Localized

• 1. Crevice Corrosion

  • gaps where stagnant solution stays
  • solution containment --> gaps
  • less o2 inside=anode
  • o2 out= cathode=protected
  • Often happens to metals with passive films- contaminents destroy passive films
  • Anodic: M --> M+ + e-
  • Cathodic: O2 + 2H2O + 4e- --> 4OH-

  Protection:

  • welded joints
  • design for drainage
  • avoid sharp corner
  • inspect equipment- clean deposits
  • use non-absorbent gasket (seal)

  Special crevice corrosion:

  Filiform corrosion- coating not protecting
• 2. Pitting

  • highly localised
  • form holes
  • may cause sudden failure- cant see clearly/hard to predict- covered by rust
  • Auto- catalytic - days-years but once start, can accelerate

  Protection:

  • same as crevice
  • avoid pitting sensitive materials
  • Inhibitors
• 3.Intergranular corrosion

  Special type for mostyl SS by depletion of Cr in GB area

  Intergranular corrosion is not the same as grain boundary corrosion

  GB corrosion= weaker than intergranular

  BUT intergranular much worse and goes very deep

  
  

  When heated to 500 to 800C

  -Cr decrease

  -GB becomes anode

  -corrosion resistance in GB

  -less than 500C reaction does not occur

  -more than 800C - diffusion active

  welding=problem

  
  

  Protection:

  • Retreat at high T then quench
  • Steel contains low %C <0.03%
  • Add strong carbide forming elements (stabilizer)
• 4. Selective Leeching

  One of the element is removed in corrosive environment

  dezinification of brass ( 70% CU + 30% Zn)

  Protection:

  • Reduce corrosivity of environment (H+, Cl-)
  • Coatings
  • Cathodic Protection
  • Allow modification