Cathodic Protection
Cathodic protection (CP) is a technique used to control the corrosion of a metal surface by
making it the cathodic side of an electrochemical cell. The simplest method to apply CP is by
connecting the metal to be protected with another more easily corroded metal to act as the
anode of the electrochemical cell.1
The principle of cathodic protection is that an external anode is connected to the metal to be
protected, then a DC current is passed between the metals so that the protected metal
becomes cathodic.
The cathodic protection is mainly done by two ways:
1. Sacrificial Anode Method
2. Impressed Current Method
Sacrificial Anode Method
In this method, the metal structure that needs to be protected is attached with a more reactive
metal (Zn, Mg, Al). The sacrificial anode, as it is more electrochemically active, gets
corrodate instead of the protected metal.
This whole setup creates a galvanic cell where the sacrificial anode acts as an anode and the
protected metal acts as a cathode. This is a self regulatory system where there is no power
supply needed from the outside.
Fig: Sacrificial Anode Cathodic Protection
Impressed Current Method
In this method, the metal that needs to be protected is exposed to a low voltage DC current
supplied from an external power source. The protected metal is turned into a cathode by
1
Just What is Cathodic Protection? - Abriox
installing an inert anode (Graphite, Pt, Mixed metal oxides) near the protected metal and
supplying current to the protected metal through the inert anode. This current flow neutralizes
the electrochemical activity which causes the corrosion.
Fig: Impressed Current Cathodic Protection
Cathodic Protection Measurements
The process of assessing the effectiveness of cathodic protection systems is known as
cathodic protection measurement. It is done to see how well the cathodic protection system is
working and if it is not then what needs to be fixed. Some common protection measurements
are given below.
Potential measurement
Measures the electrical potential between the protected structure and the reference electrode.
Cu/CuSO4 is commonly used as the reference electrode. This process shows how effectively
the cathodic protection system is working.
Current measurement
This involves measuring the amount of protective current applied by the cathodic protection
system to ensure it is enough to protect targeted metal structure. It is done by monitoring the
current density and anode current output.
Voltage drop measurement
Voltage drop or IR drop measurement measures the lost voltage caused by the resistance
from the environment that affects the accuracy of the potential measurement.
Surface condition testing
To verify the performance of the cathodic protection system structure surface is tested for the
signs of corrosion.
Polarization decay test
It involves turning off the cathodic protection system and observing at which rate the
structure depolarizes. A slow decay rate means that the structure is protected well, while a
rapid decay means it is poorly protected.
Occurring Problems & Solutions in SACP
1. Uneven anode consumption
Areas of the structure lose protection by uneven corrosion of anode and subsequently wearing
off of some anode faster than the others.
Solution
Regular inspection, replacement, proper placement and distribution of anodes. This ensures
even consumption and uniform protection.
2. Insufficient protection
In large structures, one sacrificial anode is not enough to give proper protection which leads
to corrosion.
Solution
Using more anodes, or using a more electrochemically active metal, or using an impressed
current method can solve this problem.
3. Environmental factors
Environmental factors like salinity, soil resistivity (Dry soil), temperature reduces the
efficiency of the anodes.
Solution
Appropriate anode needs to be selected for specific environmental conditions.
4. Interference from nearby structures
Other nearby cathodic protection systems or power sources stray current can cause corrosion
instead of preventing it.
Solution
The system needs to be designed with adequate separation between structures by installing
barriers or ground beds and regularly monitoring the current distribution.
Occurring Problems & Solutions in ICCP
1. Power supply failure
Power supply failure can leave the protected structure vulnerable as impressed current
cathodic protection needs external power supply.
Solution
Using backup power supplies, alarms to detect the power supply failure quickly and regular
maintenance of the power supply system are solutions for this problem.
2. Over protection
Over protection as a result of excessive current can cause coating damage, hydrogen
embrittlement, or excessive evolution of hydrogen gas.
