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FORM OF CORROSION (Crevice Corrosion (Ways to overcome (Use welded butt…

- - - - Anode
        
        Corrosion of the anode will accelerate
      - Cathode
        
        Corrosion of the cathode will decelerate or even stop.
  - - - The greater the difference the higher the driving electric force of corrosion. Electrode potential is based on EMF or Galvanic series position
        
        More Cathodic
        
        Eg. Platinum, Gold
        
        More Anodic
        
        Eg. Zinc, Magnesium
    - - Ratio of cathodic to anodic areas
        
        Large anode connected to a small cathode
        
        ⬇ corrosion rate.
        
        😃
    - - Example: pH, humidity, temperature, soil
        
        ⬆moisture
        
        ⬆corrosion rate
        
        ☹
  - - - Corrosion is significant if corrosion potential difference btw two metals >250 mV
- - - - Metals outside the gap are corrosion resistant
  - - - Inside of crevice: anode
        Dissolution of Fe metal
        Fe→Fe2+ + 2e
        Outside of crevice : cathode
        O2 + 2H2O + 4e→4OH-
        
        migration of chloride ions into crevice to balance charge
        
        ⬆ concentration of metal chloride
        
        Hydrolysis of ferrous ions
        Fe+2Cl- + 2H2O = Fe(OH)2 + 2H+Cl-
        
        Fe(OH)2 = insoluble hydroxide (rust)
        
        Corrosion rate on adjacent surface also ⬆ thus protects the external surface cathodically
  - - - to permit liquid entry
    - - to maintain a stagnant zone
- - - - Resulting pits can become wide and shallow or narrow and deep which can rapidly perforate the wall thickness of metal
        
        is one of the most damaging forms of corrosion
      - Aggressive ions that causes pitting corrosion on metals or alloys
        
        Cl-
        
        Br-
        
        I-
  - - - B) Mechanical damage of the passive film caused by scratches.
        
        Anodic reaction starts on the metal surface exposed to the electrolyte. The passivated surrounding surface act as the cathode.
      - C) Particles of a second phase (non-metallic inclusions, intermetallic inclusions, metallic particles, Microsegregation) emerging on the metal surface.
        
        These particles precipitating along the grains boundaries may function as local anodes causing localized galvanic corrosion and formation of initial pits
      - A) Localized stresses in form of dislocations emerging on the surface may become anodes and initiate pits.
      - D) Non-homogeneous environment may dissolve the passive film at certain locations where initial pits form
      - Pit growth.
        
        In presence of chloride ions pits are growing by autocatalytic mechanism.
        
        What is autocatalytic mechanism?
        
        Initially a small pit is formed.
        
        Dissolved oxygen is difficult to diffuse into the pit
        
        formation of very small anodic area inside the pit and very large cathodic area outside the pit
        
        1 more item...
- - - - impurities at the grain boundaries
      - enrichment of one of the alloying elements
      - depletion of one of these elements in the grain boundary areas.
  - - - The precipitation consumes alloying element,causes chromium form a narrow band along grain boundary and make it anodic.
  - - - Less than 425°C
        
        Diffusion rate of carbon atom is too low. Carbide precipitation along grain boundaries cannot occur.
      - At 425°C - 815°C
        
        Carbide precipitate along grain boundaries. Chromium depleted zone is formed adjacent grain boundaries. An unfavorable area ratio consists of a large cathode
        and a small anode forms and results in microscopic localized galvanic attack leading to IGC
      - More than 815°C
        
        Dissolution of carbide
    - - highly localized form of IGC that occurs for only a few grain diameters immediately adjacent to weld bead in stabilized austenitic stainless steels (type 321 and 347)
        
        When the material is heated > 1230°C (during welding) followed by rapid cooling, carbon atoms remain in solid solution. If the material is reheated to critical temperature, localized sensitization is taking place in that place.
        
        Consequently failure can occur by conventional IGC in the narrow sensitized region
        
        To prevent it, heating above 815°C to dissolve chromium carbide and to form Nb/Ti carbide at the same time.
    - - Because of low solubility of interstitials in ferrite, it sensitizes much more rapidly and at lower temperature.They must be soaked at 800°C or slow cooled in the region 700 to 900°C to replenish chromium-depleted grain boundaries by diffusion to surrounding grains.