
Corrosion parameters
using Tafel calculation


Abstract
The Tafel calculation gives access
to the corrosion current regularly present at the surface
of a metal which is in contact with a corroding solution.
The corrosion is expressed in loss of metal thickness per
year. The Thermodynamical Ba and Bc parameters of the Butler
Volmer equation are also determined.
A voltammetric curve i = f(Vg)
having a zero current potential
can be processed according to the Tafel method. The Tafel
method gives also access to other corrosion parameters such
as the zero current potential (corrosion potential), the corrosion
current and the polarisation resistance of the sample under
study. The Tafel calculation is a complete tool to study the
corrosion process at a metal surface.
Experimental curve
This voltammetric curve was obtained
from a Pot. Linear V method carried out on a Inox 10/18 [AISI430]
(demonstration curve: Polarisation curve Inox18 10_025.CRV).
Settings
The Atomic mass of Iron (Fe (55.85 g)),
the Valence or number of electrons involved in the oxidation
reaction (Fe  2 eŻ Fe++) and
the Density of iron (7.8) are required to calculate the corrosion
rate in mm of metal per year. Results
In blue: log (i) = f(E) Voltammetric
experimental curve.
In red: Cathodic and anodic tangents to the curve, straight
line of equation x = E (i = 0) (corrosion potential). These
three lines intercept at a point which coordinates are: (corrosion
potential, corrosion current).
The Tafel calculation gives:
 the zero current potential (E
(i=0)) as the corrosion current,
 the Rp polarisation resistance
(a parabolic regression is performed on the curve around
the zero current potential, Rp is determined as the
slope of the tangent to this parabola at the zero current
potential).
 the i corr. corrosion current,
 the Ba slope of the tangent
to the anodic branch of the curve,
 the Bc slope of the tangent
to the cathodic branch of the curve,
 the Corrosion rate
calculated as a number of mm of iron thickness corroded
per year (the Atomic mass, Valence, Density
and Working electrode area entered in the Initial
data are used to calculate the corrosion rate). For this
iron species: Corrosion rate = 2.455 mm/year
 the correlation coefficient displayed
(Coef.: 0.9997) indicates the quality level of the
parabolic regression. This coefficient is comprised between
0 and 1. Closer to 1 this coefficient is, better is the
regression.
Conclusion
The Tafel calculation is an ideal
tool to determine the corrosion rate at an iron sample surface.
When the linearity range of the log (i) = f(E) curve covers
more than one current decade of the cathodic branch and around
one decade of the anodic one. In such conditions, the corrosion
rates of iron species can be determined accurately.
Note (polarisation resistance):
The Rp polarisation resistance
can also be determined by using the 2^{nd} Stern method
(see Examine
your experiment: Corrosion parameters using 2^{nd}
Stern calculation).
