Materials science: what does “Edelstahl” actually mean?

You may recognise the situation: when buying a new kitchen, you naturally want a high-quality stainless-steel sink. In German, it is usually called an “Edelstahl” sink. But what do we generally associate with this word? In most cases, we think of steel that does not rust – in other words, corrosion-resistant steel.

From a materials engineering perspective, however, that is not entirely correct. The German term “Edelstahl” does not necessarily imply corrosion resistance. It is therefore worth taking a closer look at two fundamental questions: when is steel stainless, and when does it qualify as “Edelstahl”?

Stainless or corrosion-resistant?

A look in the kitchen cutlery drawer helps to explain stainlessness. To simplify matters slightly, cutlery can be divided into three categories: “normally stainless”, “particularly corrosion-resistant” and “dishwasher-safe”. The category largely depends on how much chromium, nickel and molybdenum the steel used to make the cutlery contains. How can you tell?

The stamped marking provides a clue. A spoon marked 18/0 belongs to the “normally stainless” category. The steel contains 18% chromium, which provides sufficient corrosion protection under normal environmental conditions.

However, conditions in a kitchen are not always “normal”. Concentrated lemon juice, for example, acts as a weak acid, and the salted water used to boil eggs is also quite an aggressive environment. The “particularly corrosion-resistant” category is better suited here. Cutlery marked 18/10 contains around 10% nickel in addition to 18% chromium, which significantly improves its resistance.

When kitchen conditions become particularly aggressive

The environment in a dishwasher is even more demanding. The alkaline washing solution can place considerable demands on many steels. Cutlery intended to remain dishwasher-safe therefore also contains around 2–3% molybdenum and very small amounts of titanium or niobium. These elements further increase corrosion resistance and stabilise the microstructure.

Interestingly, molybdenum is usually not included in the stamped marking on cutlery. Instead, the indication appears in the product description: “dishwasher-safe”.

When does a steel truly qualify as “Edelstahl”?

In materials engineering, the German word “Edelstahl” has a different meaning from its everyday use. Steel qualifies as “Edelstahl” when it is particularly pure. First of all, this means that it contains as few unwanted precipitates as possible. These include, for example, oxides that can enter the melt during steelmaking, or silicates that can form when the melt is poured.

At the same time, “Edelstahl” is characterised by very low levels of elements that are detrimental to steel, particularly phosphorus and sulphur. Both elements can significantly impair the mechanical properties of a steel.

“Edelstahl” therefore refers to high-purity steels with very low sulphur and phosphorus contents. The crucial point is that such a steel does not necessarily have to be corrosion-resistant. It can certainly rust if it does not contain suitable alloying elements such as chromium, nickel or molybdenum.

Typical applications of high-purity steels

High-purity steels are primarily used where purity and a homogeneous microstructure are essential. One example is applications with particularly demanding surface quality requirements. On highly polished surfaces, even a single microscopic precipitate can become visible and impair quality.

High-purity steels are even more important for components subject to cyclic loading. In such cases, inclusions or precipitates can act as starting points for fatigue cracks, which can lead to component failure in the worst case.

“Edelstahl”: conclusion

Why, then, does the kitchen retailer describe its products as “Edelstahl” even though they do not necessarily qualify as such in materials engineering terms? The answer is simple: “edel”, meaning “noble” or “high-grade”, sells better. In everyday German, the term has therefore become synonymous with stainless or corrosion-resistant steels, even though this is not entirely correct from a materials science perspective.

W.S. Werkstoff Service – expertise in materials science

If you suspect that a steel described as “Edelstahl” is rusting, or if you need an analysis of a metallic material in general, the colleagues at our accredited W.S. MaterialtestCenter will be happy to help with a materials or damage analysis.

If you would like to deepen your knowledge of materials testing or materials engineering for metals, it is also worth looking at the other seminars and courses offered by our W.S. TrainingCenter.