Solution
Controlling the current output, regularly monitoring the system, and automated control
systems can help maintain the optimal current levels.
3. Anode corrosion
If the system is not properly maintained, then the anodes can deteriorate over time.
Solution
Anodes need to be made from high quality corrosion resistant materials, while regularly
inspected and replaced if needed.
4. Stray currents
Localized corrosion or interference with the impressed current cathodic system can be caused
by the stray currents.
Solution
Proper shielding, grounding, or isolating methods need to be implemented. Further stray
currents need to be monitored and mitigated by adjusting the current output or using
insulating joints.
Anodic Protection
Anodic protection is a corrosion control method where the metal to be protected is
maintained in its passive state by applying a small, controlled electrical current, making it the
anode in an electrochemical cell. This is primarily used for metal like steel and alloys that can
form stable, protective oxide layers when passivated.
The principle of anodic protection depends on the concept of electrochemical polarization.
The metal that needs to be protected is polarized in the anodic direction pushing its potential
into a specific range where corrosion is minimized or halted. This specific range is known as
the passive region which is described by the formation of a stable, protective oxide film on
the metal's surface.
Fig: Anodic Protection
Below is given step by step of anodic protection method:
Initial Setup & Selection of Metal
Only metals like stainless steel and certain alloys are ideal for anodic protection as they can
be passivated. The ideal environment for anodic protection is acidic where sulphuric,
phosphoric acids and other aggressive chemicals are used.
Electrochemical Monitoring & Polarization
In anodic protection, the metal is made anode by applying a positive electrical potential from
a DC power source. This is known as polarization.The applied voltage is controlled to fall
within the passive region of the metal.
Passivation
When the applied potential falls within the passive region, then the metal transitions into the
passive state and forms a stable oxide film on the surface of the metal. This oxide film
prevents the metal from getting corroded by reducing the rate of corrosion. The supplied
current needs to be monitored as the oxide film forms the current needed to maintain the
protective state decreases.
Maintenance of the Passive State
A feedback mechanism is established to adjust the current based on the electrochemical
potential of the metal to constantly keep the metal in the passive state. Continuous monitoring
of the applied voltage and current is done.
Anode Placement
In anodic protection, the necessary current is supplied to the metal structure by placing inert
anode near the metal structure. This inert anode does not corrode themselves rather maintain
a current flow to keep the metal structure in the passive state.
System Design & Control
Based on feedback from the sensors monitoring the electrochemical potential of the metal,
automatic control circuits adjust the applied voltage and current which ensures that the
protection is always in the correct range.
Shutdown & Inspection
The system needs to be regularly inspected for the signs of passive film breakdown or over
protection.The system needs to be deactivated or shutdown with precise control to bring the
metal out of the passive state without breaking the oxide film formed on the metal surface as
uncontrolled shutdown can lead to abrupt changes.
Occurring Problems & Solutions in Anodic Protection
1. Over-protection
Applying too high a potential can push the metal into the transpassive state which can cause
damage to the oxide film causing localized, pitting,or stress corrosion.
Solution
The electrochemical potential can be maintained by a precise control system and ensured that
the potential stays in the correct passive range. Also the current output can be adjusted by
automated feedback mechanisms.
2. Under-protection
The metal can fall into the active corrosion state or can not maintain the passivation state if
the applied voltage is too low.
Solution
A precise control system, automated feedback mechanisms, regular inspection and
monitoring the current levels can help in this case.
3. Environmental factors
Changes in the surrounding environmental elements like temperature, pH, and concentration
of corrosive agents can alter the current and potential requirements affecting the passive state.
Solution
Sensors are installed to monitor the environmental factors and the applied potential is
adjusted to keep the metal in the passive state.
4. Power supply
Power supply issues can cause the metal to fall in the active corrosion state as anodic
protection needs continuous power supply.
Solution
Using backup power supplies, alarms to detect the power supply failure quickly and regular
maintenance of the power supply system are solutions for this problem.
