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Forms of Corrosion (Galvanic corrosion : (Prevention (Choose metals with…
Forms of Corrosion
Galvanic corrosion :
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Explanation
Occurs when dissimilar metallic materials are brought into contact in the presence of electrolyte. Electron will flow because of different potential with one being anode and cathode respectively. Potential difference is the driving force for an accelerated attack on the anode metal and dissolved into electrolyte. If the potential difference is more than 250mV, it will be more significant.
Factors
Environmental affect- such as moisture, soil, temperature, humidity, pH.
Electrode potential- based on the EMF/Galvanic series position. Anode (more reactive) corrodes at an accelerated rate and cathode (more noble) at a slower rate.
Area effect- it is better to have large anodes and small cathodes. Corrosion of metal will take place with high current density.
Intergranular corrosion
Explanation
Caused by impurities at the grain boundaries, enrichment of one of the alloying elements or depletion of one of these elements in the grain boundary areas. localised attack at and adjacent to grain boundaries with relatively little corrosion at the grains.
Mechanism
1) Carbide formation- At temp of 510 to 788 degree celcius, chromium carbide which is insoluble are formed at grain boundaries.
2) Chromium depletion zone- Chromium consumed result in lower chromium in area adjacent to grain boundaries and this zone corrode because of it being less corrosion resistant.
3) Localized attack- The two dissimilar composition are in contact and unfavourable area ratio occur which results in the microscopic localized galvanic attack and lead to intergranular corrosion.
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Pitting corrosion
Explanation
A form of extremely localized attack in environment containing aggressive ions that results in holes in the metal.
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Mechanism
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3) Pitt growth (if repassivation cannot occur), corrosion rate inside pitts is significantly high due to autocatalytic process
Crevice corrosion
Explanation
Localised form of corrosion caused by deposition of dirt, dust, mud and deposits on a metallic surface or by the existence of voids, gaps and cavities between adjoining surfaces.
Mechanism
2) After short interval, oxygen in crevice depleted due to convection restriction,so oxygen reduction ceases in this area. The corrosion rate within & without crevice still equal since area within crevice very small.
3) But as dissolution of metal(Fe) inside crevice continues, excess +ve charge produced inside crevice.
1) Anodic and cathodic reaction occur uniformly over entire surface of metals including crevice interior.
4) This results in migration of chloride ions into crevice and increased concentration of metal chloride.
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7) As the corrosion within crevice increases, the rate of oxygen reduction on adjacent surfaces also increases. This cathodically protects the external surface thus the attack is localised within shielded areas.
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