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CORROSION (LOCALISED (MICRO ( (INTERGRANULAR CORROSION (IGC) (Austenitic…

- - - - INTERGRANULAR CORROSION (IGC)
        
        Caused by impurities at the grain boundaries, enrichment of one of the alloying elements, or depletion of one of these elements in the grain boundary areas.
        
        Austenitic stainless steel
        
        Knife line attack (KLA)
  - - - PITTING
        
        extremely localized attack in environment containing aggressive ions that results in holes in the metal.
        
        Conditions for pitting
        
        Breaks of the passivation film or other defects such as lack of homogeneity in the passive film on the metal surface
        
        Presence of halogen ions (Cl-, Br-, I- S2O3-)
        
        Stagnancy of electrolyte
        
        MECHANISM
        
        Localized adsorption of aggressive ions
        
        Passive film breakdown (due to corrosion potential increases ≥ Epitt)
        
        Pitt growth (if repassivation cannot occur) rate of corrosion inside pits is significantly high due to autocatalytic process
      - CREVICE
        
        Caused by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence voids, gaps and cavities between adjoining surfaces
        
        **crevice must be wide enough to permit liquid entry and also sufficiently narrow to maintain a stagnant zone
        
        Mechanism
        
        Anodic and cathodic reaction occur uniformly at the interior of the crevice.
        Anodic reaction: Fe→Fe+++2e
        Cathodic reaction: O2 +2H2O+4e→4OH2
        
        Oxygen within the crevice is depleted
        
        Dissolution of metal inside crevice continues, excess positive charge is produced inside the crevice
        
        Migration of chloride ions into crevice (to balance charge). Increased concentration of metal chloride
        
        Fe+2Cl-+ 2H2O = Fe(OH)2 + 2H+Cl
        Fe(OH)2= insoluble hydroxide (rust) H+Cl-= free acid
        
        Both chloride and hydrogen ions accelerate the dissolution rates of most metals and alloys
        
        Control:
        Use welded butt joints, close crevices in existing lap joints, design vessels for complete drainage, remove deposit and solids in suspension, use solid nonabsorbent gaskets such as Teflon wherever possible.
      - GALVANIC
        
        Dissimilar metallic materials are brought into contact in the presence of electrolyte
        
        Factor affect: eletrode potential, current density, area affect, environment
        
        Control: inhibitors, avoid threaded joints, Install a third metal that is anodic
        
        Attack at the junction between two dissimilar metals/alloys