5. Passive film breakdown
Due to mechanical damage or chemical attacks from the aggressive species the protective
oxide film can degrade over time.
Solution
The passive film needs to be regularly inspected and if damage is detected then the applied
potential needs to be adjusted, or protective measures like coating and inhibitors can be
introduced. Also the environment needs to be free from contaminants that could damage the
film.
6. Inadequate anode placement
The metal surface can be inadequately protected if the inert anode is positioned poorly.
Solution
The system needs to be designed with proper anode placement to ensure uniform current
distribution across the metal surface.
7. Stray currents
Localized corrosion or interference with the impressed current cathodic system can be caused
by the stray currents.
Solution
Proper shielding, grounding, or isolating methods need to be implemented. Further stray
currents need to be monitored and mitigated by adjusting the current output or using
insulating joints.
Corrosion in Industrial Boiler Plants
Corrosion in boiler plants can take various forms, each of which is driven by different
mechanisms and environmental factors. The main forms of corrosion that occur in boiler
systems include:
1. Uniform Corrosion
This is a general form of corrosion where metal surfaces are uniformly attacked across large
areas. Uniform corrosion occurs due to the presence of oxygen, carbon dioxide, or acidic
conditions in the boiler water, leading to the gradual thinning of metal surfaces. Boiler tubes,
water tanks, and steam lines can be affected by uniform corrosion.
Prevention
Proper treatment of water to control oxygen, CO₂, and pH levels.
2. Pitting Corrosion
Pitting is a localized form of corrosion that leads to the formation of small, deep pits on metal
surfaces. It is highly dangerous because it can cause significant damage with minimal surface
area affected. Pitting is typically caused by the presence of dissolved oxygen in the feedwater,
as well as chloride ions that break down protective oxide layers on metal. Mainly boiler
tubes, especially in areas of stagnant or low-flow water, get affected.
Prevention
Oxygen scavengers are used to remove dissolved oxygen and proper water treatment to avoid
chlorides.
3. Crevice Corrosion
This form of localized corrosion occurs in confined spaces or crevices, such as at joints,
flanges, or under deposits and gaskets. These areas trap water and chemicals, leading to
corrosion. Accumulation of water or chemicals in tight spaces, where oxygen levels can drop,
leading to differential aeration and accelerated corrosion. Affected areas are under washers,
between pipe flanges, and at joints.
Prevention
Proper design is ensured to avoid crevices, and uses of effective cleaning and maintenance
practices to remove debris and deposits.
4. Erosion-Corrosion
Erosion-corrosion is the combined action of mechanical wear (erosion) and chemical attack
(corrosion). This occurs when high-velocity water or steam strips away protective oxide
layers from metal surfaces, exposing fresh metal to corrosive agents. High water or steam
velocities, especially in areas with suspended solids or aggressive chemicals causes this.
Affected areas are elbows, bends, boiler tubes, and feedwater lines where fluid velocity is
high.
Prevention
Reduction of fluid velocity, installation of erosion-resistant materials, and minimization of
suspended solids in boiler water.
5. Stress Corrosion Cracking (SCC)
SCC is the cracking of metal caused by the combined effects of tensile stress and a corrosive
environment. It usually occurs in materials under high stress, particularly in the presence of
specific chemicals like chlorides or hydroxides. Affected areas are boiler tubes and
high-stress areas like joints and welds.
Prevention
Avoiding excessive stress, using corrosion-resistant materials, and maintaining proper water
chemistry (especially to control chloride levels).
6. Caustic Embrittlement
Caustic embrittlement, also known as caustic cracking, occurs when high concentrations of
sodium hydroxide (NaOH) build up in the boiler water, leading to localized cracking at
stressed areas. This happens due to improper water treatment or leakage. Affected areas are
boiler tubes, especially at welds or stressed points.
