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A simple view on a complex matter


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CORROSION “ A Simple view on a complex matter”:

CORROSION “ A Simple view on a complex matter” J.Saranya Asst professor in Chemistry Department of S & H

Definition of Corrosion:

Definition of Corrosion “ The process of decay of metal by environmental attack” Metals exposed to atmospheric gases, water and liquid medium. Converted to their ores. Eg: Rusting of iron

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Effects of corrosion Reduced Strength Life time is reduced Metallic properties are lost Wastage of metal

Theories of Corrosion:

Theories of Corrosion 1. Dry or Chemical corrosion Metal exposed to atmospheric gases in the absence of moisture Mechanism: Corrosion by Oxygen (Metal) M  M n+ + n e - n e - + O 2  n O 2- M n+ + n O 2-  nMO (Metal Oxide)

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Al, Pb, Cu, Cr, etc. form stable oxide layer Li, Ca, Na, K, etc. form unstable oxide layer Mo, Sn, etc. form volatile oxide layer Pt, Au, Ag – no corrosion Corrosion by Chlorine: Ag + Cl 2  2AgCl (ppt)

Wet or Electrochemical corrosion:

Wet or Electrochemical corrosion Liquid is in contact with a metal (b) Dissimilar metals or alloys are immersed partially or completely in a solution

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Eg: Fe  Fe 2+ + 2e - In acidic solution: 2H + + 2e -  H 2 In basic solution: O 2 + 2H 2 O + 4e -  4OH - Fe 2+ + 4OH -  2Fe(OH) 2 In neutral solution: 2H 2 O + 2e -  2OH - Fe 2+ + 2OH -  Fe(OH) 2

Anodic & Cathodic Reactions:

Anodic & Cathodic Reactions

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Differential aeration corrosion: Metal is partially dipped in a solution less aerated area – corosion Eg: Wire mesh Accumulation of dirt, sand, scale, etc. Pitting corrosion: Surface roughness, scratches, stresses, etc.

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Caustic Embrittlement: Na 2 CO 3 + H 2 O  2NaOH + CO 2 2NaOH + Fe  Na 2 FeO 2 Na 2 FeO 2 + H 2 O + O 2  12NaOH + 2Fe 3 O 4

Factors influencing corrosion:

Factors influencing corrosion Nature of metal: (a) Purity (b) Surface film (c) Volatility of the corrosion product (d) Solubility of the corrosion product

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2. Nature of Environment: (a) Temperature (b) Humidity (c) Impurities (d) pH

Corrosion control methods:

Corrosion control methods Proper design Selection of metals Modifying the environment Sacrificial anodic protection Corrosion inhibitors Surface coatings (Paints, Electroplating, etc.)