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Forms of Corrosion (Galvanic Corrosion Galvanic-Corrosion-Terminology…

- - - - 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
  - - - 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
  - - - 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
        
        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
- - - - 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
        
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  - - - 2) Close crevice by continuous welding or soldering
        
        3) Design vessels for complete drainage
        
        4) Inspect equipment and remove deposit frequently
- - - - The surface diameter of pits are about the same or less than the depth
  - - - 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
  - - - 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
  - - - 2) Adding inhibitor including increase of pH
        
        3) Cathodic protection
- - - - 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
  - - - 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