Just that. Why is some steel stainless and some is not, what’s the difference actually?
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Steel is a mixture of different metals. What you mix together and how you temper it changes the nature of the steel, giving it different properties.
Stainless steels are steels containing at least 10.5% chromium, less than 1.2% carbon and other alloying elements. Stainless steel’s corrosion resistance and mechanical properties can be further enhanced by adding other elements, such as nickel, molybdenum, titanium, niobium, manganese, etc.
This is in contrast with something like Damascus steel, which will have a much higher carbon content and will also contain higher percentages of nickle.
Steel is a type of alloy that will contain iron and carbon but can also contain other elements like chromium. It is the mix of elements but also how it cooled down that determines the properties.
The cheapest low-carbon steel will contain around 0.2% carbon the rest will be iron, there can be other stuff in it like silicon but it might only be 0.5%. So we talk about close to 100% iron
Stainless steel will contain a large percentage of other metals, like 16% chromium, 7% Nickel, 2% Manganese. Sp we talk about 75% iron and 25% other metal. The other metal will cost more than the iron so it will be more expensive.
There are other properties that can be different between different steel like how tough it is, and how hard it is to work with. Stainless will be harder to work with low carbon steel so it is not just the material cost that differs.
So different types of steel are selected both for cost and other properties like resistance to corrosion (stainless means high corrosion resistance), toughness, hot hard it is to work with etc. So stainless might not always be an alternative even if you do not care about cost.
Though the above answers are correct on the components that make up stainless steel they don’t answer the question of the corrosion resistance…
My understanding is the chromium component essentially makes a very thin barrier at the surface of the steel that blocks oxygen from bonding with the iron thus stopping rust from forming.
Stainless steel is an alloy of iron with a lot of chromium and nickel added in.
The chromium corrodes quickly in air, to form a layer of chromium oxide. The chromium oxide is very tough and a thin, invisible layer seals the surface of the metal off, so no more air can get in.
Regular iron corrodes to form iron oxide, which is rust. However, rust doesn’t seal the surface, so air and water can get penetrate the rust and get to the metal underneath causing further corrosion and more rusting.
Chromium, the metal that is mixed with iron, oxidizes in a way that is protective of the iron atoms. Iron oxidizes to a flaky substance, chromium oxidizes nice and hard. If you distribute chromium atoms throughout iron, it makes it a lot harder for rust to weaken the alloy.
Steel will still rust; it will just take a hell of a lot longer.
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I recall listening to a podcast (The Infinite Monkey Cage) about food. I don’t know the ‘why’ or ‘how’ (am sure someone could explain it) but when your Grandma brought out the ‘good silver cutlery’, it was a mistake. Silver reacts with food at a chemical level and alters its flavour. Stainless on the other hand .. does not.
Granny should have brought out the ‘good stainless steel cutlery’ for Christmas dinner.
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