What is Corrosion and how it is dangerous to metals?

The term corrosion is defined as an act or process of gradual wearing away of a metal due to chemical or electro-chemical reaction by its surroundings such that the metal is converted into an oxide, salt or some other compound. Generally corrosion is defined as the decaying of a metal due to oxidation or reaction with moisture or water. A substance known as rust results from such a process. We can also said that in other word that, The corrosion indicates the deterioration and loss of material due to chemical attack.
Corrosion in a pipe

Generally Corrosion divided into two category:-

  1. Dry corrosion
  2. Wet corrosion
  • Dry corrosion is chemical reaction between metal and gas or liquid in the absence of electrolytes is known as the dry corrosion. An electrolyte is a substance that dissociates into ions in solution or when Fused, Thereby becoming electrically conducting.

  • Wet corrosion is the electrolyte causing corrosion is an aqueous solution of acid, salt or alkali, it is known as the wet corrosion.
                      If the corrosion is localized, severe cavities or depressions may lead to the formation of holes. Such a condition may prove disastrous in case of liquid vessels because extensive corrosion reduces mechanical strength of the section. The corrosion has been a serious problem and its effects are to be minimized.

Causes of corrosion:-
  • Congested reinforcement in small concrete sections.
  • Excessive water cement ratio.
  • Improper construction method.
  • inadequate design procedure.
  • Incompetent supervising staff or contractor.
  • Rest metal in open area or in contact with water.
  • Poor workmanship.
  • Presence of moisture.
  • Presence of salts.
  • Types of atmospheric conditions surrounding the region.
Factors affecting corrosion:-
  • Blow holes, inclusions and trapped gases.
  • Chemical nature of metal.
  • Distribution of secondary phases.
  • Internal structure of metal.
  • Impurities in metal during manufacturing.
  • Nature of engineering application.
  • Presence of dust, dirt or other foreign matter.
  • Working condition or environment like temperature, concentration of stresses, etc. 

Effects of corrosion on reinforcing bars in concrete:-

                  The action of corrosion especially on the steel used as reinforcement in concrete is very slow and except under exceptional circumstances, such corrosion does not decrease the life of concrete member. It should however be remembered that action of corrosion becomes more intensive when it is combined with adverse effects of internal and external stresses.
                   One most important effects of corrosion is the formation of cracks and these cracks usually progress or advance most rapidly where shearing stresses are the greatest and where slipping occurs due to loss of bond.

Corrosion of ferrous metals:-

                    The main constituent of the ferrous metal is iron. The term rusting is sometimes used to refer the corrosion of ferrous metals. The ferrous metals corrode most easily.

                    The three important ferrous metals are cast iron, wrought-iron and steel. It is observed that rusting of cast iron is less, that of steel is much more and that of wrought-iron is medium.

Corrosion of non-ferrous metals:-

                    There are two reason due to which corrosion is developed in non ferrous metals:-
  1. Contact between unprotected dissimilar metals where moisture is present,
  2. Contact with masonry, lime, cement and some varieties of timber which may release acids, alkalies or salts when damp. 

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