Prevention
Uses of phosphate treatments instead of sodium hydroxide for water treatment, and ensuring
even distribution of chemicals to avoid localized buildup.
7. Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in the presence
of an electrolyte (such as boiler water). The less noble metal (anode) corrodes more rapidly,
while the more noble metal (cathode) is protected. Affected areas are connections between
different metals, such as pipes and fittings.
Prevention
Uses of compatible metals or installation of insulating materials between dissimilar metals to
prevent electrical contact.
8. Hydrogen Damage (Hydrogen Embrittlement)
Hydrogen damage occurs when hydrogen atoms, formed during certain corrosion reactions,
penetrate the metal and cause embrittlement or cracking. Hydrogen is generated as a
byproduct of acid corrosion or in the presence of high-temperature water. It diffuses into the
metal, leading to cracking. Affected areas are boiler tubes, especially in high-temperature
areas.
Prevention
Maintaining proper water chemistry to prevent acid corrosion and minimizing hydrogen
generation.
9. Under-Deposit Corrosion
This form of corrosion occurs under deposits such as scale, sludge, or corrosion products.
These deposits trap water and chemicals, creating a localized corrosive environment. This
happens due to poor water treatment or lack of boiler blowdown. Affected areas are boiler
tubes and other areas where scale or deposits form.
Prevention
Regular blowdown to remove impurities, proper water treatment to prevent scale formation,
and frequent inspection and cleaning of the system.
10. Acid Corrosion
Acid corrosion occurs when acidic water or cleaning solutions are present in the boiler, either
due to improper water treatment or chemical cleaning. Affected areas are boiler tubes,
feedwater lines, and areas exposed to cleaning solutions.
Prevention
Monitoration of pH levels and uses of appropriate chemicals for water treatment. During acid
cleaning, inhibitors are used to protect metal surfaces.
11. Condensate Corrosion
This occurs when carbon dioxide (CO₂) dissolved in condensate forms carbonic acid, which
lowers the pH and leads to corrosion in condensate return lines. CO₂ is formed when
bicarbonates in the feedwater decompose under heat, and it dissolves in condensate to form
carbonic acid (H₂CO₃), leading to acidic conditions. Affected areas are condensate return
lines and steam traps.
Prevention
Uses of deaerators to remove CO₂ and addition of neutralizing amines to maintain the pH of
the condensate above 7.
12. Oxygen Pitting
This is a form of localized pitting corrosion caused by the presence of dissolved oxygen in
the feedwater. It leads to the rapid formation of pits that penetrate the metal surface. Affected
areas are feedwater systems, economizers, and boiler tubes.
Prevention
Deaeration of the feedwater to remove oxygen and addition of chemical oxygen scavengers
like sodium sulfite or hydrazine.
Fig: Corrosion in industrial boiler plants
Corrosion in Petroleum Industries
Corrosion in the petroleum industry occurs when metal equipment and structures come into
contact with corrosive agents present in crude oil, natural gas, and various process chemicals.
This leads to the gradual degradation of pipelines, storage tanks, refineries, and other
processing equipment, which can result in equipment failure, safety hazards, and economic
losses. Corrosion in the petroleum industry is driven by several factors, including the
presence of water, acids, sulfur compounds, CO₂, H₂S, and high operating temperatures.
Below are the forms of corrosion that occur in the petroleum industry:
1. Uniform Corrosion
This is the most common type of corrosion, where the metal corrodes evenly across the
surface, leading to a gradual thinning of the material. Uniform corrosion typically occurs due
to the presence of water, oxygen, CO₂, and sometimes chloride ions in crude oil or natural
gas.
Prevention
Proper water treatment, oxygen scavengers, and the use of corrosion inhibitors help prevent
uniform corrosion.
2. Pitting Corrosion
Pitting corrosion is localized and results in the formation of small but deep pits or holes on
the metal surface. It is often caused by the presence of chloride ions, sulfur compounds, or
acids like hydrogen sulfide (H₂S), which can break down protective oxide layers, leading to
localized corrosion.
