GALVANIC CORROSION

In 1980s, the Statue of Liberty is found to suffer from corrosion during regular maintenance checks. Over a dozen areas encountered this problem and it was due to galvanic corrosion.

What is galvanic corrosion?

When two dissimilar metals are connected together in an electrolyte, one metal becomes more electronegative resulting in corrosion and hence galvanic corrosion is also termed as dissimilar metal corrosion or bimetallic corrosion.

The Galvanic series

The potential difference between the galvanic couple plays a major factor in corrosion and so the materials selected from galvanic series to couple should have a low potential difference between them.

A galvanic series is a list of metals and alloys arranged according to their corrosion potentials or galvanic potentials as measured in a given environment. Zinc can act as a perfect ‘sacrificial Anode’ to protect metals like Steel from corrosion.

IS 3043 says that when a galvanized mild steel of dimensions 150 mm×25mm×3 mm was buried for 12 years, the average loss in specimens, that is corrosion is about 0.5 % per year giving a considerable and an apparently permanent protection.

Measures to reduce galvanic corrosion

- Selecting materials from the galvanic series in such a way that the potential difference between them is so small.
- Taking care of the cathode to anode area ratio as galvanic corrosion is proportional to it. A small Anode and a large Cathode will increase the current density at the Anode resulting in large galvanic corrosion.
- Insulating the dissimilar metals wherever possible.

Hence critical care and attention is to be taken while connecting dissimilar metals in order to avoid galvanic corrosion.

 

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