A PRESENTATION ON CATHODIC PROTECTION BASIC PRINCIPLES AND PRACTICES

WHAT IS CORROSION?

  • Degradation of metal by its chemical reaction with a non-metallic matter such as oxygen, sulfur etc.
  • Return of metal to the form in which it originally existed as an ore with complete loss of its metallic properties.

Metal electrode through which current flows….

From metal into electrolyte-ANODE

From electrolyte into metal-CATHODE

Effect of Corrosion… 

One Amp of current can cause a loss of 9 Kg of Steel in One Year.

Even a small amount of Current discharge, 1 mA can result in 7 holes of ¼” dia. in a 2”steel pipe of standard thickness in 1-year time.

Corrosion can be mitigated by….

  • Cathodic Protection
  • Selection of Materials
  • Coatings

Cathodic Protection?

Use of DC Current from an External Source to oppose the discharge of corrosion current from anodic areas of the structure.

Theory of Cathodic Protection…

Steel in soil / water                              Anodic

Earth / Sea water                                 Cathodic

As a result  :-

Steel loses electrons and hence corrosion

To reverse the above:

Make steel                            Cathodic

Seawater / Earth             Anodic

HOW DOES CATHODIC PROTECTION WORK?

Direct Current is forced to flow from an external source to the Structure. When the flow of this current is so adjusted to over power corrosion current discharging from all anodic areas thereby providing complete Protection.

Criteria for Cathodic Protection….

Steel in Soil: Pipe to Soil potential must be between

–0.85 V to -1.2 V w.r.t Cu/CuSo4 Reference                Electrode.

Steel in Water: Pipe to Electrolyte potential must be              between –0.8V to –1.10V w.r.t Ag/AgCl  Reference     Electrode.

BASIS FOR CURRENT DENSITY….

SOIL RESISTIVITY

>1000 ohm-cm                        10 mA/m2

100-1000 ohm-cm                     20 mA/m2

<100 ohm-cm                                     35 mA/m2

WATER RESISTIVITY

>150 ohm-cm                                           50 mA/m2

50-150 ohm-cm                                        75 mA/m2

<50 ohm-cm                                            110 mA/m2

Application of Cathodic Protection…

Cathodic Protection with Galvanic Anodes.

  • Magnesium Anodes
  • Zinc Anodes
  • Aluminium Anodes

Cathodic Protection with Impressed current Anodes.

  • High silicon chromium cast iron Anodes
  • Mixed Metal Oxide Anodes
  • Graphite Anodes
Galvanic Anode System
Fig. 1: Galvanic Anode System

Advantages of Galvanic Anode System…

  • Simple in Installation
  • No External Power Source
  • No Maintenance
  • No Power Bills
  • Easy to Design
  • No expensive accessories like cables etc
  • Economical for small structures

Limitation of Galvanic System…

  • Low Driving Voltage
  • Poor performance due to passivation
  • Limited Current
  • Low life

Typical Application of Galvanic anode System…

  • Small Pipelines with good Coating
  • Harbor Facilities, Steel piles, Jetties etc
  • Vessels, Tanks
  • Plant facilities and Equipment, Seawater intakes, Screens, Condensers, Heat Exchangers etc.
Typical Anode and Monitoring
Fig. 2: Typical Anode and Monitoring
Impressed current cathodic protection system
Fig. 3: Impressed current cathodic protection system

Advantages of Impressed Current CP System…

  • Current and Voltage can be varied
  • Can be used in almost any resistivity Environment
  • Can be designed for remote monitoring and control
  • Can be designed for measurement of Instant OFF / ON
  • No limitation of driving Voltage
  • System is extremely flexible

Limitations of ICCP Systems…

  • Regular monitoring and maintenance required
  • Requires Main supply or another source of electric Power
  • Interference Problem must be considered.

Main data required for deciding a CP System…

  • Details of Structure Dimensions
  • Surface Coating Scheme
  • Details of Soil Strata / Terrain
  • Presence of Foreign Metallic Structures.
  • Details of cased crossings
  • History of corrosive areas
  • Stray current conditions
  • Operating Temperature
  • Availability of AC Power

Planning a CP System…

Factors that govern the Design:

  • Choice of Cathodic Protection system
  • Amount of Total Current to achieve CP
  • No of CP Installations
    • Spacing between them &
    • Current Output of each Installation
  • Type of anodes and ground bed configuration
  • Any special conditions at certain locations needing modification of general CP Plan
  • Location of CP Test station.

Why do we need Plant Cathodic Protection

  • Initial Investment for Petrochemical complexes, Fertilizer Plants and Refineries are very high.
  • The corrosion problems are not detected until some leak appears.
  • Leakages can be extremely disastrous causing fatal accidents and great financial loss.
  • Increases maintenance cost of repairs of leakages.
  • The corrosion problem can also cause plant shut down there by losing production.
  • Cathodic Protection can be installed at the time of erection of the plant at a very low cost.
  • The CP cost could be 5-6 times, if it is installed after completion of project as it involves lot of excavation and restoration of structures.

