Forms of Corrosion
Galvanic Corrosion 
Occurs when dissimilar metallic materials are brought into contact in the presence of electrolyte
Electrons will flow because of differential potential (Anode&Cathode)
It is the major suspect when attack at the junction between two dissimilar metals/alloys
Corrosion is greatest on the more active metal near the junction which is the joining part between two metals
Factor affecting galvanic corrosion
1) Electrode potential - based on EMF/Galvanic series position
2) Area effect - an unfavorable area ratio consists of a large cathode and a small anode because corrosion will be taking place with high current density. Corrosion=current density (I/A)
3) Environmental affect - corrosion is greater near seashore than in dry rural atmosphere as seashore contains salts which make it more corrosive
It will be significant if the corrosion potentials difference between two metals/alloys ≥ 250mV
Hence, it is suggested to select two different metals/alloys with their corrosion potentials as close as possible (Ecathode - Eanode = small)
Ways to control it
1) Select combinations of metals as close as possible in galvanic series
2) Avoid unfavorable area effect of a small anode and large cathode
3) Insulate dissimilar metals wherever practicable
4) Applied coating with caution
5) Add inhibitors to decrease the aggressiveness of the environment
Crevice Corrosion 
It is a localized form of corrosion 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
usually associated with small volumes of stagnant solutions caused by holes, lap joints, surface deposit and crevice under bolt and rivet heads
Mechanism
1) Anodic and cathodic reaction occur uniformly over the entire surface of metals including the interior of the crevice
2) After a short interval, the oxygen within the crevice is depleted due to convection restriction, so oxygen reduction ceases in this area
3) Excess positive charge is produced inside the crevice as dissolution of metal (Fe) continues
4) This results in migration of chloride ions into crevice to balance charge
5) Hydrolysis of ferrous ions is taking place
6) Both chloride and hydrogen ions accelerate the dissolution rates of most metals and alloys
7)This will accelerate corrosion of steel inside crevice and dissolution rate of metal increased
8) Hence, the rate of oxygen reductions on adjacent surfaces increase too. This cathodically protects the external surface. The attack is localized within shielded areas
Ways to control it
1) Use welded butt joints
2) Close crevice by continuous welding or soldering
3) Design vessels for complete drainage
4) Inspect equipment and remove deposit frequently
Pitting Corrosion
It is a form of extremely localized attack in environment containing aggressive ions that results in holes in the metal
The holes may be small or large but in most cases they are relative small
The surface diameter of pits are about the same or less than the depth
Mechanism
1) Localized adsorption of aggressive ions
2) Passive film breakdown (due to corrosion potential increases ≥ Epitt)
3) Pitt growth - rate of corrosion inside pits is significantly high due to autocatalytic process
Autocatalytic processes occurring in a corrosion of pit
1) Initially a small pit is 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 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 increase in acidity inside the pit
Intergranular Corrosion (IGC) 
Ways to control it
1) Materials that show pitting should not be used to built the plant or equipment
2) Adding inhibitor including increase of pH
3) Cathodic protection
Localized attack at and adjacent to grain boundaries with relative little corrosion at the grain
It can be caused by impurities at the grain boundaries, enrichment of the alloying elements or depletion of elements in grain boundary areas
IGC of austenitic stainless steel
In intermediate temperature range carbide M23C6 is precipitated along grain boundaries. This precipitation consuming chromium atoms and results in Cr depleted zone around precipitated carbide
Chromium-depleted zone is less corrosion resistant than surrounding grains. Unfavorable area ratio consists of large cathode and small anode forms and results in microscopic localized galvanic leading to IGC
Heating of austenitic stainless steels in this temperature region result in sensitized stainless steels
Ways to prevent it
1) Alteration of environment.
2) Lower acidity and less oxidizing conditions will generally reduce the susceptibility of IGC
3) Solution annealing
4) Heating the alloy above 815 degree celsius to re-dissolve chromium carbide and followed by rapid cooling
5) Decreases carbon content to ≤ 0.03%
6) Use stabilized stainless steel
6) Avoid threaded joints for materials far apart in the galvanic series
7) Install a third metal that is anodic to both metals in the galvanic contact
