Corrosion inhibitor
A corrosion inhibitor is a chemical compound that, when added in small concentration, stops or slows down corrosion (rust) 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 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 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 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.  Another example of a rust and scale inhibitor is Aqua Clear PX, manufactured in Cape Town, South Africa. As a result of the anti-corrosive, rust and scale properties that Aqua Clear PX contains, the following conditions in engines have been experienced:
- Radiator caps, cores, tanks, thermostats, hoses, Welsh plugs and heater controls last longer, therefore, clean corrosion, rust and scale free systems should not require a lubricant.
- Clean water jackets, radiator cores and other passages dissipate heat quicker resulting in less abnormal engine operating temperatures.
- Obvious advantages of such conditions will minimize unexpected and costly fatalities such as:
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.
For fuels, various corrosion inhibitors can be used:
- 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 U.S. 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
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. Other corrosion inhibitors are Anticor 70, Albaex, Ferrophos, and Molywhite MZAP.