Many steel water tanks are made of galvanised steel—a zinc coating that protects the steel from corrosion. These tanks can be built to hold capacities from about 30,000 litres to millions of litres of water making them a suitable solution for extremely large storage requirements.
Galvanised steel tanks will rust and corrode over time as the zinc wears away allowing the iron within to come into contact with oxygen in the air. Furthermore, zinc can leak into your water leaving your stored water tasting metallic.
To help overcome this issue most steel rainwater tanks come with liners made of food-grade polyethylene. These can be quite expensive to fabricate, and over time can break and require repair. Poly liners provided often carry a much less warranty period than the tank itself.