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Causes of Corrosion

The science of corrosion is basically directed towards gaining the basic knowledge in science so that we understand the process of corrosion and the mechanism which is involved in this chemical change.

The engineering of corrosion involves both scientific knowledge and its application for preventing corrosion and the various methods that helps in reducing the process.

The science of corrosion and the engineering involved are complementary and help in preventing the process and its most effective when both the science and engineering work in tandem to reduce the bad effects.

The reasons why one should read about corrosion revolve around the four major factors which affect our daily life.

Before we understand the causes of corrosion we need to understand its importance.

  • The cost involved in corrosion and its prevention
  • Conserving materials which are prone to corrosion
  • The safety which involves human and other life forms
  • The challenges involved in the phenomenon which is both difficult to fathom and interesting
Corrosion by the simplest definition is the process of a metal which returns to the materials thermodynamic state. So the formation the oxides oxides or even the sulphides from which the metals originate when they are taken out from earth before getting refined into useful engineering materials.


General Cause of Corrosion

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These are caused by electro chemical reactions which follows the law of thermodynamics which establish the corrosion reactions or rates are affected by ion and corrodent concentrations. This explain the idea of why some of the reactions involving corrosion are reversible and controllable and in other cases these are not. Corrosion in aqueous solutions is the most common form of corrosion process and hence water, sea water and other mediums provide the aqueous forms.

The moisture in atmosphere and water in soil account for the aqueous corrosions. Water is never present in pure form as water without mixed salts won’t allow the transfer of ions and thus will never support the process of corrosion. The gases and salts dissolved in water and their respective dissociation helps the water in conduction and hence chemical nature of the electrolyte could either become acidic, alkaline or neutral.

The basic corrosion reaction involves the oxidation of a pure metal when these are exposed to strong acids.

The reaction could be written as follows.
$Fe + 2 HCl \rightarrow FeCl_2 + H_2$

$Fe + 2 H^{+} + 2 Cl^{-} \rightarrow Fe^{2+} + 2 Cl^{-} + H_2$ 

The iron gets converted to iron ion by giving up two electrons (process of oxidation) which are basically picked up by hydrogen ions. By the gain of electrons the hydrogen ion was reduced and formed hydrogen gas.

The transfer of electrons takes place on metal surface. The spots where the electrons are given up are called the anodes while the spot where the electrons are given up are known as cathodes. The difference in the electrode potential which exists between these two spots gives rise to electrical circuit.

The negatively charged electrons flow in the direction of cathode from anode while the positively charged hydrogen ions move towards cathode to complete the circuit. The higher the flow of current the faster the dissolution of metal or corrosion occurs.

Anodic reactions:
The reactions at anode in metallic corrosion are simple and metals are oxidized to higher valence state. This results in formation of metallic ions. Metals which show multiple valence states exhibit several stages of oxidation during the process of corrosion.

Cathodic reactions:
These are difficult but are categorized into five types of reduction reactions.

The evolution of hydrogen                                                  
$2 H^+ + 2 e_-  \rightarrow H^2$ 

Metal ion reduced
$M^{3+} + e^-  \rightarrow  M^{2+}$

Oxygen reduction in acids
$O_2 + 4 H^- + 2e^- \rightarrow 2 H_{2}O$

Metal deposition
$M^{2+} + 2e^{-} \rightarrow M$

Oxygen reduction neutral solutions
                                                                                  $O_2 + 2H_2O + 4e^- \rightarrow 4 OH^-$

Factors that Cause Corrosion

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The general cause of intergranular corrosion is mainly due to the presence of galvanic elements which results in difference in concentration of impurities or the elements responsible for alloying.
  • Impurities segreggated to the grain boundaries
  • Larger amount of dissolved alloy element at grain boundaries
  • Smaller amount of dissolved element of alloying at grain boundaries

Corrosion Protection

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The corrosion prevention is basically aimed at removing or reduces the effect of the conditions which leads to corrosions.
  • Selection of material that does not corrode in actual environment
  • By removing the oxygen or adding some anti-corroding chemicals
  • Developing designs that help prevent corrosion
  • Changing the potential by making the metal more negative and help natural metal ions tendency to move from metal to environment
  • By applying coatings or anti-corrosion coating on metal surfaces in order to create a barrier between the metal and the corroding environment

What Causes Battery Corrosion?

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During summer the extreme heat usually accelerates the corrosion in batteries and causes the battery fluid to evaporate faster. This leads to shortening of the battery life. 

Rust Prevention Methods

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These are nothing but methods which help in reducing the contact between the metal surface and moisture laden environment. There is several advance technology which are now employed to prevent waste in the metal and drain the economy of such wastage which not only put strain on running cost but also put human life in danger.
  • Galvanizing methods
  • Painting
  • Oiling the surface to prevent the moisture to attack the metal surface
  • Anodizing the metal surface
  • Using of anti-corrosive designs

What Causes Corrosion of Steel?

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Corrosion of steel structures in soils is basically a electrochemical process and in nature it’s caused by the presence of oxygen and moisture in the soil. The process of corrosion is most likely to occur at or may be above the water table in any stratified abnormal soil with low resistivity.

The stray electrical currents that flow in the ground from sources like transit systems could also become the reason behind the corrosion of steel structures.

Anti-Corrosion Coating

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The anti-corrosion coating acts a as a barrier between the metal and atmospheric corrosive materials. The list of coating that helps against corrosion of metals
  • Fluoro-polymer basically a resin based material provides very good protection against corrosion.
  • PTFE materials help in resisting high temperatures and are non-sticking coating
  • Epoxy materials helps against abrasion as well as corrosion
  • Polyurethane helps in against abrasion and oxidizing agents and is basically an epoxy and zinc coating
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