Prevention
Use of high-quality alloys with good resistance to pitting, corrosion inhibitors, and proper
water treatment to reduce chloride concentrations helps to prevent this.
3. Crevice Corrosion
This form of corrosion occurs in confined spaces or crevices where oxygen levels are low,
such as under gaskets, flanges, and deposits. Crevices trap water and corrosive substances
like sulfur compounds and chlorides, creating a localized corrosion cell.
Prevention
Proper sealing, regular maintenance to avoid deposits, and the use of non-corrosive materials
in critical areas helps to prevent this.
4. Erosion-Corrosion
Erosion-corrosion is a combination of mechanical wear (erosion) and chemical corrosion. It
happens when high-velocity fluids or abrasive particles strip away protective oxide layers
from metal surfaces. The flow of crude oil, natural gas, or water with suspended solids at
high velocities accelerates erosion. This can occur in pipelines, valves, and pumps.
Prevention
Reducing fluid velocity, avoiding transporting solids, and using erosion-resistant materials
like hardened steel.
5. Stress Corrosion Cracking (SCC)
SCC occurs when a material under tensile stress is exposed to a corrosive environment,
leading to cracking and eventual failure. High levels of hydrogen sulfide (H₂S) in sour gas or
crude oil, combined with tensile stress, can lead to hydrogen embrittlement and cracking of
steel.
Prevention
Uses of corrosion-resistant alloys, stress-relieving techniques, and limiting H₂S content in gas
and crude oil helps to prevent this.
6. Sulfide Stress Cracking (SSC)
A specific form of stress corrosion cracking, SSC occurs in the presence of hydrogen sulfide
(H₂S) and is a major concern in sour service conditions. The combination of high levels of
H₂S and tensile stress causes steel to crack and fail.
Prevention
Uses of materials resistant to SSC (such as alloys designed for sour service), cathodic
protection, and controlling H₂S levels in the petroleum fluids helps to prevent this.
7. Microbiologically Influenced Corrosion (MIC)
MIC is caused by bacteria, such as sulfate-reducing bacteria (SRB), that produce corrosive
byproducts like hydrogen sulfide (H₂S) in the presence of organic matter and water. Bacteria
colonize metal surfaces, forming biofilms that trap moisture and corrosive chemicals, which
accelerate corrosion underneath the biofilm.
Prevention
Regular cleaning, the use of biocides, and controlling microbial growth through water
treatment and monitoring helps to prevent this.
8. Galvanic Corrosion
Galvanic corrosion occurs when two dissimilar metals are in electrical contact in a corrosive
environment, resulting in the more anodic metal corroding faster.
Prevention
Use of similar metals in contact or electrically isolating dissimilar metals with coatings or
using non-conductive materials helps to prevent this.
9. Naphthenic Acid Corrosion
Naphthenic acid corrosion occurs in high-temperature refinery processes and results from the
presence of naphthenic acids in certain crude oils. Naphthenic acids become more aggressive
at high temperatures (e.g., in distillation or catalytic cracking units), causing corrosion in heat
exchangers and furnace tubes.
Prevention
Uses of high-temperature resistant alloys, chemical treatment to neutralize acids, and
controlling the content of naphthenic acids in crude oil helps to prevent this.
10. High-Temperature Corrosion (Sulfidation)
Sulfidation corrosion is a high-temperature corrosion process in which sulfur compounds,
often found in crude oil, react with metal at elevated temperatures, forming metal sulfides.
Prevention
Use of high-temperature-resistant materials such as nickel-based alloys, controlling sulfur
content in the crude oil, and temperature regulation helps to prevent this.
11. Acid Corrosion
Acid corrosion happens when acidic compounds such as hydrogen sulfide (H₂S) or organic
acids (like naphthenic acids) are present in crude oil or natural gas. Acids react with metal
surfaces, leading to dissolution and degradation of the material.
