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Definition of what is corrosion

Definition of what is corrosion, The process of slowly turning metals into unwanted compounds by gases and moisture present in the atmosphere is called corrosion.

12th, Chemistry, Lesson-3


Metals The atmospheric moisture reacts with gases like CO₂, SO₂, H₂S, NO₂ etc. to form a layer of unwanted compounds, which spoils the surface of the metals. This action is called corrosion.

types of corrosion

Corrosion is divided into the following four parts, atmospheric, underground, underwater, and chemical corrosion.


Metals kept open in the atmosphere are affected by moisture, particles suspended in dust and other substances and active gases, for example CO₂, O₂, H₂S etc. The effect of moisture on a metal like iron produces hydroxide which later turns into an oxide.


It depends on the nature and concentration of ions present in the medium, the product as a result of corrosion, the conductivity of the medium, the concentration of dissolved oxygen, the temperature, the concentration of other salts formed and the pre-treatment of the metal. If the product formed by corrosion cracks, the new surface of the metal becomes available for corrosion.


If the amount of water is more than that of air and the metal is in close contact with water, for example in a pipeline, steam boiler, etc., then the corrosion of the metal is called water corrosion. The rust formed in it will be soft and uniform.


Corrosion of metal structures used in construction and pipelines used for water falls in this category. This corrosion is very similar to water corrosion. Its speed increases in the presence of active salts.

factors affecting corrosion

  1. If the value of electrode potential of metals is negative or less than 1.23 volts, then the total potential of that cell will be positive and corrosion of that metal will occur.
  2. Most metals react with oxygen to produce thin oxides. Due to which the new surface of the metal starts corrosive after coming in contact with the atmosphere. In addition, corrosion also depends on the solubility of metal salts.
  3. The corrosion of flat or smooth surface is comparatively less and the corrosion of rough curved metal is more. This is probably due to the high pressure on the metal atoms or molecules at these locations.
  4. If there is an impurity in the metal with a higher electrode potential value, then the corrosion of that metal will happen at a faster rate, because in this situation the metal starts acting as the anode and the impurity cathode, which increases the corrosion speed.
  5. Corrosion occurs quickly when the humidity in the atmosphere is high, H₂S, SO₂ etc.

corrosion prevention measures

  1. oil or greaseRust can be avoided by applying a thin layer of oil or grease to the metal surface.
  2. by applying paint varnishCorrosion can be prevented by painting on metals, due to which the metal does not come in contact with air and moisture.
  3. electroplating– The metal surface is coated with nickel or chromium by electroplating, which can prevent rusting.
  4. electrical protectionDiesel tanks, petrol tanks, which are buried under the ground, are joined by a bandage to a plate of a metal with a low ionization potential, such as Mg or Zn, so that the active metal continues to act while the metal in the tank remains safe.

chemical theory of rusting

According to this principle, the surface of impure iron behaves like an electrochemical cell. does Is. Such a cell is also called a corrosion cell. Pure iron acts as anode and impure iron as cathode in these cells.

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