Methods to prevent corrosion

1. Electrochemical Protection or
CathodicProtection
•Principle-Toforcethemetaltobehavelikeacathodeby
connectingittoapowerfulanodicmetalplate
•TwoKinds
Sacrificialanodicprotection-
Metallicequipmenttobeprotectedisconnectedtoamore
anodicmetal(Zn,Mg..)(undergroundpipelinesanbe
protected)
Impressedcurrentcathodicprotection
•Anelectricalmethod,achievedbyforcingadefinedquantityof
directcurrent
•Positiveterminalisconnectedtoaninsolubleanodelike
graphite
1(underground pipelines can be protected)

2. CathodicProtection
Sacrificialanodicprotection Impressedcurrentcathodicprotection

3. Methods for application of Metallic Coating
•Hot Dipping
•Electroplating
•Metal spraying
•Metal cladding
•Cementation
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Prevention …

4. Hot Dipping
•BaseMetals(Fe,Cu)havinghighermeltingpointare
immersedinthetankofmoltenmetallikeZn,Sn,Pb,Al
etchavinglowmeltingpoint
–CoatingofZnonironbyhotdippingiscalledGalvanization.
–CoatingofSnonironiscalledTinning.
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5. Electroplating
•Depositionofcorrosionresistantmetal(Au,Ag,Ni,Cr,Zn,Pb)over
thebasemetalbypassingelectriccurrentthroughanelectrolytic
solutioncontainingsolublesaltofcoatingmetals.
•Anode:rodofmetaltobedeposited
•Cathode:basemetal(thoroughlycleaned)
•Electrolyte:metallicionssolution
•e.g.ElectroplatingofSilveronanothermetal
–AtAnode- Ag(s)→Ag
+
(aq)+e
-
–AtCathode- Ag
+
(aq)+e
-
→Ag(s)
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Whencurrentispassedmetalionsfrom
electrolytedepositoncathodeandequal
numberofmetalionsareaddedtothe
electrolytefromanode

6. Electroplating
Before -dull
After -shiny
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7. Metal Spraying
•Sprayingguns-forcoatingofmetals.
•Zn,Al,Sn,Pb(lowmelting)aresprayedonthebase
metalinmoltenstate
Wiregunmethod-wireofmetaltobecoatedismeltedby
oxy-acetyleneflameandblastofcompressedairrelease
moltenmetalasspray.
Advantage:Canbedoneonroughsurfacesandrequired
thicknessiseasilyobtained
Techniquecanbeusedwheremetaldippingisnotpossible
likebridges.
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8. Wire gun
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9. Metal-Cladding
Processinwhichthinlayerofcoatingmetalgetbonded
tobasemetalononeorbothsidespermanentlywhen
theyarepassedthroughheavyrollersundertheactionof
heatandpressure
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
•Basemetalisheatedafterpackingitinpowderof
coatingmetalinabox.
•Heatingdiffusesthecoatingmetalintobasemetal
forminganalloy
•Thicknessofcoatingiscontrolledbyvaryingthetemp
andtimeoftreatment
–CoatingofCroverbasemetal:Chromising
–CoatingofSioverbasemetal:Siliconising
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12. Organic Coatings
•Paints
•Varnishes
•Enamels
•Lacquers
Mechanicaldispersionmixturesoffinelydividedsolidscalled
pigmentsinafluidmedium.
Pigmentse.g.Cr
2O
3forGreenPigment,ZnOforwhitepigment
•Fluidmediumconsistofnon-volatilefilmforming
materialcalleddryingoil(glycerylestersofhigherfatty
acids)andahighlyvolatilesolventcalledthinner
(kerosene,benzenetoreducetheviscosity)
Paints

13. Dryingoilisusedtoholdthepigmentonmetal
surfacebyformingaprotectivefilm
Plasticizerscanbeaddedinpaintstoprovideelasticitytofilm
andtominimizethecrackingoffilm

14. •Colloidal solution of Natural & Synthetic resins
–RESIN-Crosslinked,highmolweightorganicpolymers
–Natural:RosinC
15H
20O
6-asolidformofresinobtainedfrompinesand
someotherplants
–Synthetic:Phenolformaldehyde
Varnishes
Enamels
Dispersion of pigments in a varnish-provides lustrous finish
Lacquers
Colloidaldispersionofasolutionofcellulosederivatives
likecellulosenitrate,resins&plasticizersinasolventlike
acetone Providesawaterprooffilm