Stainless Steel was invented in the early part of the 20th Century. It's supposed to resist corrosion, and it does for a while. There are different grades of Stainless Steel, and depending on how much Nickel or Chromium it's got, it's better at resisting one type of corrosion or another.
Stainless Steel actually benefits from exposure to oxygen, and if it doesn't have access to it, things start to fuck up. It's got this thing called a passivation layer, which protects it from the outside, acutally adapts to different molecules that come at it. There are different ways of messing up this layer, like you could weld it away, or under the right conditions for long enough it could be worn off. like, you could deprive it of oxygen. it's weird to think how it needs to breath, almost like a living thing.
When they make stainless, they start by rolling it to a certain thickness. Then they do something called annealing, which is they heat it and cool it, sometimes repeatedly, to get rid of bubbles and impurities (things that could be pockets inside the metal that are deprived of oxygen, which work like a cancer to corrode from within.) Then they 'pickle' it, which in this sense means they soak it in acid, which removes the extra molecules on the surface, creating the passivation layer.
316L is the alloy used for most watches and for cutlery. Fitting for a watch manufacturer that charges thousands of dollars for its watches, Rolex uses the more expensive 904L, which is especially resistant to corrosion. But they could have done better -- types 6Mo and 'super duplex' are even more corrosion resistant. Meanwhile type 2205 has mostly replaced the need for 904L, because it's got similar corrosion resistance, and it's stronger. But if Rolex changed metals, someone would notice and then they'd realize they didn't have the best availabe.
I wonder what grade was Ultima, the robot from the film Metropolis.
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