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.
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