Factors affecting the rate of corrosion

1. Nature of metal
2. Nature of the corroding environment
3. Solution pH
4. Oxidizing agent
5. Temperature
6. Velocity
7. Surface films
8. Other factors
Factors affecting the rate of corrosion
1. Nature of the metal

• Position in the Galvanic series

• Relative areas of the cathodic and anodic parts
• Purity of metal (tiny electrochemical cells)
• Physical state of the metal (grain size, crystal faces)
• Nature of the surface film
• Passive character of the metal
• Volatility of the corrosion product
• Solubility of the corrosion product
Factors affecting the rate of corrosion
2. Nature of the corroding environment

• Temperature
• Humidity of air
• Presence of impurities in atmosphere
• Nature of ions present in environment
• Conductance of corroding medium
• Amount of oxygen in atmosphere
• Velocity of ions which flow in the medium
• pH value of the medium
• Suspended impurities
 Factors affecting the rate of corrosion
3. Solution pH

• Metals such as Fe dissolve rapidly in acidic solution. In the middle pH

range the concentration of H+ ions is low. Hence, the corrosion rate
is controlled by the rate of transport of oxygen.

• Certain amphoteric metals dissolve rapidly in either acidic or basic

solution. e.g. Al and Zn.

• Noble metals are not affected by pH e.g. Au and Pt.

• H+ ions capture electrons and promote anodic corrosion.

 Factors affecting the rate of corrosion
4. Oxidizing agents

• Oxidizing agents accelerate the corrosion of one class of materials,

whereas retard another class.
• Oxidizing agent retard corrosion due to formation of surface oxide
films, which makes the surface more resistant to chemical attack.
• Thus a balance between the power of oxidizing agent to preserve the
protective layer and their tendency to destroy the protective film
determine the corrosion of metal.
Factors affecting the rate of corrosion
5. Temperature
• Rise in temp increase rate of corrosion.
• Increase in temp reduce the solubility of oxygen or air. The
released oxygen enhances the corrosion.
• Increase in temp induces phase change, which enhance the
rate of corrosion.
 Factors affecting the rate of corrosion
6. Velocity
• High velocity of corrosive medium increases corrosion.
• Corrosion products are formed rapidly, because chemicals are
brought to the surface at a high rate.
• The accumulation of insoluble film on the metallic surface is
prevented. So corrosion resistance of these films decreases.
• The corrosion products are easily stifled and carried away, thereby
exposing the new surfaces for corrosion.
 Factors affecting the rate of corrosion
7. Surface Films
• The oxide films are formed on the surface of S.S. these films absorb
moisture, which delay time of drying and hence increases the
extent of corrosion.
• Insoluble slats such as carbonates and sulphates may be
precipitated from hot solution on the metal surfaces. These
protects the metal surfaces.
• If the film is porous (e.g. ZnO) corrosion continues. Nonporous
films (CrO on iron) prevents further corrosion.
• Oil and grease films may occur on the surface either intentionally
or naturally. These films protect surface from direct contact with
corrosive substance. e.g. metals submerged in sewage .
Corrosion control methods
• Selection of metals
• Proper design
• Use pure metal
• Modifying the environment
• Sacrificial anodic protection
• Corrosion inhibitors
• Surface coatings (Paints, Electroplating, etc.)
Corrosion control methods
1. Proper design
• Minimal contact with medium
• Prevention from moisture
• Adequate ventilation and drainage
• Welding
• Avoid cervices between adjacent parts
• Bend should be smooth
• Bimetallic contacts should be avoided
• Paint cathodic portion
• Prevent uneven stress
Corrosion control methods
2. Modifying the environment
The corrosive nature of the environment can be reduced either
1. By the removal of harmful constituents
2. By the addition of specific substances which neutralize the effects of
corrosive constituents of the environments
Deactivation involves the addition of chemicals, capable of
combining with the oxygen in aqueous solution by using
• Sodium sulphite
𝑁𝑎2 𝑆𝑂3 + 𝑂2 → 2𝑁𝑎2 𝑆𝑂4
• Hydrazine hydrate
𝑁2 𝐻4 + 𝑂2 → 𝑁2 + 2𝐻2 𝑂
Also,
• Dehumidification: Silica gel, Alumina
• Alkaline neutralization: NH3, NaOH
Corrosion control methods
3. Cathodic protection
The method of protection given to a metal by forcibly making it to
behave like a cathode

