Explain why

iron sheet is riveted with copper bolt

An iron pole is partly burried underground

Both steel and zinc react in different ways when in contact with soil and an understanding of the performance of each material when in contact with soil allows structure service life to be determined with reasonable accuracy.

Steel requires oxygen, moisture and the presence of dissolved salts to corrode. If any one of these is absent, the corrosion reaction will cease or proceed very slowly. Steel corrodes quickly in acidic environments and slowly or not at all as alkalinity is increased.

Zinc requires the presence of stable oxide films on its surface to provide its corrosion resistance. It performs best in neutral pH environments although it can tolerate exposures in the range from pH 5.5 to pH 12. In the absence of air, the stable oxide films do not form on the zinc surface, and corrosion can be accelerated if moisture is present under these conditions.

For this reason, galvanized steel is the best combination where structures are partly buried and partly exposed to the atmosphere, as the zinc provides the durability above ground while the steel performs predictably in-ground

Regards

Arun (askIITians forum expert)