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Metal Corrosion
Source：Corrtest
Time：2021-12-22
View：0
Metal Corrosion
When the metal material is in contact with the surrounding medium, the material is destroyed due to chemical or electrochemical action. Metal corrosion is a thermodynamic spontaneous process, converting a metal of high-energy state into a metal compound of a low-energy state. Among them, the corrosion phenomenon in the petroleum and petrochemical industry is more complicated, including the electrochemical corrosion of brine, H2S and CO2.
The nature of most corrosion processes is electrochemical. The electrical properties of the metal/electrolyte solution interface (electric double layer) are widely used in corrosion mechanism studies, corrosion measurement, and industrial corrosion monitoring. The electrochemical methods commonly used in metal corrosion research are: open circuit potential (OCP), polarization curve (Tafel plot), electrochemical impedance spectroscopy (EIS).
CS potentiostat/galvanostat electrochemical workstation can be used to monitor the real-time electrode potential of the metal material in the system for a long time. After the potential is stabilized, the open circuit potential of the material can be obtained.
To express the polarization performance of an electrode process completely and intuitively, it is necessary to experimentally determine the over-potential or the electrode potential as a function of current density, which is called “polarization curve”.
The icorr of the metal material can be calculated based on the Stern-Geary equation..png)
B is the Stern-Geary coefficient of the material, Rp is the polarization resistance of the metal.
.png)
Principle to obtain icorr through Tafel extrapolation method
Corrtest CS studio software can automatically do fitting to the polarization curve. The tafel slop on anode segment and cathode segment, i.e., ba and bc can be calculated. icorr can also be obtained. Based on the Faraday law and in combining with the electrochemical equivalent of the material, we can convert it to metal corrosion rate (mm/a) .
The impedance spectrum is a curve drawn from the impedance data measured by a test circuit at different frequencies, and the impedance spectrum of the electrode process is called an electrochemical impedance spectrum. There are many types of EIS spectrum, but the most commonly used are the Nyquist plot and the Bode plot.
The user studied the corrosion resistance of Ti-6Al-4V alloy stent prepared by conventional wrought method(specimen #1), selective laser melting method(specimen #2) and electron beam melting method(specimen #3). The stent is used for human implantation, so the corrosion medium is simulated body fluid (SBF). The temperature of the experimental system also needs to be controlled at 37℃.
Instrument: CS350 Potentiostat/galvanostat
Experimental device:CS936 jacketed flat corrosion cell, Constant temperature drying oven
Experimental drugs: Acetone, SBF, Room temperature curing epoxy resin
Experimental medium:
Simulated body fluid (SBF)：NaCl-8.01,KCl-0.4,CaCl2-0.14,NaHCO3-0.35,KH2PO 4-0.06, glucose -0.34, unit is: g/L
Specimen(WE)
Ti-6Al-4V Alloy stent 20×20×2 mm,
Exposed working area is 10×10 mm
The non-test area is coated/sealed with room temperature curing epoxy resin.
Reference electrode(RE): Saturated calomel electrode
Counter electrode(CE): CS910 Pt conductivity electrode
.png)
2.1 Experiment steps and parameters setting
2.1.1 OCP
Before testing. the working electrode needs to be polished from coarse to fine (360 mesh, 600 mesh, 800 mesh, 1000 mesh, 2000 mesh in order) till the surface is smooth. After polishing, rinse it with distilled water and then degrease it using the acetone, put it in a constant temperature drying oven and dry at 37℃ for use.
Assemble the specimen onto the corrosion cell, introduce the simulated body fluid into the corrosion cell, and insert the saturated calomel electrode (SCE) with a salt bridge into the flat corrosion cell. Be sure that the tip of the Luggin capillary right face the working electrode surface. The temperature is controlled at 37℃ by water circulation.
Connect the electrodes with the potentiostat by the cell cable.
Experiment→stable polarization→OCP.png)
You should enter a file name for the data, set the total time of the testing, and start the test. The OCP of metal material in the solution changes slowly, and it takes a relatively long period to keep stable. So it’s suggested to set time no shorter than 3000s.
