3. Faraday's Laws and Rate of Corrosion

3.1 The I and II Laws

The classical electrochemical work conducted by Michael Faraday in the Nineteenth century produced two laws published in 1833 and 1834 named after him. The two laws can be summarized below.

First Law

The mass of primary products formed at an electrode by electrolysis is directly proportional to the quantity of electricity passed. Thus:

m I t

m = Z I t

I    =  current in amperes
t    =  time in seconds
m  =  mass of the primary product
Z   =  constant of proportionality (electrochemical equivalent). It is the mass of a substance liberated by 1 ampere-second of a current (1 coulomb)

Second Law

The masses of different primary products formed by equal amounts of electricity are proportional to the ratio of molar mass to the number of electrons involved with a particular reaction:

m1, m2    =  masses of primary product
M1, M2    =  molar masses (g/mol)
n1, n2  =  number of electrons
Z1, Z2   =  electrochemical equivalent

combining the First Law and the Second Law, we get

where F = Faraday’s constant. It is the quantity of electricity required to deposit the ratio of mass to valency of any substance and expressed in coulombs per mole (C.mol-1). It has a value of 96,485 couloumbs per mole.