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Corrosion

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hisoki
18 diapositives · 548 Views · Dec 30, 2012

Corrosion . What constitutes an anode/cathode pair?. In summary– Anything that makes two half-cells different gives anode & cathode ––> corrosion. We can call these types macro cells. Corrosion . Then what about micro cells? anode & cathode within single material

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Corrosion
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  1. Corrosion What constitutes an anode/cathode pair? In summary– Anything that makes two half-cells different gives anode & cathode ––> corrosion We can call these types macro cells

  2. Corrosion Then what about micro cells? anode & cathode within single material operating on a micro scale Grain boundary - high energy region…anodic wrt bulk of grain Differently oriented grains - some planes of atoms dissolve faster than others Segregation (coring)

  3. Corrosion Then what about micro cells? anode & cathode within single material operating on a micro scale Segregation (coring)

  4. Corrosion Another type or micro cell - 2nd phase 2nd phase will tend to react differently than matrix phase - sets up anode/cathode pair

  5. Corrosion Example - welds

  6. Corrosion corrosion Example - welds 304 stainless steel plate arc- welded to other plates Note corrosion in HAZ Why?

  7. Corrosion Remember…. What makes stainless steel stainless? tight, tenacious film of Cr2O3 on the surface film occludes electrolytes, oxygen

  8. Corrosion Corrosion in stainless steel HAZ due to "sensitization"

  9. Corrosion Decrease C content - no Cr carbide formation Tie up C as Ti carbide (321) or Nb carbide (347) - no Cr carbide formation How to avoid sensitization – get rid of C

  10. Ol' Harold's story 1913 We find Ol' Harold Brearley in England industriously testing alloy steels for gun barrels

  11. Ol' Harold's story Ol' Harold's records showed that the unrusted steel sample contained 14% Cr! The birth of stainless steels! Moral: behind every successful person, there may be a junk pile Stolen from: Tales from Sheffield by Stan Lessteal Cutter Also see Brearley: Steel-makers; and Knotted String (EMS Library)

  12. Suppressing corrosion Components of a cell: anode cathode electrolyte connection

  13. Suppressing corrosion Occlude electrolyte - coatings asphalt greases, oils paints polymers ceramic coatings "noble" metals

  14. Suppressing corrosion Occlude electrolyte - coatings "noble" metals: unless scratch heals, a galvanic cell is set up

  15. Suppressing corrosion Cathodic protection galvanizing: Zn coating applied by hot-dipping electroplating

  16. Suppressing corrosion Cathodic protection galvanizing: presence of scratch does not promote corrosion of protected metal but steel helps Zn corrode

  17. Suppressing corrosion Cathodic protection set up cell by adding metal which is more anodic than metal or alloy to be protected

  18. Suppressing corrosion Cathodic protection example: protecting ship plate