Corrosion report and comments on the various Tafel Plots for Aluminium sample with different inks and without ink

1. Report 2 : Corrosion studies
Group 4:
Ahmed Shumayal
Mohammadreza Shamshiri

2. 1-Introduction
Corrosion is defined as a chemical deterioration of materials such as metals due to
exposure to their environment, which is either liquid or gas. This process usually
occurs in the surface of materials, however it could progress within their bulk
[1].From economic point of view, corrosion has been along with a huge cost for us. It
is estimated that the expense of corrosion in the areas including commercial,
residential and transportation is annually about 276 billion dollars. Only $121 billion
of this cost has been allocated to control of corrosion [2].
One of the most important types of corrosion is electrochemical corrosion.
Undoubtedly, electron transfer reactions play the main role in this kind of corrosion.
Surface groups or adsorbed ions could charge the aqueous interfaces and the situation
is controlled and affected by a static electric field. As a result of interaction between
different components, namely solvent, mobile charges and fixed particles, a
complicated interfacial boundary layer forms (figure 1). This layer is called ‘’the
double layer’’. The first layer created by the closest absorbed molecules of solvent
and other component is known as inner Helmholtz layer. This layer which forms on
the solid surface, has a certain charge.
Figure 1: Layer at solid/ liquid interface including: (1)
Inner Helmholtz layer, (2) Outer Helmholtz layer,
Solved cations, (4) Diffuse layer, (5) Electrolyte solvent,
(6) adsorbed ions [3].
The thickness of inner Helholtz layer is d1. The second layer which contains particles
with apposite charge is called the outer Helmholtz layer. As it is seen in the figure (1)
free ions could just approach until distance d2 form the surface where the outer
Helmholtz layer is located. There is another layer called ‘’the diffuse layer’’ which is

3. drawn from outer Helmholtz layer to the bulk. The solvated ions could distribute in
this layer due to thermal motion in the system. This layer plays an important role in
corrosion[3].
When the flow of these reactions reaches a balance, they would be in equilibrium and
no electrical current exists [4]. The change of electrical potentials of anodic and
cathodic reactions against absolute current are represent by Evans diagram (Figure 2).
In this graph the total and theoretical currents for both reactions are depicted using
curved and straight lines respectively. The total current could be measured by means
of potentiostat. Due to need for the investigation of a wide range of current values,
logarithmic scale for current is used.
Figure2: Anodic and cathodic
current in Evans diagram [4]
When there is no connection to the metal, the voltage is called Open circuit potential
(OCP) and it is shown by Eoc. OCP refers to the difference that exists in electrical
potential. It normally occurs between two device terminals when detached from a
circuit involving no external load. Once a potential relative to the open circuit is made
present, the entire system gauges the potential of the open circuit prior to turning on
the cell. We should allow reach this potential before starting any experiment. In fact
this situation is a steady state. The current value attributed to Eoc is called corrosion
current (Icorr).Whenever the potential of an electrode is forced away from its value at
open-circuit, that is referred to as “polarizing” the electrode. When an electrode is
polarized, it can cause current to flow through electrochemical reactions that occur at

4. the electrode surface. The amount of current is controlled by the kinetics of the
reactions and the diffusion of reactants both towards and away from the electrode.
In cells where an electrode undergoes uniform corrosion at open circuit, the open
circuit potential is controlled by the equilibrium between two different
electrochemical reactions. One of the reactions generates cathodic current and the
other generates anodic current. The open circuit potential equilibrates at the potential
where the cathodic and anodic currents are equal. It is referred to as a mixed potential.
If the electrode is actively corroding, the value of the current for either of the
reactions is known as the corrosion current.
Mixed potential control also occurs in cells where the electrode is not corroding.
When there are two, simple, kinetically-controlled reactions occurring, the potential
of the cell is related to the current by the following equation:
where, Icorr = the corrosion current in amps, Eoc = the open circuit potential in volts, βa
= the anodic Beta coefficient in volts/decade, βc = the cathodic Beta coefficient in
volts/decade.
Using corrosion current, we are able to calculate corrosion rate. The corrosion rate
depends of several factors including temperature, solution composition, metal history
and the like [5].We can calculate corrosion rate as below:
Corrosion Rate (
mm
decade
) = CR =
3.27×10−3
(𝑖 𝑐𝑜𝑟𝑟)(𝐸𝑊)
𝑑
Where icorr is corrosion current density in µA per cm2, d is alloy density in g per cm3,
and EW is the equivalent weight for metal.
Three-electrode Cell
As corrosion is a slow phenomenon and occurs gradually, we need some testing
methods which expose material in harsher environments than normal ones to
accelerate the deterioration, so we would readily be able to quantify the materials’
resistance against corrosion. These kinds of tests are used to have a better material
selection and find suitable corrosion control strategies. A normal corrosion testing can
consume a lot of time while using accelerated corrosion test (ACT) would take less

