A corrosion inhibitor is a chemical compound that, when added to a liquid, decreases the corrosion rate of a material, typically a metal or an alloy.

The effectiveness, or corrosion inhibition efficiency, of a corrosion inhibitor is a function of many factors, including but not limited to: fluid composition, quantity of water, and flow regime. If the correct inhibitor and quantity is selected then it is possible to achieve high, 90-99%, efficiency.

Some of the mechanisms of its effect are formation of a passivation layer, that is a thin film on the surface of the material that stops access of the corrosive substance to the metal, inhibiting either the oxidation or reduction part of the redox corrosion system (anodic and cathodic inhibitors), or scavenging the dissolved oxygen. The suitability of any given chemical for a task in hand depends on many factors, from the material of the system they have to act in, to the nature of the substances they are added into and their operating temperature.

Anodic Inhibitors
An example of an anodic inhibitor is chromate which forms a passivation layer on aluminum and steel surfaces which prevents the oxidation of the metal. Unfortunately, chromate is carcinogenic in humans; the toxicity of chromates was featured in the film Erin Brockovich. Like hydrazine, the use of chromate to protect metal surfaces has been limited; for instance it is banned from some products. ScaleAway^TM does not use Anodic Inhibitors Nitrite and Nitrate are further anodic inhibitors. If anodic inhibitors are used at too low concentration, they can actually aggravate pitting corrosion, as they form a non-uniform layer with local anodes. Other types of anodic inhibitors are based on phosphates.

Another anodic inhibitor is pertechnetate. Unfortunately technetium is a radioactive isotope with a much higher radioactivity than uranium and may be used only in very safe areas like nuclear reactors.

Cathodic inhibitors
An example of a cathodic inhibitor is zinc oxide, which retards the corrosion by inhibiting the reduction of water to hydrogen gas. As every oxidation requires a reduction to occur at the same time it slows the oxidation of the metal. As an alternative to the reduction of water to form hydrogen, oxygen or nitrate can be reduced. If oxidants such as oxygen are excluded, the rate of the corrosion can be controlled by the rate of water reduction; this is the case in a closed recirculating domestic central heating system, where the water in the radiators soon becomes anaerobic. This is a very different situation to the corrosion in a car door where the water is aerobic. For instance, cars suffer from the fact that water can enter the cavity inside the door and become trapped there. The fact that the oxygen concentration is not uniform within the layer of water in the door then creates a differential aeration cell leading to corrosion. A cathodic inhibitor would be of little use in such a situation as even after inhibiting the reduction of water, the reduction of dioxygen would still be able to occur. A better method of preventing corrosion in the car door would be to improve the design to prevent water being trapped in the door and to consider using an anodic inhibitor such as phosphate.

One very good example of a cathodic inhibitor is a volatile amine present in steam; these are used in the boilers used to drive turbines to protect the pipework in which the condensed water passes. Here the amine is moved by the steam in a steam distillation to the remote pipework. The amine increases the pH thereby making proton reduction less favorable. It is also possible that with correct choice, the amine can form a protective film on the steel surface and, at the same time, act as an anodic inhibitor. An inhibitor that acts both in a cathodic and anodic manner is termed a mixed inhibitor.

Oxygen scavengers
Hydrazine and ascorbic acid (vitamin C) both help reduce the rate of corrosion in boilers by removing the dissolved oxygen from the water. However, as hydrazine is a highly toxic carcinogen, its use is being discouraged. Another oxygen scavenger is sulphite. Some other antioxidants can be used as well. ScaleAway^TM does not use Hydrazine

Uses
Corrosion inhibitors are commonly added to coolants, fuels, hydraulic fluids, boiler water, engine oil, and many other fluids used in industry.

Corrosion
Corrosion is the disintegration of an engineered material into its constituent atoms due to chemical reactions with its surroundings. In other words, corrosion is the wearing away of metals due to a chemical reaction.