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Form of Corrosion (Crevice Corrosion (Control Methods (Use welded butt…

- - - - Wide enough to permit entry of liquid.
      - Narrow enough to maintain a stagnant zone.
  - - - Convection restriction occurs and the oxygen within the crevice depletes.
        
        Excess positive charge produced inside the crevice due to dissolution of metal inside the crevice.
        
        This causes negative electrolyte charges to migrate into the crevice to balance the charge and form metal salt.
        
        Hydrolysis of metallic ions take place and form rust.
        
        Hydrogen ions and negative ions from the electrolyte promotes the dissolution rate and accelerates the corrosion process of the metal inside the crevice.
        
        Corrosion within the crevice increases, causes oxygen outside to deplete, avoid corrosion at external surface, form localized corrosion wiithin shielded area.
- - - - Increase pH
  - - - Localized adsorption of aggressive ions.
        
        Corrosion potential exceeds electrode potential of pitting.
        
        Breakdown of passive film.
        
        Pit growth if no repassivation occur (high corrosion rate of pit due to autocatalytic process).
    - - Formation of small pit.
        
        Dissolved oxygen hard to diffuse into the pit.
        
        Small anodic area form inside the pit, large cathodic area form outside the pit.
        
        Excess metallic postive ions accumulate inside the pit.
        
        Aggressive negative ions migrate into the pit to balance the charge.
        
        Metal ions diffuse outwards and hydrolyzed, H+ ions diffuse inside to replace metal ions and increase acidity of the pit.
- - - - Microscopic level
  - - - Alter environment condition
        
        Lower acidity
        
        Reduce oxidizing condition
      - Solution annealing
      - Heat above 815˚C to redissolve chromium carbide, then perform rapid cooling.
      - Reduce carbon content to less than or equal to 0.03%.
      - Use stabilized stainless steel.
    - - Heat above 815˚C to dissolve chromium carbide and form Nb/Ti carbide.
    - - Soak at 800˚C or slow cool from 700˚C-900˚C to replenish Cr-depleted grain boundaries by diffusion to surrounding grains.
  - - - At temperature from 425˚C-815˚C, carbide precipitate along the grain boundaries.
        
        Carbide precipitate consumes chromium atoms.
        
        Forms Cr depleted zone (less corrosion resistant) around precipitated carbide.
        
        Form large cathodic area to small anodic area.
        
        Result in dissolution of carbide, microscopic localized galvanic attack occur, stainless steel become sensitive to IGC (sensitized stainless steel).
    - - Occurs only a few grain diameters adjacent to weld beads.
        
        When material is heated over 1230˚C during welding, it is then rapid cooled to remain carbon atoms near weld beads as solid solution.
        
        The material is reheated to critical temperature.
        
        Localized sensitization takes place.
        
        IGC occur in the narrow sensitized region.
    - - Material is sensitized only after being heated above 925˚C.
        
        Low solubility of interstitials in ferrite sensitizes the material faster and at lower temperature.
      - Duplex stainless steel is resistant to IGC
        
        Insufficient carbon remains to precipitate carbide at austenite grain boundaries.
    - - Aluminium alloys depend on precipitation phases for sthrengthening but are susceptible to IGC.
        
        When solution-quenched, resistant towards IGC increased but strength is reduced.
      - Die cast zinc alloys containing aluminium is not resistant towards IGC under steam and marine atmosphere.
- - - - Different potential between the two materials form anodic or cathodic reaction.
        
        Promote flow of electron.
  - - - The further the distance between the two metals in EMF series, the greater the tendency of corrosion.
      - When the corrosion potential difference between the two metals is at least 250mV, galvanic corrosion is significant.
    - - High ratio of cathodic to anodic area will produce higher current density, the greater the tendency of corrosion.
        
        Unfavourable area ratio effect
    - - Seashore promote higher corrosion rate than in dry rural atmosphere due to presence of salt.
      - Presence of electrolyte to produce current between two electrodes.