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Corrosion: A Discussion and Empirical Demonstration
1. Joseph W. Lehmann
Corrosion: A Discussion and Empirical Demonstration
2015
Oxford, Ohio-Miami University Engineering Laboratories
2. Abstract
Corrosion is a major issue in today’s chemical industry for both the cost and safety aspect of it.
With that said, predicting corrosion, specifically the corrosion rate is vital. This experiment
investigated a method utilizing electrical potential and currents to estimate the corrosion rate.
The rate was obtained from three different methods: polarization resistance, tafel method, and
the computer software. The experiment concluded that of the three metals investigated
(aluminum, brass, and steel), the steel had the highest corrosion rate with an average of 292.4
mills per year and brass had the lowest with an average of 12.1 mills per year. It was also
important to note that steel had a very wide range of values between the methods used to
calculate the corrosion rate, from 181 mills per year to 44.7 mills per year.
Introduction
Under the right condition, all types of metal will corrode. The corrosion of some metals species
even happen in the open air, under standard conditions. The current understanding of this
phenomena is that in the presence of very concentrated acidic solutions, currents, or other harsh
conditions the rate of corrosion increases. It is understood that this occurs because more
electrons are able to flow from one medium to the other in such conditions, versus in standard
atmospheric conditions. Interestingly, the corrosion of metals can be calculated and represented
quantitatively. This is often represented as a mass per unit time, but can sometimes be shown as
a length per unit time. Using this data, polarization curves for metals can be produced, which
basically plot the current values against the electrochemical potential values. The applications of
these ideas are very important and worth researching. Some of the applications of corrosion, and
rates of corrosion expand into areas of structural engineering, petrochemical engineering,
medicine and health, and even technology and aeronautical engineering. This report will attempt
to quantify the corrosion rate, and potential for specific metals to corrode, known as the
corrosion potential. Also it will address the active-passive behavior of some metals, using the
polarization curves mentioned previously.
Experimental Procedure
To begin the setup, a glass reactor vessel is setup with 3 electrodes connected to it. The three
necessary electrodes were a working electrode, a counter electrode and a reference electrode in
order to make measurements of the vessel’s environment. These were attached to an
electrochemical testing unit, supplied by Gamary Instruments. The working electrode was setup
so that each metal being studied can be connected and act as a working electrode. The counter
electrode was a simple platinum mesh. The reference electrode was setup so that it maintained
the same potential of the working electrode, to avoid any unwanted flow of current between the
two. A representative picture of the setup can be seen in figure 1. After the apparatus was set up,
3. and checked for accuracy, each metal species was cleaned and prepared, and secured into the
reaction vessel. Also, worth noting is that a current resistant rubber tape was paced around the
sides of the metal specimen, so as to keep the area by which current flows relatively constant.
From this point, each metal was tested using the electrochemical testing unit, and the data from
each test was saved. After each run, the next metal specimen was cleaned and prepared as
aforementioned. A total of three metals were tested.
Figure 1: Diagram of reaction vessel with attached electrodes, and metal species (2).
Theoretical and Discussion
Active-passive metals demonstrate a particular behavior in solutions with such that their s-
shaped polarization curves. When trying to understand what exactly we are dealing with when
studying passivity, we need to understand what this is describing. The passivity of a metal is a
condition that occurs during corrosion in which a resistance to corrosion is formed as a thin film
forms over the surface during oxidation. In order to clarify what is going on with the metal we
can look at what happens at different portions of a graph. (3)
Vessel
4. Figure 2: Graph which depicts areas of active-passive metals (3)
We can see that low potentials the corrosion rate increases with anodic currents, which is the
active portion. Above this lies the passive portion of which is cause the development of a passive
film that protects the metal from corrosion. Next the metal goes into the transpassive where it is
supposed that this is the breakdown voltage or the pitting potential is reached.
5. Figure 3: Steel Active-passive graph
Here in Figure 3 the we can see the steel is active until roughly -225 mV, then the steel develops
the protective film. We can only see the beginning of the passive region in our experiment, we
see that the graph is beginning to level out.
Figure 4: Brass active-passive graph
Figure 4 shows the experiment involving brass, the active region is seen to be increasing until
about 50 mV which is higher than the steel, however the passive area is very small. The
protective film does not last very long as it degrades quickly and goes almost immediately into
the transpassive area.
