Chemical Corrosion Inhibitors, Redux

November 18, 2016

A chemical corrosion inhibitor is a compound that, through many different mechanisms, can decrease the corrosion rate of a material (typically an alloy or pure metal) when added to a liquid or gas in contact with said material.

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A chemical corrosion inhibitor is a compound that, through many different mechanisms, can decrease the corrosion rate of a material (typically an alloy or pure metal) when added to a liquid or gas in contact with said material. Commonly, corrosion is inhibited through the formation of a passivation layer, which prevents the corrosive compound from contacting the surface of the metal beneath. However, it is important to remember that corrosion inhibitors are additives to the fluids that surround the material which must be protected from corrosion – the passivation layers formed do not result from properties in the metal (such as the chromium oxide layer that is formed over the surface of stainless steel) and instead form from properties of the inhibitor. Usually, the corrosive materials that need to be inhibited are oxygen, hydrogen sulfide, or carbon dioxide. However, carbon dioxide is not generally corrosive on its own but when mixed with water forms carbonic acid, which is much more corrosive.

Synthetic or Natural Compounds

Corrosion inhibitors can be either synthetic or natural chemical compounds and can be classified into groups either through their mechanism, as organic or inorganic substances, or as oxidants or reductants. Typically, inorganic corrosion inhibitors have either cathodic or anodic mechanisms while organic corrosion inhibitors can have either cathodic, anodic or a protective film mechanism adsorbed from the fluid surrounding the metal.

Examples of Chemical Corrosion Inhibitors

There are many examples of chemical corrosion inhibitors used in standard industrial applications. Organic inhibitors are typically dissolved in acid solutions to prevent attack of metals known as acid pickling. Also, in the oil industry, sodium carbonates and organic amine complexes are used to reduce the corrosiveness of carbon dioxide, dihydrogen sulfide, and organic acids typically found in crude oil. The use of chemical corrosion inhibitors in the oil industry thus allows the use of carbon steel in the crude oil wells instead of stainless steel, which is much more expensive but also more resistant to corrosion than carbon steel.

Chemical corrosion inhibitors are a cheap and easy way to reduce corrosion without the need for expensive materials. Using inhibitors, corrosion can be reduced by additives in the process fluid, which is particularly helpful in plants which have not been designed or constructed to accommodate corrosive materials and therefore have not been constructed using the proper corrosion resistant materials. They are used in a wide variety of applications and industries with varying degrees of effectiveness. Therefore, it is important that an engineer take into account the mechanism of inhibition to improve the inhibitor’s efficiency, protect the environment from potential hazards, and to avoid any undesirable side reactions.

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