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Form of corrosion (Pitting Corrosion (Form of localized attack containing…

- - - - Autocatalytic process
        
        1) Initially small pit formed
        2) Dissolved oxygen is difficult to diffuse into the pit leading to the formation of very small anodic area inside the pit and very large cathodic area outside the pit which is still covered by a passive film
        3) Excess of positive charge (due to metal ions accumulation) inside the pit is necessarily balanced by the migration of aggressive ions into the pit.
        4) Because of metal ions are diffusing outward and being hydrolyzed and deposited just in front of the pit, H+ are diffusing inward to replace the metal ions that results in gradualy increase in acidity inside the pit
- - - - Due to the chromium depleted region which is less corrosion resistance than the surrounding grain. Unfavorable area ration between large cathode and small anode.
      - Prevent IGC
        
        Alteration of environment
        
        Lower acidity and less oxidizing conditions
        
        Solution annealing
        
        Use stabilized stainless steel
    - - Localized form of IGC occurs for only a few grain diameters immediately adjacent to weld bead in stabilized austenitic stainless steels
        
        Prevent KLA
        
        Heating above 815 degree celcius to dissolve chromium carbide and to form Nb/Ti carbide at the same time
    - - Due to low soluility of interstitial in ferrite, it will sensitizes more rapid at lower temperature. They must be soaked at at 800°C or in the region 700 to 900°C to replenish chromium-depleted grain boundaries by diffusion to surrounding grains.
    - - High strength of aluminium alloys depend on precipitated phases for strengthening and susceptible to
        IGS. This happen when an alloy is solution quenched to keep copper in the solution, their susceptibility to intergranular corrosion is very small but they have low strength.