1.11: Summary

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Corrosion is a problem facing us every day and in almost every activity. Corrosion wastes material and energy, and could prevent objects from doing the job they were made to do, possibly with dangerous consequences.

The rate at which corrosion occurs depends on the kinetics of the reactions taking place and so the electrical double layer is important.

Applying an overpotential to an electrode drives the reaction in one direction and away from equilibrium. Tafel’s law governs the new rate and as long as the reaction kinetics are activation controlled, the overpotential is proportional to the log of the corrosion current.

Other factors may limit the maximum rate of corrosion, with oxygen depletion limiting the speed of the cathodic reaction to the rate at which oxygen can be supplied from the bulk. The anodic reaction may be limited by passivation, if a sufficiently large overpotential is applied to form a passive layer. Passive layers separate the metal from the electrolyte and slow the corrosion reaction.

Faraday’s law can give meaningful results from the predicted corrosion current, i.e. giving the mass loss per unit time.

Corrosion can be slowed by either adding an inhibitor to remove hydrogen ions and move to a passivating region of the Pourbaix diagram, by adding an inhibitor to form a passive layer on the anode, or by adding an inert barrier to the surface of the anode.

Going further

Books

J M West, Basic oxidation and corrosion, Ellis Harwood (1936)

K R Trethewey and J Chamberlain, Corrosion for students of science and engineering, Longman (1988)

Websites

A really good website with definitions and explanations
http://www.corrosion-doctors.org/

Another website that covers most things in this TLP. A bit more detailed but needs more prior knowledge
http://www.corrosionist.com/

Corrosion control on the aluMATTER website