The passive layer: how stainless steel protects your food
A film a few atoms thick is the reason steel is safe to store food in. Here is how it works, and how to avoid damaging it.
The single most important thing about stainless steel is invisible and is a few atoms thick. Everything else follows from it.
What it is
Stainless steel contains chromium. At the surface, that chromium reacts with oxygen — from air or from water — to form chromium oxide. Unlike iron oxide, which is rust and which flakes away exposing more metal to corrode, chromium oxide is:
- Dense and tightly bound to the metal beneath it.
- Chemically inert, so it does not react with food acids or salts.
- Self-repairing. Scratch it and, provided oxygen is available, it re-forms within moments.
That last property is what separates stainless steel from every coated cookware material. A non-stick coating that is damaged stays damaged. A passive layer that is damaged heals itself.
Why chromium content matters
There is a threshold — conventionally around 10.5% chromium — below which the layer is not continuous enough to protect the metal. Above it, protection improves with chromium content.
Our range uses Krome 16+, a ferritic grade whose chromium content sits comfortably above that threshold. The "16" is the clue to the chemistry, and the magnetic behaviour is the clue to the crystal structure — which is also what makes the whole body induction-capable.
What can damage it
The passive layer needs oxygen to maintain itself, and it dislikes chlorides. So:
- Undissolved salt on a hot surface. The single most common cause of pitting. Salt into moving liquid, never onto dry hot metal.
- Brine or salted water left standing, especially overnight.
- Chlorine bleach, particularly at strength or left in contact.
- Anything that excludes oxygen for a long period — a wet cloth left lying in a vessel, or food debris under a rim.
- Contamination with ordinary carbon steel. Steel wool leaves iron particles embedded in the surface; those particles rust and appear as spots.
How to look after it
- Rinse and dry after use, particularly after salty or acidic preparations.
- Store dry and with air circulation.
- Use nylon or steel-safe scourers, never carbon steel wool.
- Let the vessel see air. That is all it needs to keep re-passivating.
If a vessel has been abused and is spotting, it can usually be restored — our article on passivation explains how.
The reassuring part
You do not really have to manage any of this. Ordinary use, ordinary washing, ordinary drying, and the layer looks after itself for the life of the vessel. It is worth understanding only so that the few genuine hazards are obvious.
Specifying cookware for your kitchen?
Every product on this site publishes its full size chart — gauge, dimensions and weight for every size. If you would rather just describe the job, we will tell you what to order.
