At first glance, they look the same: the noble rust-brown fire bowl from the discount store and the Corten steel raised garden bed from a specialist. Both have that coveted "rust look" – but only one of them will still be in your garden in ten years. This is because many cheap products marketed as having a "rust look" are simply made of untreated, standard structural steel. In this article, we explain the difference between genuine Corten steel and ordinary structural steel St37 (now S235) – and how to spot inferior goods before you buy.
What is Corten steel?
Corten steel (COR-TEN®) is a weather-resistant structural steel that is specifically alloyed with copper, chromium, nickel, and phosphorus – typical amounts are about 0.8% chromium, 0.5% copper, 0.5% nickel, and 0.1% phosphorus. It is standardized, among others, as S355J2WP (Corten A, material no. 1.8946) and S355J2W (Corten B, material no. 1.8965). The name represents its two core properties: CORrosion resistance and TENsile strength.
What is St37 or S235?
St37 is the old designation for what is now S235JR – classic, unalloyed structural steel. It is an excellent material that we also work with daily: when galvanized or powder-coated, it is perfectly protected for outdoor use, and is suitable for indoor use regardless. The problem only arises when S235 is left untreated and exposed to the elements – which is exactly what many discount suppliers do in order to sell a "Corten steel look" without the extra cost.
The crucial difference: the patina
How genuine Corten steel protects itself
Due to its alloying elements, Corten steel forms a firmly adhering barrier layer of sulfates and phosphates directly beneath the rust layer when exposed to weather. This patina seals the surface and largely stops further corrosion – the rust becomes a protective shield. This requires an alternation between wet and dry phases, which is typical in a garden environment. After a few weeks, the characteristic rust-brown coloring appears; it is fully developed after two to three years – and the component will last for decades.
How normal structural steel rusts through
With untreated S235, on the other hand, the rust is porous and flakes off. Moisture penetrates deeper and deeper into the material, the wall thickness decreases year after year – leading to pitting and eventually rusting through. With the thin sheets used in many cheap products (often under 1.5 mm), this happens quickly: after just a few seasons, the supposed bargain is scrap. In short: not all rust is created equal.
Corten steel and St37 in direct comparison
| Property | Corten steel (S355J2WP/J2W) | Structural steel St37/S235 (untreated) |
|---|---|---|
| Alloy | Cu, Cr, Ni, P – weather-resistant alloy | unalloyed |
| Rusting behavior | firmly adhering protective patina, sealed | porous rust, flakes off, eats deeper |
| Outdoor lifespan | decades, maintenance-free | a few years until rusted through |
| Appearance | uniform, warm noble-rust patina | patchy, flaking rust |
| Price | higher material price – one-time investment | cheap – but a disposable product |
How to identify inferior "rust look" products
- No material specifications: If it just says "metal," "steel in rust look," or "noble rust" without a material standard (S355J2WP/S355J2W or Corten A/B), you should be skeptical.
- Suspiciously low price: Genuine Corten steel as a raw material costs significantly more than S235 – a "Corten steel" product at a structural steel price is usually not what it claims to be.
- Very thin sheets: Anything under 1.5–2 mm wall thickness lacks the substance for stability and longevity.
- Patchy, flaking rust: If the rust layer peels off over large areas, it is not a patina but progressive corrosion.
- No manufacturer proof: "Made in Germany" with identified production is a strong sign of quality.
Corten steel in practice: our firewood racks
How much this difference matters in everyday life is evident in products that stay outside permanently – for example, our firewood racks and log stores made of Corten steel. They stand unprotected in the garden or on the terrace all year round, hold heavy stocks of wood, and are also expected to look good. Our VESTA firewood rack, for example, is made of 2 mm thick Corten steel, is constructed with screws, and is produced in Germany – the patina protects it for decades, while the open design ensures the firewood dries optimally. From the compact FRAME model to the 1.80 m high VESTA XL, you will find the right size with us.
For raised garden beds, the difference is particularly clear because moist soil is constantly in contact with the wall. Our custom-welded Corten steel raised bed comes fully welded in lengths up to 240 cm, and the Corten steel raised bed kit comes as a screw-together version up to 10 m. You can read about how to properly plan dimensions, sheet thickness, and filling in our raised bed guide.
Conclusion
Corten steel and untreated structural steel St37/S235 do not differ in their appearance during the first few months – but they do in what happens afterward. The alloy-induced protective patina makes Corten steel a durable, maintenance-free investment; untreated structural steel, by contrast, rusts through relentlessly. When buying, pay attention to the material specifications, material thickness, and manufacturer – then you will enjoy the noble rust look for decades.
Would you like to delve deeper into material science? Then also read our article Stainless Steel V2A & V4A – Differences and distinction from inferior quality. We are happy to advise you personally if you have questions about materials and design.