Two methods –
1. Sacrificial anodic protection
2. Impressed current cathodic protection
 Corrosion control methods
3. Cathodic protection
The method of protection given to a metal by forcibly making it to
behave like a cathode

1. Sacrificial anodic protection:

In this method of protection , the metallic structure to be
protected called “base metal” is connected to more anodic
metal through a wire.
2. Impressed current cathodic protection:
In this method an impressed current is applied in the
opposite direction to nullify the corrosion current, thus the
anodic corroding metal becomes cathodic and protected from
corrosion.
Corrosion control methods

Cathodic protection of a buried steel pipeline:

(a) A sacrificial magnesium anode assures that the galvanic cell makes the
pipeline the cathode.
(b) An impressed voltage between a scrap iron auxiliary anode and the
pipeline assures that the pipeline is the cathode.
Corrosion control methods
3. Cathodic protection
Examples: Mg sacrificial anode of an iron storage tank

Anodic protection (AP) is a technique to control the corrosion of a

metal surface by making it the anode of an electrochemical
cell and controlling the electrode potential in a zone where the
metal is passive.
Corrosion control methods
3. Cathodic protection
Examples: Zinc is attached to the steel hull of the vessel
Impressed current cathodic protection (ICCP) is a corrosion protection system consisting of
sacrificial anodes connected to an external power source. The external power source, often a DC
power supply, provides the current necessary to drive the electrochemical reaction required for
cathodic protection to occur.

In the absence of a DC power source, a transformer-rectifier connected to AC power can be used.

Alternative sources may also be used to power the electrochemical cell, including wind, solar or
gas-powered thermoelectric generators.

Impressed current cathodic protection systems are typically used in relatively large structures,
where passive cathodic protection methods are ineffective or impractical. For example, passive
cathodic protection is suitable for protecting individual structural members and appurtenances.
However, this method generates minimal current, making it ineffective for protecting larger
structures such as oil and gas pipelines and storage tanks. Rather than depend on the natural
potential difference between the anode and cathode material, ICCP systems use external sources
to provide the necessary current. This increased current makes it possible for the cathodic
protection to span a wider effective area.

While ICCP systems are more effective than their passive counterparts, they require more
equipment and are also costlier to maintain.
Corrosion control methods
3. Cathodic protection
Examples:
Corrosion control methods
Corrosion inhibitors
A substance which when added in small quantities to the
aqueous corrosive environment, effectively decrease the
corrosion of a metal.

Anodic inhibitors:
These inhibitors avoid the corrosion reaction occurring at the
anode, by forming the sparingly soluble compounds. They are
adsorbed on the metal surface , forming a protective film or
barrier, thereby reducing the corrosion rate
Ex: Chromates , Phosphates , Tungstates
Corrosion control methods
Corrosion inhibitors
Cathodic inhibitors:
In acidic medium corrosion may reduced by:
2𝐻 + 𝑎𝑞 + 2𝑒 − → 𝐻2 (𝑔)
1. Slowing down the diffusion of H+ ions by adding organic inhibitors like
amines, mercaptans, heterocyclic nitrogen compounds.
2. By increasing the over voltage of hydrogen evolution by adding inhibitors
like Antimony and Arsenic oxides
In neutral medium corrosion may reduced by :
1
𝑂2 𝑔 + 𝐻2 𝑂 𝑎𝑞 + 2𝑒 − → 2𝑂𝐻−
2
1. Eliminating oxygen from the corrosion medium by the addition of Na2SO3,
Na2S ;
Corrosion control methods
Electroplating
Electroplating is a process of depositing a thin layer of a fine and
superior metal (like Cr, Zn, Ni, Au etc.) over the article of a baser
and cheaper metal (like iron), with the help of electric current.

