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Engineering chemistry  Unit  6: Corrosion science Topic :  Dry corrosion

Suman S. Kahandal Assistant Professor   K.K.Wagh Institute of Engineering ,Research Nasik

Corrosion : Factors Affecting Corrosion (Chapter 1) (Animation) - YouTube

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Corrosion is the destruction of a metal as a result of chemical reactions between it and the surrounding environment. Both the type of metal and the environmental conditions, particularly gases that are in contact with the metal, determine the form and rate of destruction of metal . Eg.Formation of rust on surface of iron ,formation of green film on the surface of copper

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Corrosion Introduction : - ppt video online download

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Dry corrosion  Corrosion due to oxygen ( oxidation corrosion )  Corrosion due to other gases  Liquid metal corrosion  2) Wet corrosion  Galvanic corrosion  Concentration cell corrosion

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It is  occurs  due to  direct attack  of  atmospheric  gases  like  oxygen,  CO 2 ,  SO 2 ,   H 2 S,NH 3 , NO 2 ,moisture  and  by hot  flowing  liquids.  Mechanism  of dry  corrosion: ( due to oxygen )   It involves following steps  1.Adsorption  of  O 2  molecules (Physical  Adsorption)  2.Dissociation  of  O 2   molecule Loss  of  electrons by  metal  atom to  oxyen   atom,  resulting in  formation  of  metal  and  oxide   ion. Formation  of  MO  by union of  ions, and  form  thin  layer  of  metal  oxide  (Chemical   Adsorption)

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Porous  film   : If the oxide  film  is porous, then through the pores,  oxygen  gas  molecules penetrate and   continue the corrosion.  e.g.  oxide  films  of  iron  (steel), Mg,  Na,  K,  Ca  metals  are  porous.   4Na   +   O 2	 2Na 2 O	 2Fe   +   O 2	 2FeO	 2Mg   +   O 2	 2MgO Non  porous  film   : If the oxide  film  is stable, nonporous, strong,  well  adhered  to the  metal surface  and self  healing,  then  further  corrosion stops,  as  the.  Various  metals can safeguard themselves  by   such impervious  metal  oxide  films  eg.  Cr,  Sn, Zn,  Pb,  Al,  Ni,Cu, Monel  metal,  bronze   etc.

2Al  + 3/2   O 2 Al 2 O 3 2Cu +   O 2 2CuO 2 Zn +   O 2 2ZnO   2 4Cr +   3O 2 2Cr 2 O 3

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Unstable   oxide   film	 : When  the oxide  film formed  is  unstable and  it  decomposes  back  into the  metal   and oxygen,  No corrosion  take  place.  e.g.  metals  like  Au,  Pt  &Ag.    4Ag + O 2  ⇌   2Ag 2 O Volatile  oxide film   : If  the  metal  oxide film  is  volatile, then its  evaporation takes place   simultaneously. The   underlying fresh  metal is  exposed for the  further attack  by  oxygen.  e.g. molybdenum   oxide (Mo 2 O 3 )  film  is   volatile. 4Mo   + 3O 2	 2Mo 2 O 3  ↑  (evaporation   slowly)

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Pilling-Bedworth   Rule: T he  rule  states  that, if the  volume  of oxide is  smaller  than the  volume  of  metal  consumed   in The  metal  oxide  formation,  then the  film is  porous.  On  the other hand  film is   non-porous if  metal  oxide  formed  has greater volume  than the  metal  consumed  during   corrosion. Volume  of  metal   oxide Pilling-Bedworth  ratio  (PBR)   = Volume  of  metal surface   consumed

If  the  PBR is greater  than one but  smaller  than 1.45 then the  metal oxide  film  is   non-porous and protective  from further  corrosion  by  any gases  or aqueous   media.

Engineering chemistry Unit 6: Corrosion science Topic : Dry corrosion

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