2.1.2 Polarization curve
Experiment→stable polarization→potentiodynamic.png)
Set the initial potential, final potential and scan rate, select the potential output mode as “vs. OCP”.
The “Use” can be checked to choose the vertex E#1 and vertex E#2. If it’s not checked, then the scan will not go through the corresponding potential.
There are up to 4 independent polarization potential set points. The scan starts from the initial potential, to “vertex E#1 ” and “vertex E#2”, and finally to the final potential. Click the "Enable" check box to turn on or off "Intermediate Potential 1" and "Intermediate Potential 2". If the check box is not selected, the scan will not pass this value and set the potential scan to the next one.
It is noteworthy that the polarization curve measurement can only be conducted on the condition that the OCP is already stable. Usually after 10 minutes’ quiet time, we will open the OCP stable function by clicking the following:
.png)
→ .png)
The software will start the testing automatically after the potential fluctuation is lower than10mV/min
In this experiment example, the user set the potential -0.5~1.5V (vs. OCP)
You can set the condition to stop or reverse the scan. This mainly used in pitting potential measurement and Passivation curve measurement.
2.2 Results
2.2.1 OCP
By open circuit potential test we can obtain the free corrosion potential Ecorr , from which we can judge the corrosion resistance of the metal material. Generally speaking, the more positive the Ecorr is, the harder the material is corroded..png)
From the graph we can conclude that the corrosion resistance of specimen #1&2 are better than #3.
2.2.2 Tafel plot analysis(corrosion rate measurement)
The polarization of this experiment is as follows:.png)
.png)
As is shown, from the calculated corrosion rate value we can get the same conclusion as what we obtained by OCP measurement. The corrosion rate is calculated by Tafel plot. We can see the values of corrosion rate comply with the conclusion we obtained by OCP method.
Based on the Tafel plot, we can obtain the corrosion current density icorr by the analysis fitting tool integrated in our CS studio software. Then according to other parameters such as working electrode area, material’s density, the equivalent weight, the corrosion rate is calculated.
Steps are:
Import the data file by clicking.png)
.png)
Data fitting
Click cell info. , and enter the value accordingly..png)
If you’ve already set the parameters in the cell &electrode setting before testing, then you don’t need to set cell info. here again.
Click “Tafel” to the Tafel fitting. Choose the auto Tafel fitting or manually fitting for the data of anode segment/cathode segment, then the corrosion current density, free corrosion potential, corrosion rate can be obtained. You can drag the fitting result to the graph. .png)
3. EIS measurement
Experiments → Impedance → EIS vs. Frequency.png)
.png)
Equivalent circuit fitting for the Q235 carbon steel EIS results
Steps are as follows:
Draw the equivalent circuit of the capacitance arc - use the model in the “quick fit” to obtain R1, C1, R2.
Draw the equivalent circuit of Warburg impedance part - use the model in the “quick fit” to obtain the specific value of Ws.
Drag values to the complex circuit→ change all the elements type to be “Free+” →click Fit
From the results, we see the error is less than 5%, indicating that the self-defined equivalent circuit we draw is in accordance with the impedance circuit of actual measurement. The Bode fitting plot is generally in accordance with the original plot..png)
.png)
When the metal material is in contact with the surrounding medium, the material is destroyed due to chemical or electrochemical action. Metal corrosion is a thermodynamic spontaneous process, converting a metal of high-energy state into a metal compound of a low-energy state. Among them, the corrosion phenomenon in the petroleum and petrochemical industry is more complicated, including the electrochemical corrosion of brine, H2S and CO2.
The nature of most corrosion processes is electrochemical. The electrical properties of the metal/electrolyte solution interface (electric double layer) are widely used in corrosion mechanism studies, corrosion measurement, and industrial corrosion monitoring. The electrochemical methods commonly used in metal corrosion research are: open circuit potential (OCP), polarization curve (Tafel plot), electrochemical impedance spectroscopy (EIS).