5. time. Selecting the right conditions causes these testing methods would be more
beneficial and their data would be more reliable [6].
Figure 3: Schematic of a Three-electrode cell: (1) working electrode; (2) auxiliary
electrode; (3) reference electrode [7]
The three-electrode cell is the basis of most of the accelerated tests. A three-electrode
cell consists of three main parts including working electrode, a reference electrode
and an auxiliary electrode (Figure 3).

6. In fact, the working electrode is a
sample which is investigated in the test.
In the other word the electrochemical
reaction occurs in the working
electrode. This electrode could be made
by materials which are inert like Au,
Ag, glassy carbon, Pt and the like. In a
corrosion application, the investigated
material plays the role of working
electrode. Dependence on the
application, the physical characteristics
of the electrode like shape and size
would be varied. The other main part
of a three-electrode cell is called the
auxiliary electrode which sometimes known as counter electrode. Using this electrode
closes the current circuit in the electrochemical cell. However, this electrode does not
take part in the reaction. Due to the fact that the total area of the source of electron
should be larger than the area of the working electrode; so this could not limit kinetic
conditions governing on investigated electrochemical process and the current readily
flows between the working electrode and the source of electrons. Similar to the
working electrode, the auxiliary one is made from inert materials like Au, glassy
carbon, Pt and the like. Moreover, a reference is needed which has a stable potential.
It is essential for potential control and measurement. A redox system with constant
concentrations of components could be a suitable candidate as a reference. Such a
system is found in buffered or saturated concentrations. Furthermore, it is usually
tried that current flow through the reference electrode is kept close to zero [8]. Figure
4 shows some of common reference electrodes with their Figure 4. The
reference electrodes
measured potentials at 250C [8].
Electrochemical impedance spectroscopy (EIS) is a method for the non-destructive
characterization of thin layers, membranes, surfaces and films. Typically a small
amplitude AC voltage (or current) signal is applied to a sample while the amplitude
and phase relationship of the corresponding current (or voltage) signal is monitored.
Figure 4: Common reference electrodes

7. For an impedance spectrum, measurements are made over several decades of AC
frequency with up to about 10 measurements per decade.
The results can be mathematically modeled, typically by treating the sample as a
number of sub-layers each with its own electrical resistance and capacitance. In
particular, the capacitance and dielectric constant of each sub-layer enables the
thickness of that layer to be determined. EIS data for electrochemical cells such as
fuel cells are most often represented in Nyquist and Bode plots. Bode plots refer to
representation of the impedance magnitude (or the real or imaginary components of
the impedance) and phase angle as a function of frequency. Because both the
impedance and the frequency often span orders of magnitude, they are frequently
plotted on a logarithmic scale. Bode plots explicitly show the frequency-dependence
of the impedance of the device under test.
A complex plane or Nyquist plot depicts the imaginary impedance, which is
indicative of the capacitive and inductive character of the cell, versus the real
impedance of the cell. Nyquist plots have the advantage that activation-controlled
processes with distinct time-constants show up as unique impedance arcs and the
shape of the curve provides insight into possible mechanism or governing phenomena.
However, this format of representing impedance data has the disadvantage that the
frequency-dependence is implicit; therefore, the AC frequency of selected data points
should be indicated. Because both data formats have their advantages, it is usually
best to present both Bode and Nyquist plots.
2- Methodology
In the laboratory, we aimed to set up a three-electrode electrochemical corrosion cell
and used it to make our measurements. The samples include aluminium without
coating (Al), aluminium coated with commercial ink1 and aluminium coated with
commercial ink2 .Working electrodes were made from aluminium cylinders, exposed
disc electrode surface area 0.90 cm2.
We used Ag/ AgCl (3 M KCl) and Pt as the reference electrode and counter electrode
respectively. In addition, a circle-shaped carbon steel plate (A= 4.9cm2) is taken as a
working electrode. It should be mentioned that the surface of plate was pre treated
using 400, 600, 800 and 1500 grade silicon carbide papers, then cleaned with acetone.
The set up is built as seen in the figure 5.

