All metals react with oxygen in the atmosphere to form an oxide film on the surface. Unfortunately, the iron oxide formed on ordinary carbon steel continues to oxidize, causing the rust to continuously expand, eventually forming holes. The carbon steel surface can be secured by electroplating with paint or oxidation-resistant metals such as zinc, nickel and chromium, but, as is known, this protection is only a thin film. If the protective layer is destroyed, the underlying steel begins to rust.
The corrosion resistance of stainless steel depends on chromium, but because chromium is one of the components of steel, the protection method is not the same. When the amount of chromium added exceeds 11.7%, the atmospheric corrosion resistance of the steel significantly increases. However, when the chromium content is higher, although the corrosion resistance can still be improved, it is not obvious. The reason is that the alloying treatment of steel with chromium changes the type of surface oxide to a surface oxide similar to that formed on pure chromium metal. This tightly adhered chromium-rich oxide protects the surface from further oxidation. This oxide layer is extremely thin, through which the natural luster of the steel surface can be seen, giving stainless steel a unique surface. Moreover, if the surface layer is damaged, the exposed steel surface reacts with the atmosphere and undergoes self-repair, and the oxide “passivation film” is re-formed to continue protection. Therefore, all the stainless steel elements have a common characteristic that the chromium content is above 10.5%.
The term "stainless steel" refers not only to stainless steel alone, but to more than one hundred industrial stainless steels. Each type of stainless steel developed has a good performance in its specific application area. The key to success is first to understand the use, and then determine the correct steel.
There are usually only six types of steel related to the application of building construction. They all contain 17-22% of chromium, and the preferred steel also contains nickel. The addition of molybdenum can further improve the atmospheric corrosion, especially to the chloride containing atmosphere.