Corrosion |Definition, Cause, Types & Protection

Corrosion is an unwanted, undesirable natural process which involves various chemical reactions resulting in the destruction of metal surface. The metals surface is gradually converted into their stable forms of oxides, carbonates, sulfide. It occurs due to the chemical or electrochemical reactions between metal surface and environmental factors like air, moisture, acidity etc. It is an example of redox type of reaction.

 

Table of Contents

Definition

Corrosion is defined in different ways. It is simply the undesirable deterioration of a materials. In scientific understanding, corrosion is the result of charge transfer from the electrified interface between metallic surfaces and environment.

IUPAC defines corrosion as "An irreversible interfacial reaction of materials (metallic, ceramic or polymer) with their environment which results in the consumption of materials"

Cause of corrosion

Corrosion is the deterioration of materials when external environmental factors come in contact with it. It causes loss of material, its lustre, beauty as well as economic loss. There are many factors which increases the rate of corrosion. They are:

  • Low pH
  • High temperature
  • Presence of electrolyte.
  • Surface of metal itself
  • Moisture
  • Scratch and cracks on metal surface

 

Types of corrosion

Corrosion is classified into different types based on the corroded substance, corrosive environment and mechanism.

Types of corrosion based on corroded surface

There are 8-types of corrosion based on corroded surface.


1. Uniform or general corrosion

It is the most common form of corrosion. It occurs uniformly on the available exposed surface. It is also called as general corrosion. It is the major and greatest types of corrosion that destruct metal especially steel. It contributes about 30% of total corrosion making the major type of cause of corrosion.


It is easier to handle and protect from these types of corrosion as compared to other. It is prevented and controlled by using surface coatings, inhibitors cathodic protection and recoating rusted surfaces. 

 

2. Pitting Corrosion

It is a localized form of corrosion which occurs by the formation of cavities and pits in the metals. These pits and cavities slowly decompose making an internal damage to the metallic structures like bridge, metal engines and other metal infrastructures. It is also difficult to identify due to small cavities and even hidden by corrosion produced compounds in the pits.

pitting corrosion

Pitting corrosion in steel plate Source: researchgate

These types of corrosion are fatal and more dangerous due to formation of internal cavities. It causes sudden and unexpected failure to the structures made with irons and steel.

 

3. Galvanic Corrosion
It is a widely known type of corrosion which occurs between dissimilar elements in presence of electrolyte. When 2 metals with different potentials such as copper (+ 0.334V) and iron (- 0.44V) are physically or electrically joined, a galvanic cell is formed.

galvanic corrosion

Image source: princeton

The corrosion which causes due to such process is called as galvanic corrosion. This type of corrosion is mainly found in metal structure found in marine areas and metal with salty water. The driving force for corrosion comes from the potential differences between the metals.

The more active metal acts as anode and corrode at higher rate while the lower active metals (i.e. Nobel metal) acts as cathode and corrodes at lower rate.

 

There are many factors which accelerates galvanic corrosion such as size & type of metals, surface ration of metals and external factors like humidity, salinity, temperature etc. It is prevented by use of sacrificial anodes.

 

4. Selective Leaching
Selective leaching is the removal of selective metals from alloys by means of corrosion process. This process is also called as dealloying, demetallification or parting. Some example of selective leaching are dezincification and graphitization etc.

 

5. Intergranular Corrosion

intergranular corrosion
It is a type of corrosion which occurs in the intergranular spaces present between the particles (grains) of a metallic material. Grains are the building block of metals and alloys and each grain is separated by respective grain boundaries. Intergranular corrosion is localized attack along the grain boundaries without affecting the bulk of the grains.

It is usually associated with chemical segregation effect or specific phases precipitated on the grain boundaries 

 

6. Crevice Corrosion
Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the micro-environment levels. It occurs in crevice or shielded areas such as those formed under gaskets, washers, insulation materials, lap joints, clamps etc. Due to the restriction of oxygen diffusion in the cervices, a differential aeration cell is set up between cervices and external surface.