8. OCP measurement of samples last 2 h following immersion in 0.05 M NaCl. In
addition, the Tafel plots were plotted from (-0.25 V vs OCP) until (+0.25 V vs OCP)
at 2 mV s-1. Tafel plots were prepared by a Autolab PGSTAT30
potentiostat/galvanostat connected to a computer with general purpose
electrochemical system software (GPES V4.9) from Metrohm-Autolab. The EIS
measurements, also, were carried out using a PC-controlled Solartron 1250 Frequency
Response Analyzer, coupled to a Solartron 1286 Electrochemical Interface using
ZPlot 2.4 software. In these measurements a sinusoidal voltage perturbation of
amplitude 10 mV rms was applied in the frequency range between 65 kHz and 0.1 Hz
with 10 frequency steps per decade.
3-Results and analysis
3-1-Tafel
Using potentiodynamic polarization measurements, the corrosion resistance
performance of the samples were evaluated. As seen in figure 5, it is clear that both
ink1 and ink2 have more positive values in the corrosion potential and applying these
coatings causes significant shifts in both anodic and cathodic tafel curves to lower
current densities. The ink1 sample has the greatest corrosion potential value among
two other samples confirms the best protection of the aluminum.
Figure 5. The Tafel curves of the bare Al, ink1 and ink2
-12 -11 -10 -9 -8 -7 -6 -5 -4 -3
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0.0
0.1
Aluminium
ink1
inki2
E/vs.Ag/AgCl
[Log (i)]

9. Table 1 illustrates the electrochemical kinetic parameters of all three samples,
cathodic and anodic polarization curves. Using these values and Stern–Geary equation
the polarization resistance (Rp) values of each sample could be obtained.
Furthermore, the protection efficiency (PEF%) of samples is estimated using the
following equation:
100
)(
(%) 


corr
corrcorr
EF
i
cii
P
The calculation of protection efficiency (table 2) shows that the Ink2 sample has a
greater PEF than that of Ink1 sample, which means it has the better performance than
Ink1 sample.
Table 1. Electrochemical kinetic of the samples.
Table 2. The protection efficiency values of samples.
3-2-Electrochemical Impedance spectroscopy
The figures 6 and 7 illustrate the Nyquist plots of the all samples and equivalent
circuit of these plots, respectively. According to table 3, which shows the Impedance
Samples βa ( V/dec) Βc ( V/dec) Ecorr (V vs. Ag/AgCl) icorr ( A cm-2)
Aluminium 0.091 0.001 -0.64 2.61 x 10-7
Ink 1 0.018 0.022 -0.17 8.74 x 10-10
Ink 2 0.023 0.028 -0.32 3.26 x 10-12
Samples icorr ( A cm-2) RP PEF
Aluminium 2.61 x 10-7 1645.5 -
Ink 1 8.74 x 10-10 4.9x 106 99.6
Ink 2 3.26 x 10-12 1.6x 109 99.9
6000
11

10. parameters related to equivalent circuit for different samples, the Rct value of Ink 2
sample is higher than those of the aluminum and Ink 1 sample. In fact, the higher
value of Rct is attributed to the effective barrier behavior of this coating.
Figure 6. Nyquist plots of the bare Al, Ink1 and Ink 2 samples.
Figure 7. The equivalent circuit for presented Nyquist plots.
Table 3. Impedance parameters related to equivalent circuit for different samples
The inhibition efficiency (ηR%) values can be determined from the following
equation:
1001
)(
)(
% 






uncoatedR
coatedR
CT
CT
R
Using this equation, the inhibition efficiency values of sample were calculated and
presented in table.
3-3- Open circuit potential (OCP)
Sample CPEdl /µF cm-
2sα-1
α Rct / kΩ cm2 ηR%
Al 34.33 0.80 13.80
Ink 1 0.10 0.91 4.43 x103
Ink 2 0.12 0.92 7.53x103

