Electrode potential

• based on EMF/ galvanic series position

Area effect

• ratio of cathode to anode area
• unfavorable when the ratio consists of large cathode and small anode
• the corrosion will take place with high current density

Environmental affect

• corrosion is greater at seashore than dry rural atmosphere
• seashore contain salts-more corrosive
• not occur when 2 metal are completely dry as no electrolyte to produce current
  

• Galvanic corrosion is significant when the potential differences between two metal alloys ≥ 250 mV
• Ecathode – Eanode = small
  

• select combination of metals
  close as possible in galvanic series
• avoid unfavourable area effect
• insulate dissimilar metals
• apply coating
• add inhibitors
  

• Caused by deposition of dirt, dust, mud and deposits on a metallic surface by existence of voids, gaps and cavities between adjoining surface
• Associated with small volume of stagnant solutions caused by holes, gasket surface and surface deposites
• crevice sites must be wide and narrow to permit liquid entry and to maintain stagnant solution

Mechanism of Crevice Corrosions
1) Anodic and Cathodic reaction occur on the entire metals surface

• Anodic reaction : Fe→Fe2++2e
• Cathodic reaction : O2 +2H2O+4e→4OH
  2) After short interval, the oxygen is depleted due to convection restrictions.
  3) As dissolution of metals (Fe) inside crevice continues, excess e charge is produced inside the crevice.
  4) Results in migration of chloride ions into crevice and Increased concentration of metal chloride.
  5) Hydrolysis of ferrous ions
• Fe+2Cl-+ 2H2O = Fe(OH)2 + 2H+Cl
  6) Chloride and hydrogen ions accelerate the dissolution rates of most metals and alloys.
  7) As the corrosion within the crevice increases, the rate of of oxygen reductions on adjacent surfaces also increases. This cathodically protects the external surface. Thus, the attack is localized within shielded areas

How to control Crevice Corrosion ?

• Use welded butt joints
• Close crevices in existing lap joints by continuous welding or soldering
• Inspect equipment and remove deposit frequently
• Use solid nonabsorbent gaskets such as Teflon

Holes can be small or large in diameter but mostly small
Surface diameter of pits are about the same or less than the depth

Aggresive anion that cause 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
• Stagnancy of electrolytes

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

• Not using materials that show tendency to pit
• Adding inhibitor
• Cathodic protection

• localized attack at and adjacent to grain boundaries
  Can be caused by impurities at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements in grain boundary area.
• depletion of chromium in the grain boundary region result in intergranular corrosion of stainless steels.

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

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.