Explain Differential metal corrosion & Water-line corrosion

3.B] Explain: (i) Differential metal corrosion & (ii) Water-line corrosion


(i) Differential metal corrosion

“The corrosion of metals when two dissimilar metals are in contact with each other in a corrosive environment is called galvanic corrosion”.
One of the metals having lower reduction potential acts as an anode and hence undergoes oxidation and the metal corrodes. The other metal has high reduction potential acts as cathode hence undergoes reduction and the metal is protected.

Example: As shown in figure, if the two different metals (Fe & Cu) are placed in a corrosive environment. Iron corrodes and copper is protected. Because Iron has lower reduction potential hence undergoes corrosion.

Differential metal corrosion

(ii) Water-line corrosion

Water-line corrosion

When water is stored in a metallic tank as shown in figure, it is observed that corrosion takes place just below the water level in the container.

The area above water is more oxygenated, acts as cathode. Area below the water level is less oxygenated, acts as anode. Therefore, area below the water level is corroded and area above water level is protected. Hence, a distinct brown line is formed below the water line due to corrosion. It is seen in steel water storage tanks, floating ships etc.

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