6. Figure 5: Aluminum active-passive graph
In Figure 5 the active area is all that we see for the duration of our experiment. This means there
was not enough time to reach the voltage where the passive protection film begins to develop. So
we don’t know when the active area is left and the passive area begins.
Polarization Resistance
One method to determine the rate of corrosion during this experiment is through polarization
resistance. It is the resistance that exists at the interface of the electrode and electrolyte. It is
represented as.
𝑅 𝑝 = lim
∆𝐸→0
∆𝐸
∆𝑖
This resistance exists as a limit to the chemical reaction that takes place between the electrode
and electrolyte. During the corrosion of this experiment, a reaction in the following form occurs.
𝑀𝑒𝑡𝑎𝑙 + 𝐻2 𝑆𝑂4 → 𝑀𝑒𝑡𝑎𝑙 𝑆𝑂4 + 𝐻2
In this case, the polarization resistance is the limit by which this reaction can occur. Different
metals offer different resistance. This is true for both the cathodic site and anodic site. At the
anodic site, the metal reacts with the sulfate group and, in doing so, electrons move to the
cathodic site. At the cathodic site, the electrons are consumed by the free hydrogen ions to form
hydrogen gas. The limit between these simultaneous processes is represented as the polarization
resistance.
7. The following relation is shown in experimentation.
𝑖 𝑐𝑜𝑟𝑟 = 2.303
𝛽 𝑎 𝛽𝑐
𝛽 𝑎 + 𝛽𝑐
1
𝑅 𝑃
Where
𝑖 𝑐𝑜𝑟𝑟 − 𝑡ℎ𝑒 𝑐𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑐𝑢𝑟𝑟𝑒𝑛𝑡
𝛽 𝑎 − 𝑎𝑛𝑜𝑑𝑖𝑐 𝑇𝑎𝑓𝑒𝑙 𝑠𝑙𝑜𝑝𝑒
𝛽𝑐 − 𝑐𝑎𝑡ℎ𝑜𝑑𝑖𝑐 𝑇𝑎𝑓𝑒𝑙 𝑠𝑙𝑜𝑝𝑒
Rearranging the equation, polarization resistance can be calculated.
𝑅 𝑃 = 2.303
𝛽 𝑎 𝛽𝑐
𝛽 𝑎 + 𝛽𝑐
1
𝑖 𝑐𝑜𝑟𝑟
The calculated polarization resistance and corrosion rate of the different metals is shown in Table
1.
Table 1 – polarization resistance and calculated corrosion rate of the three materials
RP
Corrosion
(mpy)
Aluminium 1992 95
Steel 310 181
Brass 11643 11
From this, it can be seen that steel has the lowest corrosion resistance, brass has the highest
resistance, and aluminum is between both. This correlates with the calculated corrosion rates,
the higher rate of corrosion corresponding to a lower resistance. This relation is expected.
Tafel Method
The second way the corrosion current can be determined using electric potentials is through the
Tafel method. In this method, the Tafel equation is applied to both the anode and cathode. The
equation is as follows:
∆𝑉 = 𝐴 ∗ ln(
𝑖
𝑖0
)
8. In the above equation, ∆𝑉 refers to the overpotential (Volts), A is the “Tafel Slope” (Volts), i is
the current density (A/m^2), and 𝑖0 refers to the “exchange current density”, which in this
experiment is rate of reaction at 0 voltage potential (1). In order to apply this equation, it is
assumed that the reverse reaction is negligible compared to the forward reaction. To practically
apply this method, the overpotential (Volts) is graphed versus the logarithmic of the current.
When the equilibrium is established, there will be a linear slope for each the cathode and anode.
Where these two slopes intersect determines the corrosion current and corrosion potential. This
was applied to the three metals investigated in the lab. The method in action is shown below:
Figure 6. Overpotential (V) versus logarithmic current (A) for Aluminum. The two red lines
represent the Tafel slopes.
From the above figure it is possible to estimate that the corrosion current is approximately 110
micro amps and the corrosion potential being -0.456 Volts.