Types of corrosion based on corrosive environment

There are 4-types on the basis of corrosive environment. They are atmospheric, aqueous, soil and concrete corrosion.

 

1. Atmospheric Corrosion

Atmospheric corrosion is the degradation of materials exposed to atmospheric air and its pollutants. It occurs when electrolytes such as humidity, rain, melting snow facilitates the degradation. It takes place in humid environmental conditions.

 

2. Aqueous Corrosion
It is a corrosion of materials which are immersed inside water. Water contains different types of impurities dissolve in it. These impurities attack on metal surface or on the crack parts leading to slowly degradation of the materials.

aqueous corrosion

Aqueous corrosion Image source: corrosionengineering

Dissolve oxygen, minerals ores, hardness of water, pH, temperature of water are responsible for corrosion of materials, pipe lines and equipment present inside water. Some microbes present in water are also involved in corrosion activity.

 

3. Soil Corrosion
Soil corrosion occurs in the buried steel, iron pipes and equipment that are used for underground supply of gas, petroleum products etc. Soil plays important role in corrosive due to the presence of moisture contents, pH, resistivity, chemical species micro-organism etc. in it.

 

All types of soil are generally corrosive due to the presence of these things in it. It is found that all types of materials corrode with approx. similar rate in any given soil sample. However, It’s rate may be different for different nature of soil.

Soil corrosion cause fatal destruction to underground metals. It Is also hard to deal and prevent materials from soil corrosion.

 

4. Concrete Corrosion

It is the corrosion of materials such as steel, iron etc. that are used in manufacturing building and bridges along with concrete. Concrete is complex composite & widely used for building purposes. Concrete corrosion occurs in all type of concrete structure such as bridges, building, highways, tunnels, dams etc.

Types of corrosion based on mechanism

There are 2-types of corrosion based on mechanism. Dry and wet corrosion

 

1. Dry Corrosion

dry corrosion

It is caused by the direct interaction of metals with atmospheric gases like O2, N2, H2S, halogens (X2) SO2, oxides etc. Out of which oxygen is the main atmospheric corrosion gas. When these gases are in close contact with metal surface, the surface slowly begins decolorizing leading to degradation. The decolorizing is due to the formation of metal oxides, halides or sulfide firms.

It generally begins in ambient temperature (temp. at which materials is kept) and accelerates with increasing temperature in dry environment.

 

2. Wet Corrosion
It is electrochemical in nature. It occurs in aqueous (wet) environments. The moisture present in wet environment when comes in contact with metal surface gets oxidized into metal ions. In this type of corrosion, wet air/moisture acts as cathode and reduced into hydroxide or hydrogen ions and finally into hydrogen gas. In this process, flow of electron is from anode (metal) to cathode.

Rusting of iron is an example of wet corrosion where iron is corroded to reddish bone rust in presence of moisture.

Corrosion Protection methods

There are various ways to control materials from corrosion. Some of them are:

  1. Using less corrosive metal like stainless steel, aluminum etc.
  2. Using a surface coating like grease, oil, paints or carbon fiber coating. 
  3. Lowering temperature and increasing pH of metal surroundings.
  4. Planting metal materials and industry in dry environment if possible.
  5. The major step to prevent from corrosion is surface coating of metal.
  6. Zn coating is effective way to prevent steel from corrosion.
  7. Nickel and Chromium coating are used in making smooth surface of steel parts.
  8. Lead coating is effective for materials exposed to H2SO4 environment. It is also effective in protecting automobiles and tanks.

References

1. https://en.wikipedia.org/wiki/Corrosion

2. https://www.usna.edu/NAOE/_files/documents/Courses/EN380/Course_Notes/Corrosion%20Basics.pdf

3. https://www.princeton.edu/~maelabs/mae324/12/introduction.htm

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