What Are Shipping Containers Made Of?

The eight corner castings are the exception to thin-and-corrugated: solid cast steel fittings standardised by ISO 1161, with the oval apertures that twist-locks, crane spreaders and lashing gear engage. Every lift, stack and lock in the container’s life passes through these eight castings.

The corrugations — about 36 mm deep — turn flat sheet into a deep-ribbed panel many times stiffer than the same steel laid flat; the walls act as structural shear webs between the corner posts. This is the fact every conversion runs into: the walls are not cladding, they are structure, and every opening cut for a door or window removes load path that framing must replace. The engineering consequences for homes are covered in container home dimensions.

Underneath, the steel story continues: cross-members at roughly 450 mm spacing span between the two bottom side rails and carry the 28 mm plywood floor.

The factory finish is a marine coating system — zinc-rich primer under polyurethane or acrylic topcoats, engineered for salt spray. Corten resists corrosion but is not immune, and rust on containers is predictable: it starts where the coating is broken and water sits — dents (which also pool water on the roof), welds, door hinges and the bottom rails. On a sound unit, surface rust at these spots is cosmetic: a wire brush and zinc primer stop it. Rust that flakes in sheets or perforates is structural and prices the box accordingly.

In ocean service — salt spray, stacking loads, twisting in a seaway — a container works 10–15 years. Retired to static duty the same box relaxes: no stack above it, no salt bath, and the Corten shell routinely runs 25 years and more with nothing but drainage and paint touch-ups. The wear items are not the steel at all: door gaskets and the plywood floor age out first.