Did you know that stainless steel can rust? Stainless steels are primarily alloys of iron and chromium. When treated properly, the iron in the surface layer is removed and the remaining chromium is converted to chromium oxides. This process is known as PASSIVATION. This chromium oxide layer renders the metal “stainless” in that it prevents oxygen and moisture from reaching the underlying layers that are rich in iron. If this layer is damaged or stripped off, there is nothing to prevent the underlying iron from being attacked by oxygen and forming iron oxides or rust.
Tank Wash operators should inspect and consider passivating a tank every time that a chemical with a name like chlorobenzene, vinylidine chloride, bromobutane, benzyl bromide, is shipped or stored in it (after a thorough cleaning, of course). These chemical species will break down with water (a process called HYDROLYSIS) and release acids that are particularly damaging to the chromium oxide surface film of stainless steel. Even an alkaline washing cannot prevent these “halogens” from attacking the metal…it only slows the process. Therefore, it is highly recommended that the tank be inspected for any signs of corrosion after each wash, especially if it has been used to ship a material with “brom”, “chlor”, or “fluor” in the name. If there is any question, the owner of the tank should be able to determine prior loads.
As a side note, do not fall into the “Quick-Fix Trap” of using aluminum brightener to de-rust or even clean stainless steel tanks. These materials often contain hydroFLUORic and/or hydroCHLORic acids and using them will impact the chrome oxide layer and may set the tank up for even more corrosion later on.
For more information about stainless steels, click here http://en.wikipedia.org/wiki/Stainless_steel
