About Cathodic Protection

In contrast to coating of pipes in order to prevent corrosion, cathodic protection directly interfere with the ongoing electro chemical process of corrosion. For cathodic protection electrons provided from main source or galvanic cells are send to protected structure and the potential of metal is decreased (For instance decreasing the potential of metal from -500 mV to minimum -850 mV). The intensity of cathodic protection current which will force metal to behave like a cathode depends on bare (uncoated) portion of pipeline and electrolytic environmental (soil) resistivity.

A cathodically protected metallic structures potential shifts to negative values and reaches to certain potential values. In order to determine whether a metallic structure is properly protected or not measurement of these potential values is a benchmark. These potential measurements could be performed using saturated copper/copper sulphate reference electrodes. The protection potential for iron and steel (which is effectively the potential difference between metal and electrolyte) must be -850 mV or a more negative value. Potential values more positive than -550 mV indicates that corrosion is started at structure made of iron or steel. In case the protection potential is more negative than -2000 mV, production of hydrogen gas increases and as a result cracking of coating occurs if coating is well done. At some applications after applying protection current to a structure having potential of -450 mV before cathodic protection, -850 mV potential value may not be achieved. For such cases a potential shift of 300 mV towards negative values from the initial potential value also means that the cathodic protection criteria are fulfilled (TSE, Turkish Standards Institute, 5141).

CATHODIC PROTECTION SYSTEMS

1. Protection with Impressed Current Systems

This type of system is consisting of a main power source (Transformer/Rectifier) and an anode ground-bed which is connected to main source. In this case the supply current is provided from an outside source. At this system iron/silica, metal oxide plated titanium, graphite or scrap iron could be used as an anode. Also anode ground-beds could be located horizontally or vertically. On the other hand possible power shortage and Transformer/Rectifier failures left pipe unprotected.

2. Protection with Galvanic Systems

This type of protection is based on gathering electrons from galvanic cells such as zinc and magnesium. Since there is no need for an external current source, this type of cathodic protection is also referred as sacrificial anode protection system. Although the initial expenses of such protection are more; this kind of protection system requires less operation and maintenance expenses. In continuous operation mode power consumption and Transformer/Rectifier failures are not issues. In case that there is no suitable place for a anode ground-bed or due to administrative issues, protection with a galvanic system could be a necessity. At this kind of sacrificial protection systems maximum protection current depends on capability of output current that anodes could supply. On the other hand for impressed current systems with potentiometers on Transformer/Rectifier units one can adjust the protection current and increase the current as required.

COMPARISON OF CATHODIC PROTECTION SYSTEMS

Impressed Current Protection Systems

1. 220V AC power is needed for operating Transformer/Rectifier unit.
2. Cost of electric current is cheaper and there is no limit for this current.
3. High resistivity is not a problem for these applications. By decreasing ground-bed resistance and increasing number of Transformer/Rectifier Units. Desired protection current intensity could be applied (under reasonable limits).
4. One cannot exceed current capacity of Transformer/Rectifier unit.
5. In case of change in required protection current, the protection current and voltage must be adjusted from Transformer/Rectifier unit automatically or manually. Otherwise if the required current for protection increased, potential of structure may be under protection criteria.
6. At regions close to anode ground-bed due to excessive voltage there might be cracks at pipe coatings.
7. Interference should be considered at the regions around anode ground-bed and intersections with other pipelines that are already cathodically protected.

Galvanic Anode Protection Systems

1. There is no external power supply and necessary protection current is supplied from galvanic anodes. This kind of system is the only protection options where electricity is not available.
2. Cost of protection current is high and due to that fact this kind of system is not preferable in applications where protection current for pipelines is high.
3. Since potential of the system is low, this kind of system cannot be used soil having high resistivity. The application is appropriate for soil having resistivity up to 5000 ohmxcm.
4. Application is easy and an increase in protection current could be compensated by adding extra anodes.
5. It is impossible to adjust current output of anodes. Galvanic anodes adjust necessary protection current by themselves. If there is an increase in protection current, potential would decrease so current output of galvanic anodes would increase.
6. No cracking of coating is observed in the regions where anode grounded is close to pipe.
7. Since anode/soil potential is less, effect of interference could be neglieble.