ELEKTRO INDONESIA                      Edisi ke Dua, April 1996

ELEKTRONIKA

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
If part of the steel become more positive than the other, which meant anode and cathode is present, and if the steel is immersed in the water or located underground (water/soil as electrolyte) then the four components of corrosion cell is exist. Part of the steel may become more positive than other part due to the following conditions :
Corrosion take place at the place where the electric current go out of the steel (anode area) and corrosion will not take place at the place where the current come into the steel (cathode area).
If all surface of the steel become cathode area there will be no corrosion, this condition can be achieved by using cathodic protection system.

Table .1 - Electromotive Series of Metals


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.
IV. Cathodic Protection Survey
  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.

V. Cathodic Protection Design
  1. Cathodic Protection System Selection Cathodic protection system selected base on technical and economical reason such as :
  2. Anodes Quantity Calculation
    Anodes quantity can be calculated base on the following tecnical data :
  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 :
  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.

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