Steel is actually not a material in it's own right. It's the generic name for a group of ferrous metals principally composed of iron. So the majority content af steel is actually iron, which is, as we know, prone to rusting. In fact, most modern steels contain about 98% iron!
Stainless Steel.
Part of the manufacturing process for the type of steel we know as "stainless" is to introduce corrosion inhibiting elements (usually Chromium) into the process. This creates an alloy with, typically, the following elements:
Carbon: 0.03%
Manganese: 2%
Silicon: 1%
Chromium: 18-20%
Nickel: 8-12%
The rest is Iron.
So you see, whilst stainless steel is manufactured with corrosion inhibiting properties, it cannot be claimed to actually prevent corrosion altogether because this would depend upon the ammount of Chromium and the accuracy of the process. And nothing is perfect.
So this is why, as Nelly says, it can happen to steel discs too!
