Forum member Jacob, who is studying materials science at LSU, provides this technical information: “The basic resistance of stainless steel occurs because of its ability to form a protective coating on the metal surface. This coating is a ‘passive’ film which resists further ‘oxidation’ or rusting. The formation of this film is instantaneous in an oxidizing atmosphere such as air, water, or other fluids that contain oxygen. Once the layer has formed, we say that the metal has become ‘passivated’ and the oxidation or ‘rusting’ rate will slow down to less than 0.002″ per year
(0.05 mm per year).
Unlike aluminum or silver, this passive film is invisible in stainless steel. It’s created when oxygen combines with the chrome in the stainless to form chrome oxide which is more commonly called ‘ceramic’. This protective oxide or ceramic coating is common to most corrosion resistant materials.
Halogen salts, especially chlorides, easily penetrate this passive film and will allow corrosive attack to occur. The halogens are easy to recognize because they end in the letters ‘ine’. Listed in order of their activity they are: fluorine, chlorine, bromine, iodine, astatine.
These are the same chemicals that will penetrate Teflon and cause trouble with Teflon coated or encapsulated o-rings and/ or similar coated materials. Chlorides are one of the most common elements in nature and if that isn’t bad enough, they’re also soluble, active ions. These provide the basis for electrolytes. The presence of electrolytic solutions can accelerate corrosion or chemical attack.”