Corrosion inhibitor

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A corrosion inhibitor is a chemical compound that, when added in small concentration, stops or slows down corrosion of metals and alloys.

A typical good corrosion inhibitor will give 95% inhibition at concentration of 80 ppm, and 90% at 40 ppm. Some of the mechanisms of its effect are formation of a passivation layer (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.

Some corrosion inhibitors are hexamine, phenylenediamine, dimethylethanolamine, sodium nitrite, cinnamaldehyde, condensation products of aldehydes and amines (imines), chromates, nitrites, phosphates, hydrazine, ascorbic acid, and others. 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.

An example of an anodic inhibitor is chromate which forms a passivation layer on aluminium and steel surfaces which prevents the oxidation of the metal. Sadly chromate is carcinogenic in humans; the toxicity of chromates was featured eg. 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.

Nitrite is another anodic inhibitor. If anodic inhibitors are used at too low concentration, they can actually aggravate pitting corrosion, as they form a nonuniform layer with local anodes.

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 differental 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 so making proton reduction less favorable. It is also possible that with the correct choice of an amine the amine forms a protective film on the steel surface and so acts at the same time as an anodic inhibitor. An inhibitor which acts both as a cathodic and anodic manner is a mixed inhibitor.

Hydrazine and vitamin C both help reduce the rate of corrosion in boilers by removing the dissolved oxygen from the water. Sadly hydrazine is very toxic and is a carcinogen, therefore its use is being discouraged.

Antiseptics are used to counter microbial corrosion. Benzalkonium chloride is commonly used in oil field industry.

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

For fuels, various corrosion inhibitors can be used: [1]

• DCI-4A, widely used in commercial and military jet fuels, acts also as a lubricity additive. Can be also used for gasolines and other distillate fuels.
• DCI-6A, for motor gasoline and distillate fuels, and for US military fuels (JP-4, JP-5, JP-8)
• DCI-11, for alcohols and gasolines containing oxygenates
• DCI-28, for very low-pH alcohols and gasolines containing oxygenates
• DCI-30, for gasoline and distillate fuels, excellent for pipeline transfers and storage, caustic-resistant
• DMA-4 (solution of alkylaminophosphate in kerosene), for petroleum distillates