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Form of Corosion (INTERGRANULAR CORROSION (Way to prevent IGC (Alteration…
Form of Corosion
GALVANIC CORROSION
Definition
• Occurs when dissimilar metallic materials are brought into contact in the presence of electrolyte
• Occurs when there is any attack at the junction of dissimilar metals or alloys
• Also occur between metal and other conducting materials such as carbon and graphite
• Led to a “knife-cut” effect.
Factor affecting
• Electrode potential-refer the EMF/Galvanic series position
• Area effect, or the ratio of the cathodic to anodic areas as corrosion of metal will take place with high current density.
• Environmental affect such as moisture, soil and others. Presence of salt in condensate also increase corrosion
Properties
• Significant when the corrosion potentials difference between two metals/alloys ≥ 250 mV.
• Two different metals/alloys have to be used in contact selected with their corrosion potential as close as possible.
Control
• Insulate dissimilar metals wherever practicable.
• Applied coating with caution to avoid unfavourable area defect.
• Add inhibitors to decrease the aggressiveness of the environment.
• Select combinations of metals as close as possible in galvanic series.
• Avoid the unfavourable area effect .
• Install a third metal that is anodic to both metals in the galvanic contact. • Design for the use of readily replaceable anodic parts or make them thicker for long life.
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CREVICE CORROSION
Definition
-Corrosion by the deposition of dirt, dust, mud and deposits on a metallic surface or by the existence voids, gaps and cavities between adjoining surfaces
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-Must be wide enough to permit liquid entry, sufficiently narrow to maintain as tagnant zone
Mechanism
- Anodic and cathodic reaction occur uniformly over the entire surface of metals
- Rate of corrosion within and without the crevice remains equal.
- Dissolution of metal inside crevice
- Results immigration of chloride ions into crevice
- Hydrolysis of ferrous ions is taking place
- Chloride and hydrogen ions accelerate the dissolution rates
- Rapidly accelerate corrosion of steel inside crevice
- Rate of oxygen reductions on adjacent surfaces increases.
- The attack is localized within shielded areas
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PITTING CORROSION
Definition
• A form of localized attack in environment containing aggressive ions results in holes in the metal
• Holes are relatively small
• Surface diameter of pits ≤ depth
Mechanism
- Localized absorption of aggressive ions.
- Passive fil breakdown to corrosion potential increase ≥ Epitt
3.Pitt growth if repassivation cannot occur *rate of corrosion inside pits is high due to autocatalytic process
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INTERGRANULAR CORROSION
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Cause
• Impurities at grain boundaries
• Enrichment of one of the alloying elements
• Depletion of alloying elements in grain boundaries3
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Knife Line Attack(KLA)
KLA is a highly localized form of IGC that occurs for only a few grain diameters immediately adjacent to weld bead in stabilized austenitic stainless steels (type 321 and 347).
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