrete structures can be applied in a similar manner.
Cathodic protection can be achieved in two ways:
- by the use of galvanic (sacrificial) anodes, or
- by “impressed” current.
Galvanic anode systems employ reactive metals as auxiliary anodes that are directly electrically connected to the steel to be protected. The difference in natural potentials between the anode and the steel, as indicated by their relative positions in the electro-chemical series, causes a positive current to flow in the electrolyte, from the anode to the steel. Thus, the whole surface of the steel becomes more negatively charged and becomes the cathode. The metals commonly used, as sacrificial anodes are aluminium, zinc and magnesium. These metals are alloyed to improve the long-term performance and dissolution characteristics.
Impressed-current systems employ inert (zero or low dissolution) anodes and use an external source of dc power (rectified ac) to impress a current from an external anode onto the cathode surface.
The connections are similar for the application of Cathodic protection to metallic storage tanks, jetties, offshore structures and reinforced concrete structures.
Advantages and Uses of Cathodic Protection
The main advantage of Cathodic protection over other forms of anti-corrosion treatment is that it is applied simply by maintaining a dc circuit and its effectiveness may be monitored continuously. Cathodic protection is commonly applied to a coated structure to provide corrosion control to areas where the coating may be damaged. It may be applied to existing structures to prolong their life.
Specifying the use of Cathodic protection initially will avoid the need to provide a “corrosion allowance” to thin sections of structures that may be costly to fabricate. It may be used to afford security where even a small leak cannot be tolerated for reasons of safety or environment. Cathodic protection can, in principle, be applied to any metallic structure in contact with a bulk electrolyte (including concrete). In practice, its main use is to protect steel structures buried in soil or immersed in water. It cannot be used to prevent atmospheric corrosion on metals.
However, it can be used to protect atmospherically exposed and buried reinforced concrete from corrosion, as the concrete itself contains sufficient moisture to act as the electrolyte.
Structures that are commonly protected by cathodic protection are the exterior surfaces of:
  • Pipelines
  • Ships’ hulls
  • Storage tank bases
  • Jetties and harbour structures
  • Steel sheet, tubular and foundation pilings
  • Offshore platforms, floating and sub sea structures
Cathodic protection is also used to protect the internal surfaces of:
  • Large diameter pipelines
  • Ship’s tanks (product and ballast)
  • Storage tanks (oil and water)
  • Water-circulating systems.
However, since an internal anode will seldom spread the protection for a distance of more than two to five pipe diameters, the method is not usually practical, or suitable, for the protection of small-bore pipe work.
Cathodic protection is applied to control the corrosion of steel embedded in reinforced concrete structures (bridges, buildings, port and harbor structures, etc.) – See Guide in Corrosion Control, Corrosion and Protection of Steel in Concrete and it’s Monitoring.
Cathodic protection can be applied to copper-based alloys in water systems, and, exceptionally, to lead-sheathed cables and to aluminium alloys, where cathodic potentials have to be very carefully controlled.
 
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