Electrochemical Protection or Cathodic Protection

• Principle- To force the metal to behave like a cathode byconnecting it to a powerful anodic metal plate

• Two KindsSacrificial anodic protection-

Metallic equipment to be protected is connected to a moreanodic metal (Zn, Mg..) (underground pipelines an beprotected)Impressed current cathodic protection

• An electrical method, achieved by forcing a defined quantity ofdirect current

• Positive terminal is connected to an insoluble anode likegraphite

1(underground pipelines can be protected)

Cathodic Protection

Sacrificial anodic protection Impressed current cathodic protection

Methods for application of Metallic Coating

• Hot Dipping

• Electroplating

• Metal spraying

• Metal cladding

• Cementation

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Prevention …

Hot Dipping• Base Metals(Fe, Cu) having higher melting point are

immersed in the tank of molten metal like Zn, Sn, Pb, Aletc 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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Electroplating• Deposition of corrosion resistant metal (Au, Ag, Ni, Cr, Zn,Pb) over

the base metal by passing electric current through an electrolyticsolution 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 fromelectrolyte deposit on cathode and equalnumber of metal ions are added to theelectrolyte from anode

ElectroplatingBefore - dull

After - shiny6

Metal Spraying• Spraying guns - for coating of metals.

• Zn, Al, Sn, Pb (low melting) are sprayed on the basemetal in molten state

Wire gun method- wire of metal to be coated is melted byoxy-acetylene flame and blast of compressed air releasemolten metal as spray.

Advantage: Can be done on rough surfaces and requiredthickness is easily obtained

Technique can be used where metal dipping is not possiblelike bridges.

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Wire gun

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Metal-CladdingProcess in which thin layer of coating metal get bonded

to base metal on one or both sides permanently whenthey are passed through heavy rollers under the action ofheat and pressureApplication : In aircraft industry, petroleum, equipments used for handling of

chemicals

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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

Cementation• Base metal is heated after packing it in powder of

coating metal in a box.

• Heating diffuses the coating metal into base metalforming an alloy

• Thickness of coating is controlled by varying the tempand time of treatment

– Coating of Cr over base metal: Chromising

– Coating of Si over base metal : Siliconising

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Organic Coatings• Paints• Varnishes• Enamels• Lacquers

Mechanical dispersion mixtures of finely divided solids calledpigments in a fluid medium.

Pigments e.g. Cr2O3 for Green Pigment, ZnO for white pigment

• Fluid medium consist of non-volatile film formingmaterial called drying oil (glyceryl esters of higher fattyacids) and a highly volatile solvent called thinner(kerosene, benzene to reduce the viscosity)

Paints

Drying oil is used to hold the pigment on metalsurface by forming a protective film

Plasticizers can be added in paints to provide elasticity to filmand to minimize the cracking of film

• 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 andsome other plants

– Synthetic: Phenol formaldehyde

Varnishes

EnamelsDispersion of pigments in a varnish- provides lustrous finish

LacquersColloidal dispersion of a solution of cellulose derivativeslike cellulose nitrate, resins & plasticizers in a solvent likeacetone Provides a water proof film