The corrosion resistance of the austenitic stainless steels (within the concentration and temperature conditions described above) relies on the formation and stability of a passive film. With increased temperature or for concentrations between 10% and 85%, the standard 300 series stainless steels do not develop a passive film, and are considered active, therefore high corrosion rates will occur.
The effect of flow velocity is dependent on the stability of the passive film. Under stable passive film conditions, velocities up to 20 ft/sec (6 m/sec) are possible. However, within the trans-passive region, where the passive film becomes unstable, high velocity may remove the protective film, and result in increased corrosion rates. Aerated conditions or the presence of oxidizing agents assist in stabilizing the passive film. However, excessive amounts of oxidizing agents may create trans-passive conditions where corrosion rates again increase.
Under borderline conditions, anodic protection can be used to maintain the stainless steel in the passive region and increase the useful temperature and concentration range.