Why doesn’t stainless steel rust? 🍴 Everyone knows that steel rusts, and that rust is bad. It’s ugly, it eats the base metal, it weakens stuff, it’s messy, it’s aggressive. We all hate rust. Fortunately someone invented stainless steel, which is a lot like regular steel except it doesn’t rust. But why doesn’t it rust? What is different is that stainless steel just has a bit of chromium added to it. But why? Well when oxygen attacks the surface of steel if forms an oxide layer with the most available atoms at the surface. In regular steel these are iron atoms, so iron oxide forms. Iron oxide is rust. Iron oxide is also porous so the oxygen soaks right through it and attacks more steel below the rust. It just keeps rusting. In stainless steel it’s the added chromium atoms that are available, so chromium oxide forms. Chromium oxide is nice and shiny, as a reflective surface it looks beautiful next to your spouse or a waterfall. Crucially though, chromium oxide is not porous. So atmospheric oxygen cannot soak through it to attack the steel below. Corrosion is halted. So stainless steel doesn’t rust because some clever person added chromium to steel to form a non-porous coating. Thanks for reading, you are now the thickness of a chromium oxide layer smarter than you were 60 seconds ago. Remember this next time you hold a knife and fork, as this is why you don’t die of tetanus 3 times a day.
@Object_Zero_ Slight correction. Rust doesn't cause tetanus. The animal manure which once was common to find out in fields with rusty nails does. Step on a nail, the bacterium is driven in deep.
@Object_Zero_ Stainless dose rust in the right conditions. When I was working I had a full or Stainless tools from Allen keys to screwdriver to spanner’s Using Steel tools on stainless will cause it to rust as I found out back in the 80’s
@Object_Zero_ I've seen it rust. A lot of vessels in the oil and gas industry are built from stainless. The mixture of chemicals in them causes incredible levels of corrosion.
@Object_Zero_ The Cr3O2-Oxide film that is formed essentially is a ceramic. You can say that it is very dense, since the spoon in your mouth does not taste like iron at all.
@Object_Zero_ nice explainer, but rust does not cause tetanus.
@Object_Zero_ Please keep these coming!
The question should be if chromium is only about 10% how is the entire surface still protected? Why don't you see splotches of 90% rust and 10% chromium oxide? It's actually because the chromium atoms migrate through the steel to the surface and form a complete layer because they are more attracted to the oxygen than the iron is.
@Object_Zero_ So why not mix in the lighter Aluminum and get the same effect?
@Object_Zero_ Corten looks much better.
@Object_Zero_ The irony of higher reactivity is it creates a less reactive coating
@Object_Zero_ The cosmetic, and protective, effect of the skin is enhanced by a process called passivation. A mild acid is is used to remove any free iron from the surface, leaving a clean chromium oxide layer. We used citric acid in 2008, not sure if that is still current.
@Object_Zero_ That's nice and all, but give me Ti-6Al-4V anyway. You're paying
@Object_Zero_ It's not stainless as in free of stain, it's stainless as in the just stains less
@Object_Zero_ Following the algorithm recommendations, I see. I love these posts. Keep it up!
@Object_Zero_ Well said! By the way, almost everything rusts, at the end of the day.
@Object_Zero_ The german term for stainless steel translates to something like “glorified steel” and does not say anything about that being stain-less or rust-less. There is a reason for that.
@Object_Zero_ We thermal spray coat a lot of parts for the chemical and power gen industries in particular with chrome oxide to prevent or severely reduce corrosion
@Object_Zero_ We invented plenty of carbon steels that develop surface rust, but don't rot.