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Why Metals Corrode?
Sep 18, 2018

To reach this metallic state from their various ores, it is necessary to absorb and store the energy required to release the metal from their original compound  for later return to its original state by corrosion. The amount of energy required and stored varies from metal to metal (see the position of various metal wrt oxygen , sulfur, and chloride). It is high for metals like Mg,Al & Fe but relatively low for  Cu, Ag & Au. The energy required to form iron ore hematite to metallic iron is returned when iron corrodes to form back to original iron ore. Only the rate of change of energy is different. Destruction of metals by corrosion depends on.. - Nature of metal & Alloy - Presence of inclusions & foreign materials. - Homogeneity of structure - Nature of corrosive environment as oxygen, H2S, CN, Chloride, Sulphate, SRB , Temperature & velocity etc. - Oxide scale (continuous or broken) - Stress(residual, applied, steady or cyclic) - Presence of porous/semi porous surface deposit . - Galvanic effect due to dissimilar metals. - presence of stray currents - in built crevices Corrosion failure are more often subtle and result of invisible localized effects in the form of pits, intergranular corrosion, or attack within crevices.

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