From the US Geological Services website:
"Chromium is critical in the manufacturing of stainless steel. Most stainless steel contains about 18 percent chromium; it is what hardens and toughens steel and increases its resistance to corrosion, especially at high temperatures."
From Dartmouth University's website:
"Chrome metal (the form chromium-0) is the element that makes steel “stainless.” Chromium in this form is hard, stable, and resistant to chemical changes such as oxidation or rust. Steel alloyed with chromium is harder and less brittle than iron and highly rust-resistant. This form of chromium is also used to coat or “chrome plate” the surface of other metals to produce a hard, shiny, chemically resistant surface."
From the Practical Machinist:
"Chromium is added to the iron and carbon mix, protecting the metal from oxidation. For steel to be considered stainless, it must contain 11% of chromium. However, most stainless steels contain more than that."
Stainless steels are rust-resistant, not rustproof and will corrode under the right circumstances.