11. Open circuit potential measurements were carried on samples in 0.05 M KCl
electrolyte solution during 2h of immersion and results are illustrated in figure 8.
Figure 8. Open circuit potential vs time plots 0.05 M NaCl.
As it is clear from the figure, the OCP values after 2h immersion for Ink 1 and Ink2
samples are more positive than that of Al electrode and the OCP value for Ink 2
sample is by the far the most positive value among the samples, which means either of
this coating or Ink 1 coating could be used as a protective corrosion layer on Al.
During 2h of immersion, the OCP values of Ink1 and Ink2 samples have a gradually
increasing trend, while after a dramatic rise in first 200 seconds, the OCP values of Al
sample leveled out until end of the the period.
4-Conclusion
The Tafel slope basically discloses to us the amount needed to build the overpotential
to expand the reaction rate by a factor of ten. This will be dictated by the magnitude
of the adjustment in the activation energy for a given increment in overpotential
(when a change of potential 1 V is made, the Gibbs energy of the procedure will
change by 1 eV for every electron exchanged, yet the activation energy will just
change by a small amount, and this division decides the Tafel slope). In a reaction
including just a single step with one electron exchange, the Tafel slope will be
controlled by the symmetry factor, which is normally 0.5 (comparing to a Tafel slope
of 120 mV). In a progressively unpredictable reaction including a few stages and a
few electron exchanges, the Tafel slope will be dictated by the rate-determining step
0 1000 2000 3000 4000 5000 6000 7000
-0.80
-0.75
-0.70
-0.65
-0.60
-0.55
-0.50
-0.45
-0.40
-0.35
-0.30
-0.25
-0.20
-0.15
-0.10
-0.05
Evs.Ag/AgCl/V
t / s

12. and by the number and nature (i.e., including an electron exchange or not) of the
previous steps. Thus, basically, from the Tafel slope we can reason whether the rate-
determining step obtained includes an electron exchange or not, and in addition the
quantity of electrochemical (including an electron exchange) and chemical (not
including an electron exchange) steps that go before it. Whenever we propose a
reaction system, we can compute what the comparing Tafel slope ought to be and, in
the case it doesn't agree with the test one, the mechanism we predict couldn’t be
accurate. Thus, the choice of extrapolation plays a very crucial role. [11]
From Figure 8, we can safely say that both the Ink1(Red) and Ink2 (black) have
improved corrosion resistance when compared to uncoated Aluminium. From Figure
5, they are both placed above and to the left of the Aluminium without any ink.
Depending on the specific application, it can be more reasonable to choose Ink1 over
Ink2 or vice versa.
5-References
[1]Corrosion Mechanisms in Theory and Practice, Third Edition, Philippe Marcus,
2017.
[2] http://www.corrosioncost.com/.
[3]Fundamentals of electrochemistry, Corrosion and Corrosion Protection, Andreas
Erbe, 2015.
[4]https://www.gamry.com/application-notes/corrosion-coatings/basics-
ofelectrochemical-corrosion-measurements/.
[5] Corrosion Engineering. , M. G. Fontana, McGraw-Hill, 1987.
[6]https://www.corrosionpedia.com/definition/1503/accelerated-corrosion-testact.
[7]https://en.wikipedia.org/wiki/Voltammetry.
[8] www.ecochemie.nl/download/Applicationnotes/.
[9] L. L. Shreir, Corrosion Volume 1 Metal/Environment Reactions. London,
Newnes-Butterworth, 1971.
[10] J. C. Scully, The fundamentals of Corrosion. , Pergamon Press, 1990.
[11]PalmSens – Compact Electrochemical Interfaces. Tafel Plots & Evans Diagrams
https://www.palmsenscorrosion.com/knowledgebase/tafel-plot-and-evans-diagram/
(accessed December 30th 2018)