Impressed current Cathodic Protection system Design

• Impressed current cathodic protection systems (ICCP) are used to provide cathodic protection for pipelines, ship hulls, offshore production platforms, water and wastewater treatment equipment, tank farm and of course underground storage tanks.

• The principle advantage of ICCP is its much greater output capacity as compared to galvanic anode systems. Therefore when protection is desired for large, poorly coated and bare structures ICCP is often the choice.

• ICCP systems require external DC power source that is energized by standard AC current.

•There are many anode materials available and most of them provide 100-10000 times current provided by galvanic anodes.

•The amount of Metal exposed in UST’s is very large, so to inhibit corrosion, current required is typically in excess of 4-8 amps per service station.

Advantages and Limitations

There are several advantages of using ICCP systems:

• Unlimited current output capacity: The amount of current can be designed from few amperes to several 100 amperes. The amt of current will be a function of no. of anodes provided, the rectifier voltage and amperage capacity and soil (electrolyte) resistivity.

• Adjustable output capacity: The output of rectifier can easily be adjusted to accommodate the changes in circuit resistance or current requirement. It can be automatic or manually controllable.

• Low cost per ampere of CP system: Galvanic anodes are expensive where amperes rather mill amperes are required.

The ICCP do have disadvantages as well, and can be listed as:

• They are most costly even if only a mill amperes of current is required. Their base cost itself is several thousand dollars.

• ICCP have higher maintenance cost because they are prone to failure of switches, fuses and need to be monitored frequently.

• ICCP may create stray current corrosion on other near by structures and it is inherent potential problem of system. This can be minimized sometimes by distributed anode designs where by anodes are placed near to protected structure.

Available Anode Materials

There are no. of anode materials used, which are selected depending on environment and congestion. Like HSCI and Graphite anodes are used for Pipelines while Ti based Precious metal oxide anodes are used in UST because of light wt and small diameter.

Canisters are filled with high conductivity coal or calcined petroleum breeze.Canisters are metallic spiral galvanized ducts with dia of 3’’, 4’’, 6’’ etc. Package shd be provided 2-3’’ bore.

The type and no. of anodes is dictated by user experience and current required for protection of structure.

Jobs for Corrosion engineers include:

• Current requirement test: by installing temporary current anode bed of cu electrodes and measuring the potential of structure with Cu-CuSO4 electrode versus increase in current.

• Continuity: All metallic components that are to be protected must be electrically continuous. Nearby foreign structures must also be made continuous to avert stray corrosion effect.

• soil resistivity: Resistivity test is most essential to the design and are measured with varying depths by 4 pin Resisitivity meter or two pin shepard canes device.

• Anode Location and Size: one method is to install anodes at depth of 10-20 feet in augured holes with a spacing centre to centre of 10-30 feet with first anode being placed 100 to 500 feet from the pipeline depending upon soil resistivity and structure coating.

Continued……

•Another option is to distribute the anodes along the structure with each anode being placed 10-20 feet from the structure. This is occasionally done on large diameter bare pipelines while almost always done on UST’s.

•The anodes are generally distributed based on current requirement testing and current requirement for each metallic component to be protected.• All the resistance formulas mentioned in Galvanic protection system

design equally apply to Impressed current anodes. The tables provide below are based on 1000 ohm cm resistivity spaced between 10-20 feet. Incase ,If soil resistivity is 4500 ohm cm then multiply it by 4.5, because of linear relationship with resistance.

Anodes and Ground Lead wires

• The most common insulation for anode lead wires are High MW low density polyethylene CP cable. Other cables for more arduous atmosphere are Halar, Kynar and Hypalon insulations. The cu conductor is most commonly #8 AWG 7 strand annealed copper.

• Use of individual wires has the double benefit, one is to eliminate splices and assuring only one anode become inoperative if it got damaged.

• System ground wire: Ground wire should be from structure to negative terminal of Rectifier and connection is done by thermite brazing directly to the metal.

Thermite brazing

Rectifier power supply

For selection of Power supply, these things are to be considered:

• DC output• Capacity • AC supply voltage

• In addition the method of control (manual/automatic), cabinetry, monitoring devices etc must be clearly defined and selected.

• The control selection will depend on location of Test station. Far off places with no frequent checkups will be monitored by remote methods while nearby places are usually monitored by Manual controls.

• Some features which are suggested for underground operation are constant current control and remote monitoring with the ability of requesting potential readings from permanent reference electrodes.

Resistance of Prepackaged Anodes

Graphic Representation of Resistance for 3’’ canister

Graphic Representation of Resistance for 4 and 6’’ canister

Potential required for cathodic protection

Metal Potential(Cu/CuSO4)

Steel -850 mVSteel (sulphate reducing bacteria) -950 mVCopper alloys -500 to –650 mVLead -600 mVAluminium -950 to –1200 mV

Useful Data for Protection of Steel in Various environment

Current densities required to protect steel

Environment Current densityA m-2

Acidic solutions 350 – 500Saline solutions 0.3 – 10Sea water 0.05 – 0.15Saline mud 0.025 – 0.05

Current densities

Environment Resistivity (ohm cm)

Brackish river water 1Sea water 25Town supply water 1000 – 1200Alluvial soils 1000 – 2000Clays 1000 – 5000Gravel 10000 – 25000Sand 25000 – 50000

Average resistivity's of some soils