1.Techniques in Corrosion study
1.1OCP
On an isolated metal electrode, one anode reaction and one cathode reaction is performed at the same speed at the same time, which is called the coupling of the electrode reaction. The reaction of mutual coupling is called the “conjugation reaction”, and the whole system is called the “conjugate system”. In the conjugated system, the two electrode reactions inter-couplings with each other, and when the electrode potentials are equal, the electrode potentials don’t vary with time. This state is called “stable state”, and the corresponding potential is called “stable potential”. In the corrosion system, this potential is also called “(self) corrosion potential Ecorr”, or “open circuit potential (OCP)”, and the corresponding current density is called “(self) corrosion current density icorr”. Generally speaking, the more positive the open circuit potential, the more difficult it is to lose electrons and be corroded, indicating that the corrosion resistance of the material is better.CS potentiostat/galvanostat electrochemical workstation can be used to monitor the real-time electrode potential of the metal material in the system for a long time. After the potential is stabilized, the open circuit potential of the material can be obtained.
1.2 Polarization curve (Tafel plot)
Generally, the phenomenon that the electrode potential deviates from the equilibrium potential when there is a current passing through is called “polarization”. In electrochemical system, when polarization occurs, the negative shift of the electrode potential from the equilibrium potential is called “cathodic polarization”, and the positive shift of the electrode potential from the equilibrium potential is called “anodic polarization”.To express the polarization performance of an electrode process completely and intuitively, it is necessary to experimentally determine the over-potential or the electrode potential as a function of current density, which is called “polarization curve”.
The icorr of the metal material can be calculated based on the Stern-Geary equation.
Corrtest CS studio software can automatically do fitting to the polarization curve. The tafel slop on anode segment and cathode segment, i.e., ba and bc can be calculated. icorr can also be obtained. Based on the Faraday law and in combining with the electrochemical equivalent of the material, we can convert it to metal corrosion rate (mm/a) .
1.3 EIS
Electrochemical impedance technology, also known as AC impedance, measures the change of voltage (or current) of an electrochemical system as a function of time by controlling the current (or voltage) of the electrochemical system as a function of sinusoidal variation over time. The impedance of the electrochemical system is measured, and further, the reaction mechanism of the system (medium/coating film/metal) is studied, and the electrochemical parameters of the fitting measurement system are analyzed.The impedance spectrum is a curve drawn from the impedance data measured by a test circuit at different frequencies, and the impedance spectrum of the electrode process is called an electrochemical impedance spectrum. There are many types of EIS spectrum, but the most commonly used are the Nyquist plot and the Bode plot.
2.Experiment example
Taking an article published by a user using the CS350 electrochemical workstation as an example, a concrete introduction to the method of the metal corrosion measurement system is introduced.The user studied the corrosion resistance of Ti-6Al-4V alloy stent prepared by conventional wrought method(specimen #1), selective laser melting method(specimen #2) and electron beam melting method(specimen #3). The stent is used for human implantation, so the corrosion medium is simulated body fluid (SBF). The temperature of the experimental system also needs to be controlled at 37℃.
Instrument: CS350 Potentiostat/galvanostat
Experimental device:CS936 jacketed flat corrosion cell, Constant temperature drying oven
Experimental drugs: Acetone, SBF, Room temperature curing epoxy resin
Experimental medium:
Simulated body fluid (SBF)：NaCl-8.01,KCl-0.4,CaCl2-0.14,NaHCO3-0.35,KH2PO 4-0.06, glucose -0.34, unit is: g/L
Specimen(WE)
Ti-6Al-4V Alloy stent 20×20×2 mm,
Exposed working area is 10×10 mm
The non-test area is coated/sealed with room temperature curing epoxy resin.
Reference electrode(RE): Saturated calomel electrode
Counter electrode(CE): CS910 Pt conductivity electrode
The jacketed flat corrosion cell
2.1 Experiment steps and parameters setting
2.1.1 OCP
Before testing. the working electrode needs to be polished from coarse to fine (360 mesh, 600 mesh, 800 mesh, 1000 mesh, 2000 mesh in order) till the surface is smooth. After polishing, rinse it with distilled water and then degrease it using the acetone, put it in a constant temperature drying oven and dry at 37℃ for use.
