SubMikeD: “Stainless” steel has a high chromium content. The chromium forms an oxide coating which makes it less susceptible to corrosion and chemical reaction than carbon steel.
Qwerty_Resident: It’s a steel/chromium alloy; the chromium reacts with the oxygen in water to create a protective film around the metal and prevent oxidation (rust) from forming on the steel part.
At the molecular level, it *does* corrode almost instantly on the outside, but it creates a layer of corrosion so tight and thin that it doesn’t interact with light, and it’s not detectable without modern instruments; but the layer of rust around the rest of the metal protects the rest of the metal from further corrosion.
[I learned the last paragraph from this article, which is a nice read if you want to know a bit more in-depth info about the topic](https://www.scientificamerican.com/article/why-doesnt-stainless-stee/)
krystar78: straight steel is made with a mixture of carbon and iron. iron reacts with oxygen to form brown crumbly rust.
stainless steel is made with a mixture of carbon and iron and a hint of nickel and chromium. when chromium reacts with oxygen in the atmosphere, it forms chromium oxide which blocks oxygen penetration. the surface film then protects the underlying iron from exposure and forming rust.
rednax1206: Stainless steel is a combination of iron, which is strong and magnetic but susceptible to corrosion, and chromium, which is resistant to corrosion. The combination provides the advantages of both metals.
vorpalblab: On the same theme of oxidised metal making chrome and nickel bearing steel alloys seem always shiny silver, and aluminum more like dull grey, there is an alloy of steel called HTLA for High Tensile Low Alloy which is an alloy of steel with small amounts of nickel, copper and other metals that makes the red rust extremely tight to the base metal underneath, so although the metal rusts like steel, the rust forms a protective layer that prevents penetration of more rust. Good for bridges, some parts of ship hulls where strength and lighter weight is worth an extra cost, and its cheaper than the stainless steels with fewer stress fracturing and fatigue problems long term.
BubbaTheGoat: The three main components of stainless steel are Iron, Nickel, and Chromium. When iron corrodes, it forms a large crystal structure that exposes more iron to oxygen, allowing further corrosion. With water available, the iron oxide will form even larger crystals and greatly accelerate this process. This is how rust propagates and can cause embrittlement in iron alloys.
Chromium and nickel oxidize by forming a thin layer of oxides without disrupting the surrounding lattice, and therefore do not expose more material to oxygen nor weaken the material.
To make steel stainless, we not only need to alloy with chromium (iron and nickel are still steel, just not stainless), but remove any iron from the outside layer such that only chromium is present. This process is called passivation, and there are a number of ways to do this, but the most common is with a warm bath of acid. The acid leaches iron out of the outermost layers, leaving only chromium, which will form a ‘passive’ layer of oxide which will protect deeper layers of the part.
So why do stainless steels still rust? There are a number of ways to disrupt or damage this protective chromium layer, but a very common and interesting one is salt. Chloride ions in salt will attach the chromium oxide layer and strip it off. This is why cars made of stainless steel will rust in places that use a lot of salt on the roads in winter. Another mechanism is deep scratches through the passive chromium layer can expose iron, which will begin its chain reaction corrosion process.
marcusregulus: Nickel helps change the iron crystal structure from body centered cubic to face centered cubic. A face centered cubic crystal structure has more space for other atoms such as chromium to enter the crystal lattice. Chromium has the benefits as described in previous posts.