An inhibitor is usually a material added in a small quantity to a particular environment that reduces the rate of corrosion. They may be classified a number of ways but are usually 1) Oxidizing; 2) Scavenging; 3) Vapor-phase inhibitors; Sometimes they are called Volatile corrosion inhibitor 4) Adsorption inhibitors; 5) Hydrogen-evolution retarder. Another way to classify them is chemically. As there is more concern for the environment and people are more keen to use Renewable resources, there is ongoing research to modify these materials so they may be used as corrosion inhibitors.