Shipping containers are made of Corten steel, a specialised alloy that is 75% stronger than normal steel. Also known as 'weathering steel', Corten has an incredibly low corrosion rate - it loses only 1.0 mm of surface per 100 years to corrosion when unpainted! (1.5mm in highly corrosive environments)
Fun fact - this is the same steel used for building bridges, for the same reasons!
However, when coated with epoxy paints, the lifespan of a shipping container is virtually infinite, providing a perfect weather resistant structural surface. Since the entire structure is made of steel, there is no threat of termites or fires either.
Pictured: D3 Pocket Park