An ocean container is a masterpiece of pragmatic engineering. On the surface, it looks like a simple steel box, yet it is capable of withstanding salty ocean spray, storm-induced rolling, the weight of eight identical containers stacked on top of it in a ship's hold, and temperature fluctuations ranging from -40°C to +60°C.
Many cargo owners mistakenly believe that a container is essentially a thick-walled, armored safe. In reality, the secret to its strength lies not in the thickness of the metal, but in its precise geometry, structural ribs, and specific steel grades. In this article, we will take a detailed look at the materials and thicknesses used for the components of various types of ocean containers.
1. Corten Steel: The Foundation of the Design
The vast majority of ocean containers are manufactured from Corten steel (weathering steel). In the Russian steel market, its closest equivalent is 09G2S steel or special alloys with the addition of copper, chromium, nickel, and phosphorus.
The key feature of Corten steel: upon contact with oxygen and moisture, it does not rust through; instead, it forms a dense oxide film (patina) just a few microns thick. This "rust" blocks further corrosion. This is exactly why ocean containers can last 15–25 years without rusting through, even when constantly exposed to the aggressive ocean environment.
2. Standard Dry Cargo Container (DC) Construction
Let's break down the thickness of the components using the classic 20-foot (20ft DC) and 40-foot (40ft DC, 40ft HC) containers as examples.
Walls (Corrugated Panels)
Contrary to popular belief, container walls are not smooth sheets. They feature a corrugated profile that provides immense bending rigidity with minimal weight.
Steel sheet thickness: 1.6 mm to 2.0 mm (most commonly 1.8 mm).
Corrugation pitch and depth: Engineered to absorb vertical and horizontal loads.
Fastening method: The panels are welded to the frame with a continuous seam, ensuring a watertight seal.
Roof
The container roof is also made of sheet steel but features less corrugation or a specific profile for water runoff.
Thickness: 2.0 mm.
The roof absorbs the impact of crane spreaders and the weight of snow, but its primary job is to prevent water from getting inside.
Doors
Doors are the most vulnerable point for break-ins, so they are reinforced.
Steel sheet thickness: 2.0 mm.
The doors feature double corrugation and reinforced locking rods with cam keepers to eliminate play during vessel rolling.
Frame and Corner Posts
It is the frame, not the walls, that bears all the load. When 9 tiers are stacked on top of each other in a containership's hold, the bottom container withstands hundreds of tons of pressure.
Corner posts: Hollow steel profiles 5 to 6 mm thick. They absorb up to 80% of all vertical loads.
Top and Bottom Rails: Steel profiles 4–5 mm thick.
Cross Members: Channels or I-beams 3–4 mm thick, spaced every 30–40 cm.
Corner Castings
These are the 8 massive steel blocks at the corners of the container, which cranes and twist locks latch onto.
Material: High-strength cast steel.
Wall thickness: 10 to 20 mm at the points of maximum stress. Corner castings are rated for lifting loads up to 30.4 tons and stacking loads up to 192 tons.
Floor
In standard ocean containers, the floor is not steel, but wood. This reduces the tare weight, provides better grip for the cargo, and offers shock absorption.
Material: Multi-layer marine plywood made from hardwoods (bamboo, keruing, apitong), treated with insecticides in accordance with phytosanitary standards.
Floor thickness: 28 mm (standard).
In recent years, due to environmental regulations and pest control efforts, many lines have been switching to steel floors (3–5 mm thick) or composite material floors.
3. Features of Other Container Types
High Cube (HC) — 40-foot High Containers
In terms of metal thickness, HC containers are identical to standard 40ft containers. The only difference is the height (2.89 m vs. 2.59 m). The increased height is achieved through taller corner posts and top rails, but the wall thickness remains at 1.6–2.0 mm.
Reefer Containers
The design of a reefer is radically different from a dry van. It is essentially a "thermos on wheels."
Outer cladding: Smooth sheets of stainless steel or aluminum, 0.8–1.2 mm thick (no corrugation to avoid thermal bridges).
Insulation: High-density polyurethane foam, 80–100 mm thick.
Inner cladding: Food-grade aluminum or stainless steel, 0.5–0.8 mm thick, with a T-bar floor profile for cold air circulation.
Machinery compartment (at the end): Compressor, evaporator, and control units protected by a steel casing 2–3 mm thick.
Open Top (OT)
Designed for top-loading of out-of-gauge and tall cargo via crane.
Walls and doors: Similar to standard DC (1.8–2.0 mm).
Roof: Absent. Instead, a removable canvas tarpaulin (soft top) or a removable steel roof (hard top) 2 mm thick is used.
Reinforced frame: Due to the lack of a rigid roof, the top rails and corner posts are made more massive (up to 8 mm) to compensate for the loss of rigidity.
Flat Rack (FR)
Used for transporting out-of-gauge cargo (machinery, pipes, yachts).
Floor (platform): A heavy-duty steel deck 6 to 10 mm thick with an anti-slip tread pattern.
End walls: Reinforced steel panels 3–5 mm thick, which can be folded down.
Frame: Maximally reinforced; the tare weight of a 40ft Flat Rack can reach 5,000–5,500 kg (compared to 3,700 kg for a standard 40ft).
4. Why Aren't the Walls Thicker?
A logical question arises: why not make the walls 5 mm thick, like in trucks? The answer lies in the economics of shipping.
Tare Weight. A standard 20ft container weighs about 2,200 kg, and a 40ft weighs about 3,700 kg. Increasing the wall thickness by just 1 mm would add 300–500 kg to the container's weight. This means the payload would have to be reduced by the exact same amount, as the maximum gross weight is strictly limited by ISO (usually 30.4 tons).
Metal and Production Costs. Containers are manufactured by the millions. Saving even 100 kg of steel per container yields hundreds of thousands of tons in savings on an industry-wide scale.
Rigidity is Achieved Through Geometry. A 1.8 mm corrugated panel works like an I-beam. It handles loads better than a 4 mm smooth sheet while weighing half as much.
5. Corrosion, Wear, and Repair
Despite the use of Corten steel, containers are subject to mechanical damage and corrosion:
Scratches and dents from forklift tines and cranes compromise the protective patina.
Sea salt accumulates in the lower corrugations and on the floor, causing localized corrosion.
Repair: Damaged panels are cut out with an angle grinder, and "patches" of similar 2.0 mm Corten steel are welded in their place. After welding, the repaired area is mandatory primed and painted with special epoxy paints.
According to IICL (Institute of International Container Lessors) standards, a container is deemed unfit for international shipping if the residual wall thickness in corroded areas falls below 1.2 mm, or if there are through-holes that compromise its watertight integrity.
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