Prevention
Use of acid inhibitors, corrosion-resistant materials, and regular monitoring of the pH of the
crude oil and gas helps to prevent this.
12. Under-Deposit Corrosion (UDC)
Under-deposit corrosion occurs when solid deposits like scale, sludge, or dirt accumulate on
metal surfaces, trapping water and corrosive substances underneath. These deposits, often
containing chloride or sulfur compounds, promote localized corrosion under the deposit.
Prevention
Regular cleaning of pipelines and equipment, proper filtration, and preventing solid deposits
from accumulating in the first place helps to prevent this.
13. Chloride Stress Corrosion Cracking (Cl-SCC)
Chloride stress corrosion cracking occurs when chloride ions (from seawater or process
water) combine with tensile stress, leading to cracking, especially in stainless steel and other
alloys, which is used in heat exchangers and other refinery equipment.
Prevention
Use of non-chloride-resistant materials or alloys, control of chloride levels, and stress-relief
techniques helps to prevent this.
14. Corrosion Fatigue
Corrosion fatigue occurs when a material is subjected to cyclic stress while also being
exposed to a corrosive environment, leading to the formation of cracks that grow over time.
Often seen in offshore platforms and subsea pipelines, where constant motion or pressure
variations, combined with corrosive seawater, cause material failure.
Prevention
Regular inspection, use of fatigue-resistant materials, and controlling operating conditions to
reduce stress on equipment helps to prevent this.
Fig: Corrosion in petroleum industries
Corrosion is a Reverse Process of
Extractive Metallurgy
The statement "Corrosion is a Reverse Process process of Extractive Metallurgy" highlights a
fundamental relationship between the two processes, both of which involve the interaction of
metals with their environment. Let's break down each of these concepts and see how they are
connected.
Extractive Metallurgy
Extractive metallurgy is the process of extracting metals from their ores, often through
chemical reactions like reduction (the removal of oxygen or other elements from the metal
compound) to obtain pure metals. The extraction of iron from iron ore (e.g., Fe₂O₃) in a blast
furnace, where carbon monoxide (CO) is used to reduce iron oxide into metallic iron (Fe):
Fe₂O₃ + 3CO → 2Fe + 3CO₃
Corrosion
Corrosion is the natural process where metals degrade or return to their original ore or oxide
form due to exposure to environmental elements like oxygen, water, and pollutants. It is
essentially the oxidation of metals. When iron rusts, it reacts with oxygen and moisture in the
air to form iron oxide (rust):
4Fe + 3O₂ + 6H₂O → 4Fe(OH)₃
The Reverse Process Concept:
Fig: Corrosion is a reverse process of extractive metallurgy
In Extractive Metallurgy
A metal is reduced from its ore, which involves removing oxygen (or other elements) from
the compound, using heat or chemical agents.
In Corrosion
A metal oxidizes, meaning it reacts with oxygen (and sometimes other elements like water or
sulfur) from the environment to form a metal oxide or a rust-like substance.
Thus, corrosion is the reverse of extractive metallurgy because:
Extractive metallurgy removes oxygen (or other elements) to obtain a pure metal. Corrosion
adds oxygen (or other elements) to the metal, causing it to return to an oxidized form (such as
an ore or oxide).
Example to Illustrate
Extractive Metallurgy (Iron): Iron ore (Fe₂O₃) is reduced in a blast furnace, removing
oxygen to obtain pure iron.
Corrosion (Rusting of Iron): Once the iron is obtained and exposed to moisture and oxygen,
it oxidizes to form iron oxide (rust), which is a form of iron ore again.
In essence, the corrosion process can be seen as the natural, inevitable "return" of metals to
their ore form, whereas extractive metallurgy is the process of "separating" metals from their
ores.
References
1. ScienceDirect.com
2. ResearchGate.com
3. link.springer.com
4. Abriox.com
