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LOCALIZED (Macroscopic) (PITTING CORROSION (Variations in cross sectional…
LOCALIZED (Macroscopic)
CREVICE CORROSION
What it is
localized form caused by deposition of dirt, dust, mud and deposition on metallic surface
Also by existence voids, gaps and cavities between adjoining surfaces.
Usually associated with small volumes of stagnant solutions caused by holes, gasket surfaces, lap joints, surface deposit and crevice under bolt and rivet heads
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Mechanisms
Initial: Anodic and cathodic reaction occur on the entire surface of metals including interior of crevice. After that, oxygen were depleted due to convection restriction. But, excess positive charge were produced inside crevice because of the dissolution of metal inside crevice.So to balance charge, were used migration of chloride ions into crevice and make increased the concentration of metal chloride.
Final: After that, hydrolysis of ferrous ions were taking place. The dissolution rates of most metals and alloys were accelerate by chloride and hydrogen. The reason is to rapidly accelerate corrosion of steel inside crevice. It make corrosion within crevice increase and the rate of oxygen reduction on surface increase.
PITTING CORROSION
What it is
localized attack in environment containing aggressive ions (CI-,Br-,ClO4-) that result in holes in metal.
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Conditions
presence of halogen ions (Cl-,Br-)
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Mechanism
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- Localized adsorption of aggressive ions
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Control
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some materials are more resistance to pitting than others (increasing pitting resistance like titanium)
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GALVANIC CORROSION
Factors affecting
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Environmental affect like moisture, soil etc.
Properties
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Selected of alloys with their corrosion potential occur IF 2 different metal/alloy having used in contact
Control with
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Install third metal with anodic to both metals in galvanic series like iron, cadmium, zinc
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Limited to specific area, parts and structure to be corrode.