Eli5 : How does stainless steel stay stainless?
In: Chemistry
I think the answer to your question, not what is stainless but how is stainless stainless, is a combination of the answers already here but they all are missing one key component in making stainless stain less: passivation.
When you make the stainless steel, it’s a mixture of all the ingredients pretty evenly iron, carbon, chromium, molybdenum, and whatever else. The inside and outside look the same.
You then process the SS, roll it into wire or sheet or machine it, some even forge it. Depending on what you do, you probably now added a bunch of stuff to the outside like tiny amounts of carbide or iron from the rollers, dies, hammers, anvils, tool steel… just from making the part. So now the outside looks like the inside plus all this extra stuff.
Now you have to passivate the part. You want to take all of that stuff off the outside and leave the chromium bits since they form a desirable protective layer if there’s room for them to lock together, red rover style. So we clean the part through polishing and then pickle it! There’s special machines that use acids to strip the outside of the part of iron and other contamination giving the chromium the best chance to form a protective mesh to surround the part and stop oxygen from touching the iron. ”Red rover red rover, we call Oxygen over!”
If you don’t passivate, or pickle, the SS then it’s just like any other alloy and will continuous oxidize and rust away.
This is why your SS fridge and trash cans don’t perform like marine SS parts, they manufacturing process is pretty lax when it comes to using proper grade alloys and processing the parts through the whole passivation process.
It’s an iron based alloy that contains roughly 11% chromium which is the perfect combo to prevent the iron from rusting. An alloy is a mixture of metals to create a better material with new properties that are different to the base metal. This process can increase strength, hardness and in stainless steels case the ability to not rust.
It’s called stainless because it stains LESS than regular carbon steel.
The chromium molecule is smaller than an iron molecule so the chromium creates a “matrix” around the iron, protecting the steel molecules. It’s not a perfect seal so over time the iron still breaks down from being exposed to oxygen. It just takes longer.
What’s a real mind trip is that steel is made by melting iron oxide, adding carbon, then bubbling oxygen through it. But if steel is exposed to oxygen it converts back to iron oxide. The fuck?
I’m just a welder so that’s a pretty rough run down of it. I’m sure someone much smarter than me could elaborate better.
It’s the chromium in stainless steel that basically “prevents” it from rusting (in the way that iron rusts)
Source: Saikon no Qwaser
The chrome in stainless steel oxidizes. The difference with iron is that chromium oxide has the same volume as chromium so the oxide coating is not expansive
Stainless steel is an alloy of steel that has added nickel. The addition of nickel to the steel mixture inhibits rust formation. There are different blends of compounds that make up stainless and are given different numbers like 304 or 316. These different alloy blends are engineered for different things, higher corrosion resistance, higher melting point, etc.
So all metals, other than gold really, corrode. That is they react with oxygen to form some sort of metal oxide, which is basically saying the burn very, very slowly. A metal oxide is sort of like a rock or a salt. That’s often what the ore we mine it as already is, or similar enough.
Let’s look at iron. It rusts (corrodes, but specific to iron), which turns into flaky red/black stuff, that falls off and exposes more iron to rust further.
Let’s look at copper. It doesn’t corrode as fast as iron. And it turns into this green coating (look at the statue of liberty), which doesn’t flake quite as bad as iron.
Let’s look at aluminum, it doesn’t rust/corrode, right? Wrong. Aluminum corrodes very fast and any aluminum you have ever seen is “rusted”. Except the “rust” forms very fast and sticks to the aluminum very well. This provides a very thin and very protective layer that prevents the rest of the aluminum from corroding further. So the key to being a corrosion resistant metal (gold aside) is to have a thin layer of “rust” that sticks to it well and prevents further inwards metal from “rusting”.
So what is stainless steel? It’s iron with at least 10% chromium, and some other things. 316L stainless steel (there’s many different types of stainless steel, but this is a common one) is about 16% chromium, 10% nickel, 2% molybdenum. Even regular old rusting steel is still an alloy, it has more than just pure iron, like carbon in carbon steel obviously. The chromium does the same thing aluminum does, forms a thin and hard layer that protects the chromium and iron from further corrosion. What if I scratch the layer? Shouldn’t it rust then? No, because the chromium is throughout the stainless steel. Scratching it will just form a new protective layer anywhere. This is unlike galvanized steel, where iron is just coated with zinc. Scratch the zinc off and the iron will rust, the zinc is only on the outside.
But this doesn’t mean stainless steel is corrosion proof. Given time, water mixed with some other chemicals will corrode it. Some strong acids will make it rust pretty easily. Stainless steel is only stainless under certain conditions.
Corrosion of stainless steel occurs when occur stainless steel can’t protect itself. Basically, impurities weren’t washed off (seasalt, acid rain, etc.) and the stainless’s protective barrier of chromium oxide couldn’t reform to protect itself (underwater, not exposed to air), and etc.
Sciencey Answer: stainless is a metal and suffers from galvanic corrosion. Stainless steel is highly resistant to galvanic corrosion and other metals will corrode before the stainless does. This is why we attach sacrificial pieces of zinc to metal we protect (“anodes”).
Practical Answer: Stainless can be immaculate in severe marine environments for generations if you look after it. So you must Garden hose wash the stainless regularly, and oxy-acid wash once a month. The choice of acid wash is for the environment, not because of the stainless.
Source: Working on boats
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