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Corrosion Inhibitor and some examples of it

A corrosion inhibitor is a chemical compound that reacts with a metallic surface, or the environment this surface is exposed to, giving the surface a certain level of protection (see corrosion costs study findings).

Inhibitors often work by adsorbing themselves on the metallic surface, protecting the metallic surface by forming a film. Inhibitors are normally distributed from a solution or dispersion. Some are included in a protective coating formulation. Inhibitors slow corrosion processes by either-

  • Image if Corrosion inhibitorIncreasing the anodic or cathodic polarization behavior (Tafel slopes);
  • Reducing the movement or diffusion of ions to the metallic surface;
  • Increasing the electrical resistance of the metallic surface.

The scientific and technical corrosion literature has descriptions and lists of numerous chemical compounds that exhibit inhibitive properties. Of these, only very few are actually used in practice. This is partly due to the fact that the desirable properties of an inhibitor usually extend beyond those simply related to metal protection. Considerations of cost, toxicity, availability and environmental friendliness are of considerable importance.

When added a corrosion inhibitor to a fluid or gas, it decreases the corrosion rate of a metal or an alloy.

The effectiveness, or corrosion inhibition efficiency, of a corrosion inhibitor is a function of many factors like- fluid composition, quantity of water, flow regime, etc. If the correct inhibitor and quantity is selected then is possible to achieve high, 90-99%, efficiency, but this higher values shall be documented by laboratory and field test. 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.

For example, chromate is an anodic inhibitor, 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.

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

Zinc oxide is another example of a cathodic inhibitor, 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.

Volatile amines present in steams are very good examples of a cathodic inhibitor; these are used in the boilers 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. 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.

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, engine oil, and many other fluids used in industry. Corrosion inhibitors are often added to paints. A pigment with anticorrosive properties is zinc phosphate. Compounds derived from tannic acid (e.g. Kelate) or zinc salts of organonitrogens (e.g. Alcophor 827) can be used together with anticorrosive pigments.

A particular advantage of corrosion inhibition is that it can be implemented or changed in situ without disrupting a process. The major industries using corrosion inhibitors are the oil and gas exploration and production industry, the petroleum refining industry, the chemical industry, heavy industrial manufacturing industry, water treatment facilities, and the product additive industries. The largest consumption of corrosion inhibitors is in the oil industry, particularly in the petroleum refining industry.

Inhibitors have been classified differently by various authors. Some authors, for example, prefer to group inhibitors by their chemical functionality.