Complexities of Plant CP

  • Scattered Underground Pipelines
    • -Pipes in Parallel
    • -Bifurcations
    • -Closely grouped network
  • Different pipes could be of different materials and coatings.
  • Area of influence shall vary from pipe to pipe.
  • Heavy underground civil structure reinforcements
  • Earthing network.
  • Tank Bottoms

Corrosion Control for Above Grade Storage Tanks..

  • The Tank bottom in contact with soil undergoes corrosion. This could lead to leakage, loss of product and cause environmental hazards.
  • It is much simpler and economical to install CP system during the construction stage.

Cathodic Protection for Pipelines

Main data required for deciding a CP System…

  • Details of Structure Dimensions
  • Surface Coating Scheme
  • Details of Soil Strata / Terrain
  • Presence of Foreign Metallic Structures.
  • Details of cased crossings
  • History of corrosive areas
  • Stray current conditions
  • Operating Temperature
  • Availability of AC Power

MAJOR EQUIPMENT USED FOR CP SYSTEM

  • ANODES
  • MMO ANODES
  • SILICON IRON ANODES
  • GRAPHITE ANODES
  • The anodes are installed in deep well borehole groundbeds.
TYPICAL DEEPWELL INSTALLATION WITH MMO ANODES
Fig. 4: TYPICAL DEEPWELL INSTALLATION WITH MMO ANODES

SOLAR GENERATORS:

CP Stations where input power is not available, Solar generators are used to power the CP system.

ANODE JUNCTION BOX:

The anode junction boxes are used to distribute the DC power from the T/R unit to the anodes at each CP station. The anode tail cables and the positive header (from T/R) is terminated inside the AJB.One circuits with suitably rated shunts for monitoring the current output of the anode string.

NEGATIVE DISTRIBUTION BOX:

The negative header cable (from T/R) and negative drain cable from the pipeline are terminated inside the NDB at each CP station. Each circuit shall have a variable resistor to control the current and suitably rated shunt to monitor the current.

SOLID STATE POLARIZATION CELL:

Pipeline to be earthed at all overhead powerline crossings where the voltage is greater than 100 kV. Connected across the pipeline and the system earthing. Prevents the CP current drain from the structure to the system earthing. Shunts all fault currents and induced currents to the earth.

SURGE DIVERTERS:

Surge Diverters are used across Isolating Joints Facilities. Provides surge protection to Isolating Joints  in case of a lightning strike or other faults.

MONITORING

For routine maintenance and monitoring of effectiveness of the CP system the following test facilities are provided along the pipeline.

1) POTENTIAL MEASUREMENT FACILITY: For measurement of the pipe to soil potential with respect to a portable reference cell. These facilities are provided at a regular interval of one facility every two kilometers

2)   DRAIN POINT TEST FACILITY: At each CP station a drain point test facility is provided.  Two permanent reference cells, two polarization coupons, and one corrosometer probe are used to measure the pipe to soil potential with respect to a permanent buried Cu/CuSO4 reference cell and the instant OFF potential from a buried coupon. Corrosometer allows monitoring of the corrosion rate and thereby the performance of the system. Permanent Reference cells are used to feed data for SCADA system.

3)   FOREIGN SERVICE CROSSING/PARALLEL FACILITY: To carry out interference testing at foreign pipeline       crossing or foreign pipeline running parallel to SGP. To facilitate the mitigation of Interference effects

MONITORING  OF CP SYSTEM:

Economic and safety considerations require close supervision and maintenance of all cathodic protection systems. Monitoring can be divided into three categories:

  • Monthly Monitoring
  • Quarterly Monitoring
  • Annual Monitoring

MONTHLY MONITORING:

  • Recording drain point potentials
  • Ground bed resistance
  • Anode current output
  • T/R output and T/R settings
  • Solar output settings

QUARTERLY MONITORING:

  • Recording Structure to Electrolyte Potential at measurement location. (Only ON potentials to be recorded)
  • Bonding Currents to be measured.
  • Isolation joints to be tested using Swing Test.
  • Reports to be generated.
  • Solar output settings

CLOSE INTERVAL POTENTIAL (CIP) AND DIRECT CURRENT VOLTAGE GRADIENT (DCVG) SURVEYS:

  • It is recommended that a Close Interval Potential Survey should be conducted within one year after commissioning the C.P. system. This should be repeated once every three to five years.
  • Based on the CIPS data, DCVG survey should be conducted immediately at sites where under protection has been observed during CIP survey. It is also recommended that DCVG survey is conducted for the entire pipeline once every three to five years.

 

 

Anup Kumar Dey

I am a Mechanical Engineer turned into a Piping Engineer. Currently, I work in a reputed MNC as a Senior Piping Stress Engineer. I am very much passionate about blogging and always tried to do unique things. This website is my first venture into the world of blogging with the aim of connecting with other piping engineers around the world.

2 thoughts on “A PRESENTATION ON CATHODIC PROTECTION BASIC PRINCIPLES AND PRACTICES

  1. How much our reading True Potential Power(mv) CP Criteria in Minimum & Maximum after

    conducting “ON / OFF” potential survey.
    Considering there is a present of Sulfur Reducing Bacteria.

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