Page 58 - Corrosion Engineering Principles and Practice

38 C h a p t e r 3 C o r r o s i o n E l e c t r o c h e m i s t r y 39

Some metals such as silver are univalent, while other metals such as
iron, titanium, and uranium are multivalent and possess positive
charges as high as 6 (see Fig. 2.8). Equation (3.10) is general and applies
to all corrosion reactions.

3.3 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 nineteenth 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 and the number of moles of
electrons involved. 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 Eqs. (3.11)
and (3.12):

Ag + e → Ag(s) (3.11)
+
−
Ag(s) → Ag + e (3.12)
+
−
According to Faraday’s law, the reaction with 1 mol of silver
would require 1 mol of electrons, or 1 Avogadro’s number of electrons
(6.022 × 10 ). The charge carried by 1 mol of electrons is known as 1
23
faraday (F). The faraday is related to other electrical units through the
electronic charge; the electronic charge is 1.6 × 10 coulomb (C).
−19
Multiplying the electronic charge by the Avogadro number means
that 1 F equals 96,485 C/(mol of electrons). Combining Faraday’s
principles with specific electrochemical reactions of known
stoichiometry leads to Eq. (3.13) that relates the charge Q to chemical
descriptors N and n:

⋅
Q = F ∆ N n (3.13)
⋅
where N is the number of moles and ∆N the change in that amount
n is the number of electrons per molecule of the species
being reacted
The charge Q can be defined in terms of electric current as
in Eq. (3.14),

Q = ∫ 0 t I dt (3.14)
⋅
where I is the total current in amperes (A)
t is the duration of the electrochemical process in seconds (s)