The reason why stainless steel does not rust, because it contains "chrome." Typical levels of chromium must reach 13 to 25% (by weight) in order to change the situation steel surface is oxidized. But stainless steel is not really will not rust, if observed with a microscope, its surface is actually a lot of small rust spots; closer look, these spots in all contain sulfur, and the presence of elemental sulfur is in the process of steel making no avoid. These are not the original sulfur atoms in the water becomes something like sulfuric acid cause corrosion, but in stainless steel, the cooling process, chromium steel in moving to some place, some substitution of manganese sulfide and manganese location and cause near where the chromium content is low, and when the content is less than 13% chromium stainless steel will rust.