Cathodic protection is a corrosion control method used to protect metal

surfaces from rusting or deteriorating, especially those buried in soil or

submerged in water (like pipelines, ships, offshore platforms, or storage
tanks). It works by turning the metal structure you want to protect into the
cathode of an electrochemical cell.
How it Works
Corrosion occurs when a metal becomes the anode in an electrochemical
cell and loses electrons.
Why Specific Metals?
 Metals like Mg, Zn, and Al are more "active" (more negative
electrode potential) than iron/steel, so they act as sacrificial
anodes.
 Copper (Cu) is more "noble" than iron, so it cannot be used as
a sacrificial anode—it would accelerate corrosion instead.

A solution is a mixture obtained by dissolving a substance (e.g., salt,

soda, gypsum, etc.) in water or another liquid.
These solutions act as electrolytes in cathodic protection. In other words,
they enable the flow of electric current.
Why are these solutions used?
Because:
 Soil or water alone may not conduct enough electric current.
  When solutions are added, the conductivity of the environment
increases.
 This facilitates the movement of ions and electrons between the
anode and the cathode.

Why does the anode "dissolve" over time?

In short: Because the anode is sacrificed – it oxidizes itself to protect the
metal.
More technical explanation:
 The anode loses electrons in the external circuit (oxidation).
 These electrons flow to the cathode and protect it from
corrosion.
 The atoms of the anode turn into ions and move into the
environment, meaning:
M (metal) → Mⁿ⁺ + n e⁻
So, the anode gradually “dissolves” — in other words, it disappears over
time.
Commonly used anodes:
 Magnesium
 Zinc
 Aluminum
These are active metals with a high anodic potential, meaning they oxidize
easily and protect the cathode effectively.

In the case of impressed current cathodic protection, a source of DC

current is installed in the system which provides additional energy to force
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. Graphite,
high silicon cast iron (HSCI), platinum, or mixed metal oxide are used as
anodes for impressed current cathodic protection systems as they are
slowly consumed. The anodes should be periodically inspected and
replaced if consumed.

Advantages of Impressed Current Cathodic Protection 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
the measurement of Instant OFF / ON. No limitation on driving Voltage.
Economically feasible to replace the anode system when required. The
system is extremely flexible. Limitations of Impressed Current Cathodic
Protection Systems Regular monitoring and maintenance required
Requires Main supply, or another source of electric Power Interference
Problems must be considered. Relatively large chance of premature failure
or breakdown.
1. Soil Environment
Environment: The soil is dry and poorly conductive.
What is added?
 Bentonite clay and water
 A mixture of gypsum–bentonite–sodium sulfate
These additives increase soil moisture and electrical conductivity.
  Electrons: Move from the anode toward the metal pipe
(cathode).
 Ions: Move within the soil, completing the circuit.
2. Freshwater Environment (rivers, lakes, etc.)
Environment: Water has moderate conductivity.
What is added?
 Sodium bicarbonate (baking soda)
 Salt (NaCl)
These additives enhance the water’s conductivity.
 Electrons: Similarly, mo.ve from the anode to the metal
surface.
 Ions: Freely move in the water, supporting the process.
3. Saltwater Environment (seawater)
Environment: Highly conductive due to dissolved salts.
No additives needed – seawater is already conductive.
Anodes used:
 Zinc
 Aluminum
These provide strong protection.
 Electrons: Flow from the anode to protect the metal.
 Ions: Move freely and rapidly in the saltwater.

Which environmental conditions accelerate the operation or

effectiveness of cathodic protection systems?
The effectiveness and speed of cathodic protection are significantly
influenced by the surrounding environment, particularly in high-
conductivity media, ion-rich conditions, and aggressive chemical
environments. For example, in saline environments such as seawater,
the high concentration of ions accelerates both the corrosion process and
the performance of the cathodic protection system, causing sacrificial
anodes to dissolve more rapidly. In moist and humid soils, the increased
water content enhances the soil’s electrical conductivity, facilitating the
movement of electrons from the anode to the cathode and thereby
improving system efficiency. Acidic and chemically aggressive
environments (with low pH) intensify electrochemical reactions, leading to
faster corrosion and requiring a more active cathodic protection response.
Similarly, high-temperature conditions accelerate chemical processes,
increasing both corrosion rates and the consumption rate of anodes.
Moreover, low-resistivity soils—those enriched with salts or carbon-
based materials—are highly conductive and provide ideal conditions for
cathodic protection, allowing the system to operate efficiently with minimal
energy. In such environments, it is crucial to select the appropriate
protection method (whether sacrificial anode or impressed current system)
and compatible anode materials to ensure both protection effectiveness
and the longevity of the system.

