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Corrosion inhibitors are chemical compounds that decrease the corrosion rate of metals and alloys when added to a fluid or gas.
The effectiveness of corrosion inhibitors is dependent on many factors including quantity of water, flow regime and fluid composition. In addition to these, other factors include:
o The metal or other material that the liquid or gas flows through or on, such as a pipeline or an engine component.
o The system corrosion inhibitors have to act in and the temperature at which this operates. This can include a pipeline under the sea, oil for a racing car engine, or paint for a bridge.
There are three main types corrosion inhibitors:
Anodic - These are chromates which form a passivisation layer on steel and aluminium surfaces to prevent the oxidation of the metal. These have to be used at the right concentration, otherwise, they can aggravate pitting corrosion and form a non-uniform layer with local anodes. Cathodic - These work by either slowing down the cathodic reaction or selectively precipitating on certain cathodic areas. One example of a cathodic inhibitor is a volatile amine which is present in steam, used in the boilers used to drive turbines; these protect the pipes through which the condensed water passes.
Cathodic corrosion inhibitors include the following:
o Cathodic poisons - Stifle the cathodic reduction processes to balance the anodic corrosion reaction. They can also increase the vulnerability of a metal to hydrogen-induced cracking.
o Oxygen scavengers - This refers to chemicals which react with dissolved oxygen to reduce corrosion.
Mixed - These act in both an anodic and cathodic manner. An example of a mixed corrosion inhibitor is Hydrazine, which together with ascorbic acid (vitamin C) can help to reduce the rate of corrosion in boilers by removing the dissolved oxygen from the water.
If you work with liquids or metals, or wonder why some metals corrode faster than others, perhaps it's time you thought more about Corrosion Inhibitors.