Assemble the specimen onto the corrosion cell, introduce the simulated body fluid into the corrosion cell, and insert the saturated calomel electrode (SCE) with a salt bridge into the flat corrosion cell. Be sure that the tip of the Luggin capillary right face the working electrode surface. The temperature is controlled at 37℃ by water circulation.
Connect the electrodes with the potentiostat by the cell cable.
Experiment→stable polarization→OCP
OCP
You should enter a file name for the data, set the total time of the testing, and start the test. The OCP of metal material in the solution changes slowly, and it takes a relatively long period to keep stable. So it’s suggested to set time no shorter than 3000s.
2.1.2 Polarization curve
Experiment→stable polarization→potentiodynamic
Potentiodynamic scan
Set the initial potential, final potential and scan rate, select the potential output mode as “vs. OCP”.
The “Use” can be checked to choose the vertex E#1 and vertex E#2. If it’s not checked, then the scan will not go through the corresponding potential.
There are up to 4 independent polarization potential set points. The scan starts from the initial potential, to “vertex E#1 ” and “vertex E#2”, and finally to the final potential. Click the "Enable" check box to turn on or off "Intermediate Potential 1" and "Intermediate Potential 2". If the check box is not selected, the scan will not pass this value and set the potential scan to the next one.
It is noteworthy that the polarization curve measurement can only be conducted on the condition that the OCP is already stable. Usually after 10 minutes’ quiet time, we will open the OCP stable function by clicking the following:
→ The software will start the testing automatically after the potential fluctuation is lower than10mV/min
In this experiment example, the user set the potential -0.5~1.5V (vs. OCP)
You can set the condition to stop or reverse the scan. This mainly used in pitting potential measurement and Passivation curve measurement.
2.2 Results
2.2.1 OCP
By open circuit potential test we can obtain the free corrosion potential Ecorr , from which we can judge the corrosion resistance of the metal material. Generally speaking, the more positive the Ecorr is, the harder the material is corroded.
1-OCP of Ti-6Al-4V alloy stent prepared by conventional wrought method
2- OCP of Ti-6Al-4V alloy stent prepared by selective laser melting method
3- OCP of Ti-6Al-4V alloy stent prepared by electron beam melting method
2- OCP of Ti-6Al-4V alloy stent prepared by selective laser melting method
3- OCP of Ti-6Al-4V alloy stent prepared by electron beam melting method
From the graph we can conclude that the corrosion resistance of specimen #1&2 are better than #3.
2.2.2 Tafel plot analysis(corrosion rate measurement)
The polarization of this experiment is as follows:
Based on the Tafel plot, we can obtain the corrosion current density icorr by the analysis fitting tool integrated in our CS studio software. Then according to other parameters such as working electrode area, material’s density, the equivalent weight, the corrosion rate is calculated.
Steps are:
Import the data file by clicking
Data fitting
Click cell info. , and enter the value accordingly.
Click “Tafel” to the Tafel fitting. Choose the auto Tafel fitting or manually fitting for the data of anode segment/cathode segment, then the corrosion current density, free corrosion potential, corrosion rate can be obtained. You can drag the fitting result to the graph.
3. EIS measurement
Experiments → Impedance → EIS vs. Frequency
EIS vs. frequency
EIS analysis
EIS of Q235 carbon steel in 3.5% NaCl solution is as follows:Q235 carbon steel impedance plot- Nyquist
The above Nyquist plot is composed of the capacitance arc (marked by the blue frame) and the Warburg impedance (marked by the red frame). Generally speaking, the bigger the capacitance arc, the better the corrosion resistance of the material.Equivalent circuit fitting for the Q235 carbon steel EIS results
Steps are as follows:
Draw the equivalent circuit of the capacitance arc - use the model in the “quick fit” to obtain R1, C1, R2.
Draw the equivalent circuit of Warburg impedance part - use the model in the “quick fit” to obtain the specific value of Ws.
Drag values to the complex circuit→ change all the elements type to be “Free+” →click Fit
From the results, we see the error is less than 5%, indicating that the self-defined equivalent circuit we draw is in accordance with the impedance circuit of actual measurement. The Bode fitting plot is generally in accordance with the original plot.
Bode: Fitting plot vs. actual measurement result
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