A similar process was performed for the brass metal:
9. Figure 7. Overpotential (V) versus logarithmic current (A) for Brass. The two red lines represent
the Tafel slopes.
For brass, using the figure above, the corrosion potential was estimated at -100 mV and the
corrosion current at 20.0 micro amps. In the same way the steel was analyzed:
10. Figure 8. Overpotential (V) versus logarithmic current (A) for Steel. The two red lines represent
the Tafel slopes.
From the above graph, the corrosion current was found to be 400 micro amps, and the corrosion
potential to be -340 mV.
Once all the corrosion currents were estimated, the corrosion rate could be calculated by using
the following equation:
𝐶𝑜𝑟𝑟𝑜𝑠𝑖𝑜𝑛 𝑅𝑎𝑡𝑒 (𝑙𝑒𝑛𝑔𝑡ℎ/𝑡𝑖𝑚𝑒) =
𝑖 𝑐𝑜𝑟𝑟 ∗ 𝐸𝑊
𝜌 ∗ 𝐴 ∗ 𝐹
In the above equation, corrosion rate is generally expressed in mils/year. Also, EW is the
equivalent weight (MW/charge of ion), 𝜌 is density, A is surface area, and F is Faradays
Constant (9.65*10^4 C/mol). The below table represents the summary of the results using the
Tafel method:
11. Table 2. Summary of the results from the Tafel method.
Aluminum Brass Steel
Surface Area (cm^2) 0.72 0.71 0.71
Density (g/cm^3) 2.70 8.40 7.85
Equivalent Weight
(g/mol)
8.99 32.04 27.66
i corr (Amps)
1.10E-04 2.00E-05 4.00E-04
E corr (Voltage) -0.46 -0.10 -0.34
Corrosion Rate (mpy) 65.4 13.8 254.8
As displayed in Table 2, steel by far has the highest corrosion rate, 254.8 mpy while brass has
the least with 13.8 mpy. The corrosion potential did not relate to the corrosion rate as the highest
went to aluminum at -0.46 V.
Final Results
In addition to the Tafel and Polarization Resistance Method, the computer software also ran
calculations concerning the corrosion voltage and rate. The following figure represents the
results from all three sources:
0
50
100
150
200
250
300
350
400
450
0 0.5 1 1.5 2 2.5 3 3.5
CorrosionRate(mpy)
Method
Aluminum
Brass
Steel
1 = Polarization Resistance
2 = Tafel Method
3 = Computer
12. Figure 9: Corrosion rate (mpy) for each metal tested for all three sources of calculation
From Figure 9, it shows that all three methods agree that steel corrodes the fastest with an
average of 292.8 mills per year and brass the least with an average of 12.1 mills per year. It also
shows that as the corrosion rate increases, precision between the three different measurements
decreases.
Conclusion
In conclusion, the steel metal corrodes at a rate that is twice as much or more than aluminum and
more than fifteen times that of brass. Corrosion is a very huge issue in the chemical industry
today from both a cost and safety issue. This method for calculating the corrosion rate is very
important. However, as shown in the results, there can be a great amount of variance in just how
the corrosion rate is calculated from the same experimental set-up. This means that when
applying this concept to industry a high factor of safety should be employed.
13. References
(1) "Tafel Equation." Wikipedia. Wikimedia Foundation, n.d. Web. 07 May
2015.
(2) Principles and Prevention of Corrosion, Denny Jones
(3) "Pubdoc_12_18745_228."Www.uobabylon.edu.iq/eprints/pubdoc_12_1874
5_228.docx. University of Babylon, n.d. Web.
(4) “https://www.ndeed.org/EducationResources/CommunityCollege/Materials/
Physical_Chemical/Corrosion.htm” n.d. Web. 02 May 2015
(5) “http://www.lehigh.edu/~amb4/wbi/kwardlow/corrosion.htm” n.d. Web. 07
May 2015.
Appendix
Data inputed into the software program:
Sample Mol W Oxidation
Equiv
Weight
D
(cm)
Area
density
(g/cm3)
1 Aluminum 27.0 3 8.994 0.958 0.721 2.7
2 Steel 55.3 2 27.661 0.953 0.713 7.85
3 Brass 64.1 2 32.043 0.951 0.710 8.4
Aluminum graph generated by the software:
