CATHODIC PROTECTION BASIC PRINCIPLES AND PRACTICES

Cathodic Protection is an industrial technique for controlling metallic corrosion. Corrosion is an electro-chemical process that operates as an electrical circuit. Corrosion occurs in the Anode by oxidation and metal is lost, whereas in the cathode protection occurs by the reduction reaction. So in the cathodic protection technique, the concerned metal is converted into the cathode of the electrochemical corrosion cell. In this article, we will explore the basics of cathodic protection 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” diameter in a 2” steel pipe of standard thickness in 1-year time.

Corrosion can be mitigated by….

  • Cathodic Protection
  • Selection of Materials
  • Coatings

What is Cathodic Protection (CP)?

Cathodic protection is the use of DC Current from an External Source to oppose the discharge of corrosion current from anodic areas of the structure.

Principle of Cathodic Protection

The principle of the Cathodic Protection system is determining the anode in a large corrosion cell for making the intended material as cathode overcoming smaller corrosion cells. This can be achieved by two methods:

  1. Using Galvanic Series: This method first selects a more active metal from the galvanic series. Then the metal is installed in the electrolyte and a metallic path is provided. This method of CP is called sacrificial cathodic protection, or galvanic cathodic protection. In this method, a galvanically more active metal is installed to act as the anode which sacrifices itself and protects the pipe/structure working as a cathode.
  2. Impressed current cathodic protection: A source of DC current is installed in the system which forces the current flow from an installed anode to the pipe or structural material making it a cathode. DC source can be a solar cell, rectifier, generator, battery, or some other DC power. The anode material is selected considering the cost and weight loss per ampere year of current.

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

And this is the theory of cathodic protection where the intended material is converted into cathode.

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 overpower 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 with respect to Cu/CuSo4 Reference                Electrode.

Steel in Water: Pipe to Electrolyte potential must be between –0.8V to –1.10V with respect to 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

Types of Cathodic Protection Systems

Cathodic Protection with Galvanic Anodes:

  • Magnesium Anodes
  • Zinc Anodes
  • Aluminum 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
  • The system is extremely flexible

Limitations of ICCP Systems

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

Data required for deciding a Cathodic Protection System

The following data are required for deciding a cathodic protection 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 Cathodic Protection System

Factors that govern the Cathodic Protection System 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 Cathodic Protection in Plants

  • 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 plants to shut down thereby losing production.
  • Cathodic Protection can be installed at the time of the erection of the plant at a very low cost.
  • The CP cost could be 5-6 times if it is installed after completion of the project as it involves a 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 using CP system

  • 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 for pipelines are

  • 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

Equipment used for Cathodic Protection System

Anodes

  • MMO Anodes
  • Silicon Iron Anodes
  • Graphite Anodes

The anodes are installed in deep well borehole ground beds.

TYPICAL DEEPWELL INSTALLATION WITH MMO ANODES
Fig. 4: Typical Deepwell installation with MMO Anodes

Solar Generators

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

Anode Junction Box (AJB)

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) are terminated inside the AJB. One circuit with suitably rated shunts for monitoring the current output of the anode string.

Negative Distribution Box (NDB)

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 of the Corrosion Protection System

For routine maintenance and monitoring of the 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 the 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

CP System Monitoring Frequency

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 the measurement location. (Only ON potentials to be recorded)
  • Bonding Currents to be measured.
  • Isolation joints to be tested using the 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 Corrosion Protection system. This should be repeated once every three to five years.
  • Based on the CIPS data, the DCVG survey should be conducted immediately at sites where under protection has been observed during the CIP survey. It is also recommended that DCVG survey is conducted for the entire pipeline once every three to five years.

So, we can conclude that cathodic protection is a very useful and widely used method of steel protection. However, this method is costly and requires periodic maintenance and replacement. Click to know more about Design of Cathodic Protection for Duplex Stainless Steel (DSS) Pipeline

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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 “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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