CHEM-E6185 – Applied electrochemistry and Corrosion (5 cr)
Assignment 2 (graded), 2.2.2016, computer class 144.
Topic: Determining reaction rates from voltammetric measurements
Electrochemical measurements have frequent use to estimate corrosion resistance of metallic
materials. The electrochemical nature of corrosion allows measurement of instantaneous
corrosion rates. Monitoring corrosion rate over the time interval required for weight loss
measurement is therefore possible. The mean corrosion rate obtained by integration of corrosion
rate over experiment length should equal the rate from corresponding weight loss experiments.
The basic test method in corrosion research is measurement of a potential/current density curve
or polarization curve. A polarization curve provides the first view of the behavior of the system.
Polarization curves are made by changing the electrode potential at a constant rate and recording
potential and the corresponding current density. A common practice is to use semilogarithmic
plotting of current density with units of mV or V for potential and mA/cm2 or A/m2 for current
density. All currents are often treated as positive to simplify plotting.
Homework
Polarization curves have use in several specific applications. The most common is determination
of corrosion current density with the Tafel method. This method uses the assumptions that the
polarization curve is linear and described with the Tafel equation (1). This is valid at about 100–
200 mV from the corrosion potential at current densities 100–1000 times the corrosion current
density. In (1)  is overpotential [mV], i is current density and constant b is the Tafel slope
[mV/decade].
𝜂 = 𝑎 + 𝑏 ∙ log⁡(𝑖)
The Tafel method is a high overpotential range approximation of the Butler-Volmer equation
describing a single activation controlled electrode reaction. It has constant use for mixed
potential situations with several reactions. As the sample is polarized from its corrosion potential
to the anodic direction, the rate of anodic reaction increases, and the rate of the cathodic reaction
decreases. Polarization to the cathodic direction results in an increase of cathodic reaction rate
and decrease of anodic reaction rate. At sufficiently high overpotential, the sample acts solely as
an anode or cathode. Potential vs. logarithm of current density shows a linear dependency. By
extrapolating the linear ranges of anodic and cathodic polarization curves back to the corrosion
potential, their intersections should meet and show the corrosion current density. Since the
electrode reactions are never purely activation controlled, linear ranges may be difficult to locate.
Extrapolated current densities are not too accurate. The linear part of the polarization curve
should be at least one decade in current density to ensure reliable extrapolation.
The data file worksheet “Tafel” has two replicate measurements for steel in inhibited,
closed central heating system water at temperatures T = 20 C and 80 C. Make plots of
current density vs. potential and determine corrosion current density and Tafel slopes at
both temperatures. As you have two replicate tests, calculate the best estimate and error
estimates for all parameters.
Corrosion rates can sometimes be measured rapidly using methods that do not destroy the
sample. One such method is measurement of polarization resistance. The method uses the low
overpotential approximation of the Butler-Volmer equation. Another name for the method is the
linear polarization method because the polarization curve is assumed linear in the area of
corrosion potential. This assumption is valid if the corrosion potential is sufficiently far from the
equilibrium potentials of the anodic and cathodic reactions and both reactions are activation
controlled. The polarization resistance measured in a mixed potential case is inversely
proportional to corrosion current density. It is analogous to the relationship between charge
transfer resistance and exchange current density of a single electrode reaction. Corrosion current
density in mA/cm2 is calculated from (2) where ba and bc are anodic and cathodic Tafel slopes in
mV and Rp is polarization resistance in cm2. The factor involving Tafel slopes are often
tabulated as B values.
𝑖𝑐𝑜𝑟𝑟 =
𝑏𝑎 ∙ 𝑏𝑐
1
∙
2.303 ∙ (𝑏𝑎 + 𝑏𝑐 ) 𝑅𝑝
Assuming that the anodic reaction has theoretical slopes of 30–120 mV and cathodic reaction
slopes of 120 mV for hydrogen evolution to infinity for mass transfer controlled oxygen
reduction, the factor B would have values of 10–52 mV. A two-electron dissolution (ba = 60 mV)
with hydrogen evolution as the cathodic reaction (bc = 120 mV) will produce B value of 17 mV.
Using a B value of 20–30 mV, one can usually estimate corrosion rate to at least a factor of 2. If
the measured slopes do not obey Tafel behavior due to mass transfer and resistance effects, then
the results are invalid. Poor correlation can also result with voluminous surface films, low
conductivity, and presence of other redox reactions.
The data file worksheet “Linpol” has two replicate measurements for steel in inhibited,
closed central heating system water at temperatures T = 20 C and 80 C. Make plots of
potential vs. current density and determine polarization resistances. Calculate corrosion
current densities using Tafel slopes you determined in the first part. In case you do not
have measured Tafel slopes, you can estimate the corrosion current density by using initial
values ba = bc = 100 mV used in many analysis packages. Do these values result in different
current density? As you have two replicate tests, calculate again the best estimate and error
estimates for all parameters.
In the MyCourses section “Assignment” you will find instructions and an Excel file with
polarization curves for Tafel plots and polarization resistance measurements. The first worksheet
of the file has slots for the answers. Return your Excel file in the MyCourses return box.
The graded exercise deadline is 15.2.2016 at 23.55 MyCourses time.