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Type of Corrosion (Pitting Corrosion (Causes (Damage of the protective…

- - - - Passive film breakdown
        
        Pitt growth rate of corrosion inside pits is significantly high due to
        autocatalytic process
- - - - After a short interval, the oxygen within the crevice is depleted due to convection, oxygen reduction ceases. The rate of corrosion within and without the crevice remains equal.
        
        Excess positive charge is produced inside the crevice as dissolution of metal inside crevice continues.
        
        Results in migration of chloride ions into
        crevice and increased
        concentration of metal chloride.
        
        Hydrolysis of ferrous ions take place.
        
        Fe+2Cl- + 2H2O = Fe(OH)2 + 2H+Cl
        
        Both chloride and hydrogen ions rapidly accelerate
        corrosion of steel inside crevice
        
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- - - - Mainly caused by dissolution of carbide
      - Carbide precipitate along grain boundaries leading to the formation of chromium depleted zone adjacent grain boundaries
        
        Diffusion rate of carbon atom is
        too low. Carbide precipitation along grain
        boundaries cannot occur.
        
        In intermediate temperature range, carbide M23C6 is precipitated along grain boundaries which consumes chromium atoms and results in Cr depleted zone around precipitated carbide.
        
        An unfavorable area ratio consists of a large cathode and a small anode forms and results in microscopic localized galvanic attack leading to IGC
        
        heating of austenitic stainless steels in this temperature region will results in sensitized stainless steels
    - - Highly localized form of IGC that occurs for only a few grain diameters immediately adjacent to weld bead in stabilized austenitic.
        
        Carbon has been reacted with Nb/Ti to form
        stable carbide.
        
        Chromium carbide formation does not occur.
        
        Carbon atoms in the location next to the weld bead will remain in solid solution
        
        Localized
        sensitization is taking place in that place
        
        Consequently failure can occur by
        conventional IGC in the narrow sensitized region.
    - - Low solubility of interstitials in ferrite causes the ferritic stainless steels sensitize much more rapidly and at lower temperature.
        
        Prevention
        
        Must be soaked at 800 degree celcius or slow cooled in the region 700 to 900 degree celcius to replenish chromium-depleted grain boundaries by
        diffusion to surrounding grains.