Corrosion of steel alone is a multi-trillion-dollar problem globally. Economic losses are not the only consequences of corrosion; structural failure of buildings and bridges can be fatal. This article will briefly discuss the corrosion process and methods to prevent or reduce corrosion with coatings.
Electrochemical corrosion
The most common forms of corrosion are chemical processes where metals in a high energy elemental state chemically react to attain a lower energy, more stable positively charged state. These low energy states are often in the form of metal oxides so corrosion is often referred to as oxidation.
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The corrosion process is an Oxidation-Reduction (RedOx) reaction because one participant in the reaction is oxidized (the metal) while another is reduced (oxygen). Looking back on general chemistry, we know that in order to become positively charged the metal atoms have to lose electrons (oxidize). A great mnemonic to remember the movement of electrons in oxidation-reduction reactions like those occurring in the corrosion process is “Oil Rig”.
O Oxidation i is l loss
R Reduction i is g gain
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Since electrons flow during the process of corrosion, it is an electrochemical reaction where current is generated. Three components are required to create an electrochemical cell allowing corrosion to proceed. These components are:
- an electron donor called the anode
- an electron acceptor called the cathode
- a conductive path for the electrons to flow between them.
Often in an electrochemical corrosion cell, the pathway for the electrons is a conductive liquid called an electrolyte. While pure water is not highly conductive, solutions of salts can be. This is the reason that the presence of salts often accelerates corrosion. Removing or neutralizing any of these components prevents corrosion of the substrate from occurring.
Controlling corrosion through coatings
When an object is susceptible to oxidation, one of the most common methods of corrosion control is the application of a protective anti-corrosion coating. Different mechanisms can be used to prevent corrosion but two methods are used in the majority of protective coatings. These methods are barrier formation and disrupting the electrochemical corrosion cell. Often both mechanisms are combined in a single coating to increase corrosion resistance.
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