A chemical substance or compound that can prevent or slow down the corrosion of materials when it exists in the environment (medium) in an appropriate concentration and form. Therefore, corrosion inhibitors can also be called corrosion inhibitors. Its dosage is very small (0.1%～1%), but the effect is remarkable. This method of protecting metals is called corrosion inhibitor protection. Corrosion inhibitors are used in neutral media (boiler water, circulating cooling water), acid media (hydrochloric acid to remove pot scale, acid leaching solution for rust removal of plated parts before electroplating) and gas media (gas phase corrosion inhibitor).