Reasons for Rusting of Nodular Cast Iron Pipe Fittings

What are the main reasons for the rusting of ductile iron pipe fittings?

1. The outer layer of ductile iron pipe fittings was damaged due to collision during transportation.

2. The inner and outer layers are damaged due to collision during hoisting.

3. Damage to ductile iron pipe fittings during construction.

4. Damage caused during backfilling.