Popular methods:
• Electroplating with Ni
• Electroplating with Cu

**Role of pH in electroplating
 Corrosion control methods
Electroplating – Steps:
• Before electroplating the metal surface is cleaned thoroughly.
Firstly, an alkaline solution is used to remove grease and then it
is treated with acid to remove any oxide layer. It is then washed
with water.
• The article to be electroplated is made cathode since metallic
ions are positive and thus get deposited on the cathode.
• The anode is made of pure metal, which is to be coated on the
article.
• The electrolyte is the salt of the metal to be coated on the
article.
• A direct (D.C.) current is passed through the electrolyte. The
anode dissolves, depositing the metal ions from the solution on
the article in the form of a metallic coating.
Corrosion control methods
Electroplating with Ni:
• Electrolyte - Aqueous solution of nickel sulphate
• Cathode - Article to be electroplated (nail)
• Anode - Block of nickel metal.
• Dissociation of nickel sulphate: 𝑁𝑖𝑆𝑂4 → 𝑁𝑖 2+ + 𝑆𝑂42−
Corrosion control methods
Electroplating with Ni:
 Galvanization

Galvanization is a process used to protect iron and steel from corrosion. It

involves coating the metal with a layer of zinc, which serves as a sacrificial
anode, preventing the underlying metal from rusting or corroding when
exposed to moisture or other corrosive elements. The process is named after
Luigi Galvani, an Italian scientist.
 Applications

The zinc coating provides excellent corrosion protection to the underlying

metal because zinc is more reactive than iron or steel. When the galvanized
metal is exposed to moisture or corrosive substances, the zinc corrodes
sacrificially, meaning it reacts with the corrosive elements instead of the iron or
steel beneath it. This sacrificial protection extends the life of the metal and is
commonly used in a variety of applications, such as construction materials,
automotive parts, and outdoor structures like fences and street lamps.
 Corrosion control methods

Metals Coatings
• It is possible to coat metals that are susceptible to destructive
corrosion with metals that will protect them from further corrosion.
• Probably the best-known coated material is galvanised steel, which
is steel coated with zinc.
• Zinc is a good choice for two reasons: it forms a passive layer that
prevents further corrosion, and it is anodic when compared to steel,
so in the event of the underlying steel becoming exposed the zinc
will still corrode to protect the steel.

Powder Coating: A layer of acrylic, vinyl, epoxy or other substances will

prevent moisture from reaching the metal, thereby preventing rust
galvanization protects steel from corrosion results from its physical and chemical

properties. In most corrosive environments, zinc exhibits a lower redox potential

than iron. If electrically conductive bonding is established between iron and zinc, in

the presence of an electrolyte such as moisture, zinc acts as a sacrificial anode.

Zinc ions are released from the zinc anode and are attracted to the iron cathode

Iron passes into the state of immunity and does not corrode. However, every

coating is affected by various integrity defects and pores through which moisture

penetrates to the substrate. Therefore, the effect of cathodic protection is important

even without visible coating disruptions.

Corrosion control methods
Metals Coatings
Electrode potential
The potential difference developed when an electrode of an
element is placed in a solution containing ions of that element.

The potential difference between an anode and a cathode can be

measured with a voltage measuring device. The absolute potential
of the anode and cathode cannot be measured directly. Defining a
standard electrode, all other potential measurements can be
made against this standard electrode. If the standard electrode
potential is set to zero, the potential difference measured can be
considered as the absolute potential.
How to Prevent Rusting of Iron?
Corrosion control methods
1. Proper design
Corrosion control methods
1. Proper design