What Steel Are Shipping Containers Made From?

Shipping containers are built from low-alloy weathering steel, the family usually sold under the name Corten. It is not ordinary mild steel. The alloying additions, mainly copper with chromium, nickel and in some grades phosphorus, let the surface form a dense oxide layer that stays in place instead of flaking off, so the steel beneath corrodes far more slowly than unpainted carbon steel would. That property is what makes a box that crosses an ocean for twenty years economically possible, and it is why the grade behind the word Corten has to be pinned down before any sheet is rolled.

Why weathering steel and not mild steel

A container lives outdoors, in salt air, in rain and in repeated wet-dry cycles. Ordinary carbon steel rusts in layers there: the oxide is porous and flakes away, exposing fresh metal, and the section thins until it fails. Weathering steel differs because its oxide is adherent and far less permeable, so once the surface has weathered to a stable brown-grey, corrosion slows sharply.

Two consequences follow. A thinner section can carry the same corrosion allowance, which saves weight on a structure priced by the tonne. And the paint system lasts longer, because failure at a scratch does not spread under the film as it does on mild steel. Neither effect removes the need for coating, which is why floor bearers and door bottoms are the first places an inspector looks.

The grade names behind the word Corten

Corten is a trade name that became a generic word, and a specification that says only Corten gives a mill room to choose. What belongs on the order is a designation from a published standard. The set below covers the families met most often in container and transport-equipment steel.

The designations above identify standard and grade family only. Chemistry, mechanical values and tolerances are set by the edition of the standard named on the order and are confirmed against the mill certificate for the heat supplied.

Where each part of a box uses steel, and why

A container is not one steel in one form: a welded assembly of thin formed sheet on a heavier frame, and that split explains most specification questions buyers raise.

Thin sheet, heavy frame: what that means for the order

Because panels are thin and frames are not, a container programme buys two product families at once: sheet in coil for the forming lines, and plate, strip or sections for frame and fittings. Sheet is ordered to a weathering steel standard with a surface suitable for painting, in practice either pickled and oiled or an agreed mill finish. Mill scale left on a panel is not cosmetic: paint over scale fails at the scale boundary, and the failure appears in service rather than at inspection.

The frame is a different conversation. Sections and plate are ordered by structural grade with weldability in mind, and the welding procedure has to match the grade actually supplied. Putting a phosphorus-bearing weathering grade into a joint designed around a low-phosphorus grade is the kind of change that surfaces as cracking months later.

What to write when buying the steel, not the box

Six lines carry the risk:

1. The grade designation and the standard by edition, not the word Corten on its own
2. Product form and the dimensional standard governing it: coil, cut sheet, plate or section, with thickness and width tolerance
3. Surface condition: pickled and oiled, mill finish or pre-primed, and the paint system it must accept
4. The mechanical requirements of the named standard, with test direction and the frequency the standard calls for
5. Weldability constraints where the part is welded, since phosphorus-bearing grades are not interchangeable with low-phosphorus ones in every joint
6. Certificate form, heat traceability and sea-freight packing, because thin coated sheet damages in transit faster than plate

Where substitutions creep in

Most quality problems are substitution problems, and they are hard to see once the box is painted:

• Commercial-quality mild steel against a weathering steel enquiry, which paints and forms identically and then rusts through years earlier
• One weathering grade swapped for another, changing weld behaviour and, in the phosphorus-bearing case, toughness at the joint
• Mill scale or light rust painted over at the fabricating shop, which passes a visual check and fails in service
• Second-hand boxes re-marked and sold as new steel, where the corrosion allowance is already spent
• Plate grade offered for a thin corrugated panel because the grade was in stock, ignoring that the panel is formed from sheet

Inspection and documentation on arrival

Three checks decide acceptance. The mill certificate must name the standard, grade, heat number and test results, and the heat numbers must match the markings on the coil or bundle, otherwise traceability breaks at the first step. The surface has to be examined before painting, the only moment when scale, seams and transit staining can still be fixed cheaply. Where a coating system is specified, it is verified by dry film thickness and adhesion on the finished part rather than on a coupon.

Containers entering transport service carry a further layer: the safety approval plate, which records the approval and maintenance dates. It has nothing to do with the steel specification, and buyers sourcing sheet for a new build sometimes confuse the two.

Grade availability, thickness range and the surface condition a given mill can supply vary by product form. The combination for an order is confirmed against current mill capability and the certificate for the heat supplied.

Frequently asked questions

Are shipping containers made of stainless steel?

No. The great majority are built from low-alloy weathering steel, coated on the outside. Stainless steel appears in some fittings and in specialist tanks, and aluminium is used in a minority of boxes, but the standard dry-freight container is weathering steel.

Is Corten the same as weathering steel?

Corten is a trade name that became a generic word for the family. Weathering steel is the material category, and the grades inside it are defined by standards such as ASTM A588, JIS G3125 SPA-H, EN 10025-5 and GB/T 4171. A specification should name the standard and grade, not the trade name.

Why don't containers rust through?

Partly because the steel is weathering grade, forming an adherent oxide layer rather than flaking rust, and partly because the box is painted and maintained. Coating does most of the work in service.

Can the same steel be bought for other projects?

Yes. Weathering steel sheet and plate are used for bridges, facades, planters, silos and exposed structures where the stable patina is either wanted visually or accepted as the corrosion protection. Grade and weld procedure are chosen for the application, not copied from a container drawing.

What is SPA-H?

SPA-H is a hot-rolled weathering steel grade in the Japanese JIS G3125 standard, widely used for transport equipment including containers. It is one of the designations most often quoted when Asian mills supply sheet for container production.

Are all containers steel?

No. Aluminium boxes exist and are lighter, and some specialist units use composite panels. Steel dominates on cost per unit of strength, weld repair and acceptance across the handling chain.

Related reading

• Steel plate
• Medium and thick plate
• Cold rolled steel sheet
• Pickled and oiled steel
• Hot rolled vs cold rolled steel
• How orders are inspected and certified

Send the grade and standard, the product form, the surface condition, the coating system and the destination port, and the offer is worked against those points. See the steel product catalogue or request a quotation.