Passivating Inhibitors

Passivating inhibitors are the most common type of inhibitors mainly because they are very effective in reducing the rate of corrosion. They protect the material by aiding in the formation of a thin, inert film on the surface of a metal, thereby moving its corrosion potential toward the noble region, which effectively passivates the metal. This shift in corrosion potential can be significant, and sometimes on the order of a 100 mV.3 Passivating inhibitors can be either oxidizing, which do not require oxygen to be present, or nonoxidizing, which do require oxygen to be present in the environment. Oxidizing inhibitors include nitrites and nitrates, and chromates were one of the most widely used inhibitors. Although chromate inhibitors are some of the most effective, they are currently being phased out by regulations from the Environmental Protection Agency in the United States due to health and environmental concerns. Nonoxidizing inhibitors include phosphates and molybdates. These can only be used for applications which encounter oxygencontaining environments. The primary disadvantage to passivating inhibitors is that they can actually accelerate localized corrosion on the material being protected if the concentration of inhibitors falls below a critical concentration. Therefore it may be necessary to periodically reapply the corrosion inhibitor or monitor the inhibitor concentration.