The electrolyte in a cathodic protection system is the medium that enables

ion exchange and the flow of electric current between the anode and the
protected metal. Without this medium, the protection process is not
possible.
The electrolyte environment is crucial for the effectiveness of this
process.

Different types of electrolytes affect the cathodic

protection process differently:
1. Soil Electrolytes
 Dry soil is a poor electrolyte, but moist, salty, or mineral-rich
soil conducts better.
 Used in underground pipelines.
 Higher salinity improves conductivity and protection
efficiency.
 Soil pH and moisture significantly affect protection.
2. Saltwater Electrolytes
 Very strong electrolyte.
 Common in ships, offshore platforms, underwater pipelines.
 Corrosion is rapid, requiring cathodic protection.
 Galvanic anodes dissolve faster.
 Chloride ions accelerate corrosion.
3. Freshwater Electrolytes
 Weaker electrolyte than saltwater.
 Used in drinking water systems and hydraulic structures.
 Requires higher protection potential.
 Passivation (oxide layer formation) on electrodes can reduce
protection efficiency.
4. Industrial and Chemical Electrolytes
 Variable composition.
 Used in chemical plants and storage tanks.
 Aggressive electrolytes can cause severe corrosion.
How can electrons protect metal from corrosion in cathodic
protection, but also produce light when flowing in electrical circuits?
Why do electrons have different roles in these processes?"
Electrons are the same particles, but in cathodic protection they move from
the anode to the metal surface and prevent corrosion by stopping the
chemical reactions that cause rust. In electrical systems, electrons flow
through wires carrying energy, which is then converted into light in a lamp.
The difference is that in cathodic protection electrons participate in a
chemical protection process, while in electrical systems they act as carriers
of energy.
Viscosity is a measure of a fluid’s resistance to flow or its “thickness.” It
describes how much a liquid or gas resists moving or sliding past itself. For
example, honey has a higher viscosity than water because it flows more
slowly.

Element Standard Electrode Chemical Activity

Potential (E⁰, V) (Tendency to Lose
Electrons)
Magnesium ~ -2.37 Very high, very active
(Mg)
Aluminium ~ -1.66 High activity
(Al)
Zinc (Zn) ~ -0.76 Moderate activity
Copper (Cu) ~ +0.34 Low activity (less tendency
to lose electrons)
What is the difference between the movement of electrons in
a cathodic protection system and in a standard electric circuit?
In an electric circuit, electrons move to deliver energy to a device, like a light
bulb, whereas in cathodic protection, electrons move to prevent corrosion of a
metal surface, such as a pipeline.
The source of electrons in an electric circuit is usually a battery or generator,
while in cathodic protection, the electrons come from a sacrificial anode (like zinc
or magnesium) or an external DC power source.
In an electric circuit, electrons flow through a closed loop, from the negative
terminal, through the circuit, and back to the positive terminal; in contrast, in
cathodic protection, electrons flow from the anode to the metal that needs
protection, without necessarily forming a closed power loop.
The driving force in an electric circuit is the voltage supplied by the power source,
but in cathodic protection, it’s the difference in reactivity (electrode potential)
between the anode and the protected metal.
In an electric system, electrons are used to do work, like lighting a bulb or turning
a motor, but in a cathodic protection system, electrons are used to neutralize
corrosion reactions, stopping metal atoms from turning into rust.
Also, unlike an electric circuit where no part is supposed to be consumed,
cathodic protection intentionally uses a sacrificial anode, which corrodes over
time to keep the protected metal safe.