ELEKTRO INDONESIA                      Edisi ke Dua, April 1996

Cathodic Protection System

             (Summary)

I. Introduction

One of the main problem with equipments and structures which are made of steel is corrosion. The proper corrosion problem handling may increase the life time of the equipment or structures and on the other hand the unproper handling will reduce the life time or even will cause an unexpected damage to the structures. Therefore corrosion problem handling is an important part of the industrial design. One of the most popular and proven corrosion protection system is cathodic protection. Basically corrosion process is analogical to electrolysis process which happen if the following components are present :

1. Anode
2. Cathode
3. Electrolyte
4. Metallic path between the anode and cathode

• Dissimilar Metal Cells
  Part of the steel which has more positive natural potential tend to become anode (see EMF series).
• The steel is placed in the soil with different composition
  Part of the steel which located at lower resitivity soil tend to become anode.
• The stell have parts with different temperature
  Part of the steel which higher temperature tend to become anode.
• The stell have parts with different stress
  Part of the stell with higher stress tend to become anode.
• The steel have parts with different age
  The new steel tend to become anode.

Metal	                on Formed      Potential               
			
Lithium           	Li              +2.96
Rubidium                Rb              +2.93
Potassium               K               +2.92
Strontium               Sr              +2.92
Barium                  Ba              +2.90
Calcium                 Ca              +2.87
Sodium                  Na              +2.71
Magnesium               Mg              +2.40
Alumunium               Al              +1.70
Berylium                Be              +1.69
Manganese               Mn              +1.10
Zinc                    Zn              +0.76
Chromium                Cr              +0.56
Iron (ferrous)          FE              +0.44
Cadmium                 Cd              +0.40
Indium                  In              +0.34
Thallium                Tl              +0.33
Cobalt                  Co              +0.28
Nickel                  Ni              +0.23
Tin                     Sn              +0.14
Lead                    Pb              +0.12
Iron (ferric)           Fe              +0.04
Hydrogen                H                0.00
Antimony                Sb              -0.10
Bismuth                 Bi              -0.30
Arsenic                 As              -0.30
Copper (cupric)         Cu              -0.34
Copper (cuprous)        Te              -0.56
Tellurium               Te              -0.56
Silver                  Ag              -0.80
Mercury                 Hg              -0.80
Palladium               Pd              -0.82
Platinum                Pt              -0.86
Gold (auric)            Au              -1.36
Gold (aurous)           Au              -1.50

II. Principle of Cathodic Protection

Cathodic protection is a corrosion protection method in which the corrosion cell is overcomed by providing external anode and let direct current to flow from that external anode on to all surface of the steel or to make all surface of the steel to be protected become the cathode. To stop the corrosion process, the cathdic protection driving potential must be greater then the corrosion cell. The driving potential sufficiently can be monitored by measuring the pipe to earth potential. The pipe to earth potential of -0,85 v with reference to copper/copper sulfatere reference electrode indicate sufficient cathodic protection driving potential.

1. Galvanic Cathodic Protection System In galvanic cathodic protection system the driving potential comes from the natural potential /emf different between the anode and the steel. The anode materials must have high enough natural potential in order to provide enough driving potential.
   In galvanic cathodic protection system the anode is connected directly to the steel to be protected through the anode lead wire without electric power supply.
2. Impressed Current System In impressed current system the driving potential comes from a direct current power source call as transformer rectifier. In this system the potential different between the anodes and the steel can be raised or or lowered by adjusting the transformer rectifier. In impressed current system the anode will be conneted to the positive terminal of transformer rectifier and the steel will be conneted to negative terminal of transformer rectifier.

