Brief information on corrosion and its types
In today’s article ‘Brief information on corrosion and its types’ we are going to discuss the basic introduction of corrosion, types of corrosion, corrosion by gaseous environment, corrosion by liquids or solutions, under water corrosion, under ground corrosion, theory of corrosion by liquids or solutions, Electrochemical theory, mechanism of electrochemical theory, anodic reaction, Cathodic reaction etc in detail.
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Corrosion is exactly the reverse of extraction of metals and also known as weakening of metals. When metals are exposed to the environment, they are affected by environment and start to decay. Corrosion may be defined as “The gradual disintegration, decaying or away of a metal by chemical or electro-chemical reaction with its environment is known as corrosion”.
Some examples of corrosion are:
- Formation of greyish layer on silver.
- Rusting of iron
- Formation of greenish layer on copper and brass.
Types of corrosion
The Corrosion are mainly of two types in environment.
- Corrosion by gaseous environment
- Corrosion by liquids or solutions
1. Corrosion by gaseous environment– In this types main cause of corrosion is the presence of oxidants in the environment. These oxidants attack on the metallic surface and cause decay of the metal. As a result properties of metal change.
Theory of corrosion by gaseous environment – Main postulates of this they are-
- Gas diffuses in the metal to form a solution.
- Chemical products are formed on the metallic surface. For example formation of oxide film by oxygen, sulphide film by hydrogen sulphide and carbonate film by carbon dioxide.
Three types of corrosion products may form on the surface of metal.
(a) The corrosion product is unstable as:
M+ 1/2 O2 → MO
MO → M+ 1/2 O2
This will not cause corrosion of metal as in the case of platinum.
(b) Corrosion product is volatile at high temperature. This will cause more corrosion an in the case of Mo and W.
(c) Corrosion product is stable. Most of the metals have this type of corrosion.
If the thickness of the layer formed on the metallic surface is less than 300A^° then such layer is called a film and if thickness exceeds this value thin layer is called scale. This film may be stable, unstable, volatile or prous and such nature of the film decides the further action.
(2) Corrosion by liquids or solution
In this type, corrosion occurs due to chemical reaction between metal and surroundings. This is called immersed corrosion. In this reaction an electro chemical reaction takes place between metal and liquid surrounding it. This corrosion is affected by a number of factors like concentration of liquid, depth of immersion, amount of dissolved oxygen, temperature, amount of positive and negative ions, nature of products formed during corrosion, electrical conductivity of the medium, pH of medium, presence of colloidal particles etc.
Under water corrosion– This corrosion is due to water. For example rusting of water pipes, corrosion of water boiler etc.
Under ground corrosion– Corrosion of under ground water pipes or Corrosion of iron in underground concrete pipe are included in this type of corrosion.
Theory of corrosion by liquids or solutions
Electrochemical theory– This theory is based on Nernst’s theory according to which all metals have a tendency to pass into solution. The corrosion occurs due to the presence of separate anodic and cathodic parts between which current flows through the conducting solution. At anodic part, oxidation takes place. And hence anodic metal is destroyed by either dissolving or obtaining it in combined state.
Thus corrosion always occurs at anodic part.
Mechanism : Electrochemical corrosion involves flow of electrons from anodic to cathodic areas.
(1) Anodic reaction:– At the anode, metal atom lose their electrons and pass into the solution in the form of positive ions.
Fe(s) → Fe^2+ + 2e^- (oxidation) …(1)
This process continuous as long as the electrons and ions are removed from the environment. If the process ceases, corrosion does not proceed.
(2) Cathodic reaction :- The electrons released at the anode are conducted to the cathode and are responsible for reduction at cathode.
H^+ +e^- →1/2 H2
H^+ ions are produced either by water or any other acid present in the medium.
H2O→H^+ + OH^-
CO2 + H2O→H^+ + HCO3^-
The liberated H2 causes polarisation and prevent corrosion but atmospheric oxygen takes away this hydrogen and acts as depolariser.
4H^+ O2 + 4e^- →2 H2O (l) …(2)
On adding redox reaction (1) and (2) we have
2Fe(s) + 4H^+ O2→2Fe^2+ (aq) + 2H2O
Fe^2+ ions are further oxidised by atmospheric oxygen and form rust.
4Fe^2+ O2(g)+4H2O→2Fe2O3 + 3H^+
2Fe2O3 + xH2O →Fe2O3 + xH2O (rust)
Fe^2+ ions may react with OH- ions present in the solution and form Fe(OH)2
Fe^2+ +2OH^- →Fe(OH)2
Fe(OH)2 thus, formed further reacts with atmospheric carbon-dioxite and oxygen to form carbonates.
Fe(OH)2 +2H2O + O2(g)→4Fe(OH)3
And, Fe(OH)2+ CO2 →Fe(OH)(HCO3)
Fe(OH)3 CO2 →Fe(OH)2(HCO3)
Thus corrosion is an electrochemical phenomenon in which anode and cathode are on the surface and electrolyte (water, acid or solution) on the surface of metal allows the ionic flow.
The metals which are on the top of Electrochemical series are more prone to corrosion since they act as anodes for the metals below in the series. Thus Al should have more corrosion than Fe but practically it is not true. This is due to formation of protective oxide layer on the surface of aluminium which further prevents the corrosion protective oxide layer on the surface of aluminium which further prevents the corrosion.
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