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FORM OF CORROSION (GALVANIC CORROSION (Factors affect (1) Electrode…
FORM OF CORROSION
GALVANIC CORROSION
1) When dissimilar metallic materials are brought into contact in the presence of electrolyte.
2) At the junction between two dissimilar metals or alloys. 
Factors affect
1) Electrode potential
- based on EMF/Galvanic series position.
2) Area effect
- Ratio of the cathodic to anodic areas. Corrosion is taking place at high current density.
3) Corrosion = current density (I/A)4) Environmental affect (moisture, soil etc)
- Corrosion is greater near seashore than in dry rural atmosphere.
- Condensate near seashore contains salts – more corrosive.
- Not occur when metals are completely dry since no electrolyte to
produce current between 2 electrode.
Control
1) Select combinations of metals as close as possible in galvanic
series.
2) Avoid the unfavorable area effect of a small anode and large
cathode
3) Insulate dissimilar metals wherever practicable
4) Insulate dissimilar metals wherever practicable
5) Add inhibitors to decrease the aggressiveness of the environment.
6) Avoid threaded joints for materials far apart in the galvanic series.
7) Design for the use of readily replaceable anodic parts or make them thicker
for long life.
8) Install a third metal that is anodic to both metals in the galvanic contact.
PITTING CORROSION
Condition
1) Breaks of the passivation film or other defects such as lack
of homogeneity in the passive film on the metal surface
2) Presence of halogen ions (Cl-, Br-, I- S2O3-)
3) Stagnancy of electrolyte
Mechanism
1) Localized adsorption of aggressive ions
2) Passive film breakdown (due to corrosion
potential increases > Epitt)
3) Pitt growth (if repassivation cannot occur)
rate of corrosion inside pits is significanly
high due to autocatalytic process
Controls
1) Prevent to use materials that will show pit or have
tendency to form pit
2) Add inhibitor to increase pH
3) Cathodic protection
4) Coattng the metal surface
Pitting is a form of extremely localized attack in environment containing
aggressive ions that results in holes in the metal.
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CRIVICE CORROSION
Cause
1) Deposition of dirt, dust, mud and deposits on a
metallic surface
2) Existence voids, gaps
and cavities between adjoining surfaces.
Mechanism
1) Anodic and cathodic reaction occur uniformly at the interior of the crevice.
2) Oxygen within the crevice is depleted due to convection restriction.
3) Dissolution of metal inside crevice continues, excess positive charge is produced inside
the crevice
4) Migration of chloride ions into crevice (to balance charge). Increased concentration of
metal chloride
5) Hydrolysis of ferrous ions is taking place
(except for alkali metals, Na & K)
Fe+2Cl-+ 2H2O = Fe(OH)2 + 2H+Cl
Fe(OH)2= insoluble hydroxide (rust) H+Cl-= free acid
6) Both chloride and hydrogen ions accelerate the dissolution rates of most metals and
alloys
Controls
1) Use welded butt joints
2) Close cevice in existing lap joints by continuous welding
3) Design vessel for complete drainage, avoid sharp corners
4) Inspect equipment and remove deposit frequently.
5) Remove solids in suspension early in the process or plant flow sheet, if
possible.
6) Use solid nonabsorbent gaskets such as Teflon wherever possible.
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INTERGRANULAR CORROSION
Causes
1) Impurities at grain boundaries
2) Enrichment of alloying element in grains boundaries
3) Depletion of alloying element in grains boundaries
Mechanism
Controls
1) Alteration of environment
2) Lower the acidity and less oxidising condition will reduce the tendency of IGC3) Solution annealing4) Heatinfg the alloy above 815°C to re-dissolve chromiun carbide and followed
by rapid cooling5) Decrease carbon content to <0.03%6) Use stabilized stainless steel (titanium)
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