III. Cathodic Protection Equipment

1. Transformer rectifier
   Transformer rectifier is a DC power source for impressed current cathodic protection system. Basically it is a step down transformer which has rectifier in the output side to rectify the AC current into the DC current. The transformer should have wide output voltage range for example from 2 V to 20 V (with 2 V increament). The wide output range adjustment is required in order to get the right driving potential, if it is to low the cathodic protection will not function properly and if it is to high it can damage the coating of the protected steel.
2. Anodes
   The anodes type to be used depend on the cathodic protection system and the location of the steel to be protected. For onshore galvanic cathodic protection system magnesium anodes are normally used. For onshore impressed current system graphite anodes and high silicion iron are normally used. For off shore galvanic cathodic protection system alluminium or zinc anodes are normally used.
   For onshore application the anodes can be ordered prepacked in backfilled (the anode has already packed with low resistivity backfilled material), in order to get better current output. For offshore pipeline application the anodes is bracelet type to suit the shape of the pipe. For bracelet type the thickness of the anode shall be lower than the thickness of the concrete coating of the pipe to be protected.
3. Anodes Distribution Box
   Anodes distribution box is junction box which is used to connect each anodes lead wire to one cable, before they are connected to the pipe or transformer rectifier terminal. The purpose of using this box is to provide facilities to check each anode current output, so it can be cetected if one anode is not working.
4. Test Station
   Test station or test box is a junction box consixt of a terminal block, to which the cable from the pipe (welded to the pipe) is terminated. The test station are installed along thepipe line which is protected by cathodic protection, to allow the pipe to earth potential to be tested along the pipe line roates, to check wether the cathodic protection function properly in all part of the pipe line.
5. Bonding Box
   Bonding box is a juntion box complete with adjusting resistor, which is used to connect two cathodic protection system to avoid interference.
6. Thermo Weld
   Thermo weld consist of the mould and the powder, it is normally used to connect the cable to the protected steel. There are many type of mould to suit the aplication such at cable to pipe, cable to horizontal flat steel, cable to vertical flat steel, cable to cable etc, for cathodic protection system, thermo weld is also used for cable to cable connection. It is recommended to house to cable connection in cable splicing kit with resin pouring in order to avoid current leakage.
7. Cathodic Protection Multimeter
   The cathodic protection multimeter & reference electrode is required to measure the pipe to earth natural potential. To measure the pipe to earth natural potentialone cable from the pipe (welded to pipe) connected to one terminal of the multimeter and one other cable from multimeter terminal is connected to reference electrode). Connection to the pipe can be taken from test station.
8. Insulating flange or insulating joint
   To separate the protected pipe with the other steel structure or equipment insulating flange or insulating joint is used. The purpose to separate the protected steel with other structure is to avoid current leakage.
9. Soil Resistivity Meter
   Soil resistivity is one important factor in cathodic prtection design. The soil resistivity will determine the Cathodic protection system to be used and the amount of anode required, and the location of the anodes groundbid. Nilson four pin soil resistivity meter is normally used, with this meter soil resistivity meter at certain defth can be measured by using the certain spacing of the pin.
   

1. Soil Resistivity Survey Soil resistivity survey shall be performed prior to the design. The most popular method is to used Nilson soil resitivity meter. The other way is by using soil box especially to measure the soil in deepwell groundbed system., where the soil resistivity must be measured at various defth, by taking the soil sample during drilling.
2. Current Requirement survey If the structure to be protected has already been installed at site, beside from calculation, the more exact current requiment can be found by using current requiment survey. This survey can be done by using a DC source such as car battery, variable DC source, ampermeter and temporary anodes. Temporary anodes can be made from any kind of steel available at side. The temporary anode shall be connected to the positive terminal of the battery and negative terminal of the battery shall be connected to the steel.
   When the pipe to earth potential reach - 0,85 v with reference to copper/copper sulfate reference electrode, the current requiment can be read through the ampermeter. This current requiment can be used to determine the anodes quantity and also to determine the transformer rectifier rating for impressed current system.
3. Interference Survey If teo structure is located close to the steel protected by cathodic there may be an interference problem. Interference can cause damage to foreign structure and can cause cathodic protection system do not work properly. To monitor wether there is interference or not the pipe to earth potential of both structure shall be measured.
   If there is no change in pipe to earth potential in one system while the other cathodic protection system is switch on and off, that's mean no interference. This criteria is also used to avoid interference by bonding the two structure through bonding box. The resitance at the bonding box is adjusted untill there is no change in pipe to earth potential of one system while the other cathodic protection system is switched on and off.
   The same criteria is also used to monitor the insulating flange, where the other side of the flange potential to earth shall not change while the cathodic protection system at one side is switched on off.

1. Cathodic Protection System Selection Cathodic protection system selected base on technical and economical reason such as :
   • soil resistivity value
   • pipe size and length (for short length or small soze of pipe galvanic system is more economical)
   • availability of electric power
2. Anodes Quantity Calculation
   Anodes quantity can be calculated base on the following tecnical data :
   • soil resistivity
   • pipe/steel structure size and length
   • lifetime
3. Selection of Anodes Type
   Anodes shall be selected base on the location such as onshore or offshore and current output requirement.
4. Selection of Anodes Installation Location
   Anodes location is selected based on the following :
   • soil resistivity
   • distence from the pipe (for impresed current)
   • uniform distribution (for galvanic system)
5. Selection of transformer Rectifier Rating Transformer rectifier rating is determined based on the available power source the soil resistivity and the current requirement. The current requirement will determine the output DC current of the transformer rectifier, while the output DC voltage can be calculated based on the soil resistivity, cable resistivity , anodes to earth resistivity and pipe for earth resistivity.