1. Electrochemical Protection or
Cathodic Protection
• Principle- To force the metal to behave like a cathode by
connecting it to a powerful anodic metal plate
• Two Kinds
Sacrificial anodic protection-
Metallic equipment to be protected is connected to a more
anodic metal (Zn, Mg..) (underground pipelines an be
protected)
Impressed current cathodic protection
• An electrical method, achieved by forcing a defined quantity of
direct current
• Positive terminal is connected to an insoluble anode like
graphite
1(underground pipelines can be protected)
3. Methods for application of Metallic Coating
• Hot Dipping
• Electroplating
• Metal spraying
• Metal cladding
• Cementation
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Prevention …
4. Hot Dipping
• Base Metals(Fe, Cu) having higher melting point are
immersed in the tank of molten metal like Zn, Sn, Pb, Al
etc having low melting point
– Coating of Zn on iron by hot dipping is called Galvanization.
– Coating of Sn on iron is called Tinning.
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5. Electroplating
• Deposition of corrosion resistant metal (Au, Ag, Ni, Cr, Zn,Pb) over
the base metal by passing electric current through an electrolytic
solution containing soluble salt of coating metals.
• Anode: rod of metal to be deposited
• Cathode: base metal (thoroughly cleaned)
• Electrolyte: metallic ions solution
• e.g. Electroplating of Silver on another metal
– At Anode- Ag (s) → Ag+ (aq) + e-
– At Cathode- Ag+ (aq) + e- → Ag (s)
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When current is passed metal ions from
electrolyte deposit on cathode and equal
number of metal ions are added to the
electrolyte from anode
7. Metal Spraying
• Spraying guns - for coating of metals.
• Zn, Al, Sn, Pb (low melting) are sprayed on the base
metal in molten state
Wire gun method- wire of metal to be coated is melted by
oxy-acetylene flame and blast of compressed air release
molten metal as spray.
Advantage: Can be done on rough surfaces and required
thickness is easily obtained
Technique can be used where metal dipping is not possible
like bridges.
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9. Metal-Cladding
Process in which thin layer of coating metal get bonded
to base metal on one or both sides permanently when
they are passed through heavy rollers under the action of
heat and pressure
Application : In aircraft industry, petroleum, equipments used for handling of
chemicals
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10. Some non-ferrous metals are particularly resistant to
corrosion, e.g. Cu and Zn
They form strong oxides on their surfaces (as do aluminium and lead)
and these protect the metal from further oxidation.
Cladding on the buildings above
Copper Cladding Zinc Cladding
Steel Cladding
11. Cementation
• Base metal is heated after packing it in powder of
coating metal in a box.
• Heating diffuses the coating metal into base metal
forming an alloy
• Thickness of coating is controlled by varying the temp
and time of treatment
– Coating of Cr over base metal: Chromising
– Coating of Si over base metal : Siliconising
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12. Organic Coatings• Paints
• Varnishes
• Enamels
• Lacquers
Mechanical dispersion mixtures of finely divided solids called
pigments in a fluid medium.
Pigments e.g. Cr2O3 for Green Pigment, ZnO for white pigment
•Fluid medium consist of non-volatile film forming
material called drying oil (glyceryl esters of higher fatty
acids) and a highly volatile solvent called thinner
(kerosene, benzene to reduce the viscosity)
Paints
13. Drying oil is used to hold the pigment on metal
surface by forming a protective film
Plasticizers can be added in paints to provide elasticity to film
and to minimize the cracking of film
14. • Colloidal solution of Natural & Synthetic resins
– RESIN- Cross linked, high mol weight organic polymers
– Natural: Rosin C15H20O6 - a solid form of resin obtained from pines and
some other plants
– Synthetic: Phenol formaldehyde
Varnishes
Enamels
Dispersion of pigments in a varnish- provides lustrous finish
Lacquers
Colloidal dispersion of a solution of cellulose derivatives
like cellulose nitrate, resins & plasticizers in a solvent like
acetone Provides a water proof film
