Corrosion inhibitors react with metal and form protective coatings on their surfaces to protect from harmful effects of water, polar impurities and oxygen. They are primarily used for copper and iron surfaces, and are used between 0.01% and 0.3% by weight in formulated oils. Corrosion is an electrochemical reaction between the metal surface functioning as anode and the interface between the metal surface and the lubricant as the cathode.
When a metal surface is in contact with oxygen and water, cathodic and anodic reactions take place, releasing metal into the oil. Corrosion inhibitors prevent this process from taking place by chemically insulating the metallic anode from the cathode. When they are introduced into a system, they coat the metallic surfaces due to their high affinity to the metal and form metal deactivating films. These separate the metal surfaces from oxygen and water and shut off the reactions, preventing corrosion. Many types exist: those that undergo physical adsorption (fatty amines and acid derivatives), chemical reactions (phosphorus and sulfur compounds), acid neutralization (some dispersants and detergents) and metal chelation (N-salicylidene-propylenediamine used mostly in fuels).
Tolutriazoles, thiadiazoles, triazoles and mercaptobenzothiazoles are used, with varying nitrogen and sulfur materials. Some are more effective depending on the molecular weight and their ability to get close to a metal surface and form a strong film on the surface.
The main difference between rust inhibitors and corrosion inhibitors is where they function. Corrosion inhibitors work to protect copper surfaces, and rust inhibitors protect iron surfaces. They both have polar head groups that attach to the metal surfaces and hydrocarbon tails that form a film to protect metal surfaces against chemical attack.