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Forms of Corrosion (Pitting Corrosion (Aggressive Anions that can Cause…
Forms of Corrosion
Galvanic Corrosion
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occur at
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Between metal and other conducting materials, eg. carbon and graphite.
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Crevice Corrosion
Caused by
Deposition of dirt, dust, mud and deposits on a metallic surface
By the existence of voids, gaps and cavities between adjoining surfaces.
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, including the interior of the crevice.
- After short interval, the oxygen within the crevice is depleted due to convection restriction.
Rate of corrosion within and without the crevice remains equal since the area within the crevice very small.
- As dissolution of metal (Fe) inside crevice continues, excess positive charge is produced inside the crevice.
- This results in migration of chloride ions into crevice (to balance charge). Increased concentration of metal chloride.
Hydrolysis of ferrous ions takes place:
Fe⁺2Cl⁻ + 2H₂O = Fe(OH)₂ + 2H⁺Cl⁻Fe(OH)₂ = insoluble hydroxide (rust)
H⁺Cl⁻ = free acid
- H⁺ and Cl⁻ accelerate the dissolution rates of most metals and alloys
- These are found to rapidly accelerate corrosion of steel inside crevice and the process is continued.
(Dissolution rate of metal increased)
- As the corrosion within the crevice increases, the rate of oxygen reductions on adjacent surfaces also increases. This cathodically protects the external surface.
Thus, the attack is localized within shielded areas.
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Pitting Corrosion
Extremely localized attack in environment containing aggressive ions that results in holes (relatively small) in the metals
Surface diameter of the pits are about the same as or less than the depth.
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Conditions for Pitting
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Presence of halogen ions
(Cl⁻, Br⁻, I⁻, S₂O₃⁻)
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