Shot blasting and priming of sheets.
First the steel is de-greased and shot blasted to Swedish standard SA 2.5 (near white metal) as well as being primed with zinc rich primer.
The steel used is often copper enriched (so called Corten A, a USS trade name, or equivalent).
This means that if the paintwork is penetrated by damage, the exposed steelwork will react when oxidising in a way that inhibits corrosion.
Also a process takes place called ionic transfer whereby the steel and zinc reacts to form a coating that assists durability and results in an extended lifespan of the container.