Module Three of CCE 281 Corrosion: Impact, Principles, and Practical Solutions

Faraday's Law

If the current generated by one of the anodic reactions expressed earlier was known, it would be possible to convert this current to an equivalent mass loss or corrosion penetration rate with a very useful relation discovered by Michael Faraday, a nineteen century pioneer in electrochemistry. Faraday's empirical laws of electrolysis relate the current of an electrochemical reaction to the number of moles of the element being reacted. Supposing that the charge required for such reaction was one electron per molecule, as is the case for the plating or the corrosion attack of silver described respectively in equations: (reference)

According to Faraday’s law, the reaction with one mole of silver would require one mole of electrons, or one Avogadro's number of electrons (6.022 x 10²³). The charge carried by one mole of electrons is known as one Faraday (F). The Faraday is related to other electrical units through the electronic charge, i.e electron charge is 1.6 x 10^-19 coulomb. Multiplying the electronic charge by the Avogadro number means that one Faraday equals 96,485 C/(mole of electrons). Combining Faraday’s principles with specific electrochemical reactions of known stoichiometry leads to equation:

where:

where N is the number of moles and DN the change in that amount

n is the number of electrons per molecule of the species being reacted

I is the total current in amperes (A)

t is the duration of the electrochemical process in seconds (s)

The corrosion current itself can be either estimated by using specialized electrochemical methods or by using weight loss data and a conversion chart based on Faraday’s principle.