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FAUJI FERTILIZER COMPANY LTD.

Engineering Division, Rawalpindi- Pakistan

Muhammed Azeez Sadiq Date: January 27, 2015

Cathodic Protection

Introduction

Cathodic Protection systems are a vital part of any underground piping. These are deployed

to prevent any corrosion of piping due to the potential differences that exist with the soil.

Proper installation of cathodic protection systems is essential for the pipeline to maintain its

integrity over the years.

Corrosion Phenomenon

Corrosion requires three things to occur.

1. Two dissimilar metals

2. An electrolyte (water with any type of salt or salts dissolved in it)

3. A metal (conducting) path between the dissimilar metals

If the above conditions exist, the following reaction takes place at the more active sites or

anode

2Fe => 2Fe2+ + 4e-

The free electrons travel through the metal path to the less active sites, where the following

reaction takes place:

O2 + 4e- + 2H20 => 4 OH

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Recombination of these ions at the active surface produces the following reaction, which

yields the iron-corrosion product ferrous hydroxide

2Fe + O2 + 2H2O => 2Fe (OH)2

Types of Cathodic Protection

Cathodic protection systems are divided into two categories:

1. Galvanic Cathodic Protection

2. Impressed Current Cathodic Protection

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Galvanic Cathodic Protection

Galvanic Cathodic Protection works by introducing a more active metal into the system that corrodes in preference to the metal in the piping. The activity of the metal refers to its electronegative potential. A less electronegative metal has a higher activity than a more electronegative metal. It thus acts as an electron donor and supplies electrons to the other metal.

Normally, the piping metal acts as anode and corrodes itself. With a more active metal connected to it electrically, the metal however acts as a cathode with the other more active metal becoming the anode. Cathodic protection thus prevents corrosion by converting all of the anodic sites on the metal surface to cathodic sites by supplying electrical current (or free electrons) from an alternate source.

In the case of aluminum anodes attached to steel, the reaction at the aluminum surface is

4Al => 4AL3+ + 12 e-

and at the steel surface:

3O2 + 12e- + 6H20 => 12OH

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As long as the current arrives at the cathode faster than oxygen is arriving, no corrosion will occur.

Figure 1. Sacrificial Anode System in Seawater

The anode corrodes over time and must be replaced after a period of time depending on the

corrosive conditions present. Typical metals used as anodes include magnesium, zinc and

aluminum.

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Impressed Current System Protection

For larger structures, galvanic anodes cannot

economically deliver enough current to provide

complete protection. Impressed current cathodic

protection (ICCP) systems use anodes

connected to a DC power source. Anodes for

ICCP systems include high silicon cast iron,

graphite, mixed metal oxide, platinum and

niobium coated wire and others.

A typical ICCP system for a pipeline would

include an AC powered rectifier with a maximum

rated DC output of 10-50 A and 50 V. The

positive DC output terminal is connected via

cables to the array of anodes buried in the

ground.

As in sacrificial anode systems, impressed

current systems depend on a supply of high energy

electrons to stifle anodic reactions on a metal

surface. In the case of an impressed current system these high energy electrons are

supplied by a rectifier. Low energy electrons that are picked up at a non-reactive anode bed

are given additional energy by the action of a rectifier to be more energetic than the electrons

that would be produced in the corrosion reaction.

The energy for the electron energy pump action of the rectifier is provided by ordinary

alternating current. The effect of these electrons at the structure being protected is the same

as that derived from the sacrificial anode type of cathodic protection system. However, the

anode material serves only as a source of electrons and need not be consumed in providing

protective current.

The materials used for impressed current cathodic protection can pass current into the

environment without being consumed at a high rate. Graphite and high silicon cast iron are

the most commonly used impressed current cathodic protection anode materials.

Anodes in impressed current systems must be inspected and replaced if consumed or

otherwise damaged. As is the case for any electrical equipment, rectifiers used for impressed

current cathodic protection systems require preventative maintenance to insure proper

operation.

Figure 2. ICCP System in Seawater

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Requirements for Cathodic Protection

When a cathodic protection system is installed, it must be checked to ensure that the

potential difference between the metals is acceptable. If a minimum potential difference is not

achieved, then the cathodic protection system will not function effectively. The original NACE

specification for buried utility pipelines proposed the following criteria for determining when a

steel or cast iron structure meets the required standards.

A voltage of -0.85 V relative to a copper/ saturated copper sulfate electrode

A negative (cathodic) voltage shift of at least 300 mV caused by the application of

cathodic protection current

A minimum negative (cathodic) voltage shift of 100 mV determined by interrupting the

current and measuring the voltage decay

A net protective current from the electrolyte into the surface