At first glance, they look identical: the elegantly rust-brown fire bowl from the discounter and the Corten steel raised bed from the specialist company. Both boast the coveted rust look – but only one of them will still be in your garden in ten years' time. This is because many cheap "rust-look" products are simply made of untreated, normal construction steel. In this article, we explain the difference between genuine Corten steel and ordinary construction steel St37 (now S235) – and how you can identify inferior goods before you buy them.
What is Corten Steel?
Corten steel (COR-TEN®) is a weather-resistant structural steel, intentionally alloyed with copper, chromium, nickel, and phosphorus – typically around 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 stands for its two core properties: CORrosion resistance and TENsile strength.
What is St37 or S235?
St37 is the old designation for today's S235JR – the classic, unalloyed structural steel. It is an excellent material that we also process daily: galvanized or powder-coated, it is perfectly protected outdoors, and indoors anyway. The problem only arises when S235 is exposed to the elements untreated – which many cheap providers do to sell a "Corten steel look" without 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 under the rust layer when exposed to weathering. This patina seals the surface and largely stops further corrosion – the rust becomes a protective shield. A prerequisite is an alternation of wet and dry phases, as is common in a garden. After a few weeks, the characteristic rust-brown coloring appears, fully matured after two to three years – and the component lasts 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 to rusting through. With the thin sheets of many cheap products (often less than 1.5 mm), this happens quickly: After a few seasons, the supposed bargain is scrap. In short: Rust is not just rust.
Corten steel and St37 in direct comparison
| Property | Corten steel (S355J2WP/J2W) | Structural steel St37/S235 (untreated) |
|---|---|---|
| Alloying | Cu, Cr, Ni, P – weather-resistant alloy | unalloyed |
| Rust behavior | firmly adhering protective patina, sealed | porous rust, flakes off, eats deeper |
| Outdoor lifespan | Decades, maintenance-free | A few years until rusting through |
| Appearance | uniform, warm noble rust patina | splotchy, flaking rust |
| Price | higher material price – one-time investment | cheap – but disposable product |
How to identify inferior "rust-look" products
- No material specification: If it only says "metal," "steel with rust look," or "noble rust" without a material standard (S355J2WP/S355J2W or Corten A/B), skepticism is warranted.
- Noticeably low price: Genuine Corten steel as 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: Below 1.5–2 mm wall thickness, there is insufficient substance for stability and longevity.
- Splotchy, flaking rust: If the rust layer peels off over a large area, it is not patina but progressive corrosion.
- No manufacturer's proof: "Made in Germany" with named production is a strong quality signal.
Corten steel in practice: our log stores
The extent of this difference in everyday life becomes clear with products that are permanently outdoors – for example, with our firewood racks and log stores made of Corten steel. They stand unprotected in the garden or on the terrace all year round, carry heavy wood supplies, and should also look good. Our VESTA log store, for example, is made of 2 mm thick Corten steel, screwed together, and produced in Germany – the patina protects it for decades, while the firewood dries optimally ventilated thanks to the open design. From the compact FRAME model to the 1.80 m high VESTA XL, you will find the right size with us.
Conclusion
Corten steel and untreated structural steel St37/S235 do not differ in appearance during the first few months – but in what happens afterwards. The alloy-induced protective patina makes Corten steel a durable, maintenance-free investment; untreated structural steel, on the other hand, rusts through incessantly. When buying, pay attention to the material specification, 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 demarcation from inferior quality. We would be happy to advise you personally on questions about material and design.
