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Forms of Corrosion (Galvanic Corrosion (Control (select combinations of…
Forms of Corrosion
Galvanic Corrosion
Definition
Its occurs when dissimilar metallic materials are brought into contact in presence of the electrolyte
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Factors that Affects
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Area effect which the ratio of the cathodic to anodic areas where the corrosion will take place at high current density
Environmental affect such as moisture, soil and etc
Properties
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Two different metals/alloys that have used in contact, the select alloys with their corrosion potential are as close as possible
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Pitting Corrosion 
Definition
A form of extremely localized attack in environment containing aggressive ions that results in holes in metal
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Mechanism
- Localized adsorption of aggressive ions
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- Pitt growth (rate of corrosion inside pits is significantly high due to autocatalytic process
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Autocatalytic process
- Formation of small anodic are inside pit and large cathodic area outside pit
- Excess of positive charge inside pit is necessarily balanced by migriation of aggressive ions into pit
- Metal ions are diffusing outward and hydrolyzed and deposited in front of the pit
- Weight loss methods are inadequate for pitting evaluations
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Intergranular Corrosion
Definition
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Enrichment of one of the alloying elements, or depletion of one of these elements in grain boundary
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Knife line attack (KLA)
KLA is highly localized form og IGC that occurs for only a few grain diameters immediately adjacent to weld bead
Preventing IGC
- Alteration of environment
- Lower acidity and less oxidizing conditions
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- Heating the alloy above 815oC
- Decreases carbon content to ≤ 0.03%
- Use stabilized stainless steel
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Crevice Corrosion
Definition
Corrosion that caused by the deposition of dirt, dust, mud and deposits on metallic surface or by existence voids, gaps, and cavities between adjoining surfaces
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A crevice must be wide enough to permit liquid entry and also sufficiently narrow to maintain a stagnant zone
Mechanisms
- Anodic and cathodic reaction occur uniformly over entire surface of metals including the interior of the crevice
- After short interval, the oxygen within the crevice is depleted due to convection restriction
- As dissolution of metal inside crevice continues, excess positive charge is produced inside the crevice
- the migration of chloride ions into crevice
- Both chloride and hydrogen ions accelerate the dissolution rates of most metals and alloys
- It found to rapidly accelerate corrosion of steel inside crevice
- As corrosion within the crevice increases, the rate of oxygen reduction on adjacent surfaces also increases
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