Corrosion Rate Measurement on C-Steel

Transcription

1 Measurements of corrosion rate on Carbon-steel using Electrochemical (potentiodynamic Polarization, EIS etc.) technique. Corrosion Rate Measurement on C-Steel Abdullah Al Ashraf

2 1. Introduction: The degradation of a material due to a reaction with its environment is known as corrosion. Degradation implies deterioration of physical properties of the material. This can be a weakening of the material due to a loss of cross-sectional area, it can be the shattering of a metal due to hydrogen embrittlement, or it can be the cracking of a polymer due to sunlight exposure [1]. While corrosion can take any one of the several forms, the mechanism of attack in aqueous environments will involve some aspect of electrochemistry. There will be a flow of electrons from certain areas of a metal surface to other areas through an environment capable of conducting ions [2]. Corrosion in both aqueous and atmospheric environment, is an electrochemical process because corrosion involves the transfer of electrons between a metal surface and an aqueous electrolyte solution. It results from the overwhelming tendency of metals to react electrochemically with oxygen, water, and other substances in the aqueous environment. In this context, the term anode is used to describe that portion of the metal surface that is actually corroding while the term cathode is used to describe the metal surface that consumes the electrons produced by the corrosion reaction [3]. An electrochemical reaction is defined as a chemical reaction involving the transfer of electrons. It is also a chemical reaction which involves oxidation and reduction. Since metallic corrosion is almost always an electrochemical process, it is important to understand the basic nature of electrochemical reactions [2]. There are many different techniques are used in electrochemistry to determine the behavior of corrosion and its rate. Among many of them, potentiodynamic polarization is foremost one. Potentiodynamic polarization is used to examine the overall corrosion behavior of a system. In a typical potentiodynamic polarization experiment, the potential of a metal specimen is slowly scanned over a wide potential range. During the scan, various electrochemical reactions may take place on the electrode surface, resulting in anodic and cathodic cell currents that may vary over many orders of magnitude. The graphical output of the experiment is a plot of potential vs. log current density. Analysis of the curve can often yield the following types of information [4]: Corrosion potential An estimate of corrosion current density(corrosion rate) from Tafel extrapolation Portion of potential region where the electrode is passive. Breakdown of passivity Susceptibility to galvanic corrosion

3 2. Experimental: Gamry Reference 600 Potentiostat/Galvanostat/ZRA) was used to accomplish the polarization curves (Polarization Resistance and Tafel polarization) and electrochemical impedance spectroscopy (EIS) measurements. Linear polarization curves were obtained by sweeping the electrode potential from -20 mv vs OCP to +20 mv vs OCP at a scan rate of mvs -1, whereas the Tafel polarization curves were obtained by sweeping the electrode potential -250 to +250 mv Vs OCP at a scan rate of mv s -1. Moreover, the Impedance spectroscopy measurements was carried out at a frequency range from 100 khz to 0.1 Hz with amplitude of 10 mv peak-to-peak using AC signals at open circuit potential. The experiments were done at room temperature (25 o C ±1 o C) using 0.5 M H 2 SO 4 and 3.5 % NaCl solution respectively. Three different types of electrodes were used during polarization measurements. They are: i) Working electrode: C-Steel electrode with surface area 0.3 cm2 was polished with different emery papers, starting from grade 500 to 1200 followed by ultrasonically degreasing in ethanol and finally washing with doubly distilled water, just before insertion in the electrolytic cell. ii) Reference electrode: Saturated calomel electrode (SCE) type was used as reference electrode during all polarization measurements. Fig.1: Potentiostat GAMRY Tester iii) Counter electrode: A graphite rod used as a counter electrode. It served to supply the wanted current being connected to the output of the operational amplifier. Two different types of solutions were used during polarization measurements. They are namely, 0.5 M Sulfuric acid solution which was prepared by diluting concentrated H 2 SO 4 acid to the appropriate volume of the double distilled water and 3.5% Sodium chloride solution which was prepared by dissolving 3.5 gm. of pure sodium chloride powder in 100 ml of double distilled water.

4 3. Results & Discussion: Fig.2 shows four obtained corrosion test results of C-steel in 0.5 M H 2 SO 4. Fig.2(A,B,C, & D) represents the electrical impedance spectrometry, linear polarization, Tafel polarization and potentiodynamic polarization respectively. Data analysis obtained from the curves are shown in Table 1, 2 & 3 respectively. The corrosion potential (E corr ) and corrosion current density (i corr ) from linear polarization has values of (mv, vs SCE) and (µa/cm -2 ) respectively. The slope of polarization resistance(r P ) shows values of Ω and the corrosion rate is mpy. Fig. 2A has two values that determined charge transfer resistance and the capacitance between bulk solution and electrode surface when the frequency is varying from 100 khz to 0.1 Hz. At low frequency, the real impedance is 32.5 KΩ while the imaginary part is zero. As frequency raised and reached the value that the R s & C dl have a maximum numerical results of 4.51 Ω.cm 2, µf.cm 2 respectively as shown in Table.2. The corrosion potential (E corr ) and corrosion current density (i corr ) from Tafel polarization curves has values of (mv, vs SCE) and (µa/cm-2) respectively. The corrosion rate from the Tafel polarization measurement shows mpy. The results of linear polarization and Tafel polarization measurement has almost nearby values and the corrosion potential (E corr ) has values at negative half range from which it could be illustrate that, H 2 SO 4 solution is very corrosive for C-steel where the rate of corrosion is very high which confirmed by the corrosion rate that has an average value of 260 mpy (milli-inches/year) in linear and Tafel polarization result. Fig.2: Accelerated electrochemical techniques at C-Steel immersed in 0.5 M H 2 SO 4 where A) AC Impedance, B) Linear polarization resistance, C) Tafel polarization and D) potentiodynamic polarization curves respectively

5 Table 1: Corrosion parameters obtained from linear polarization curves of C-Steel in 0.5 M H 2 SO 4 solution at 25 C -E corr i corr Rp, c a Corrosion mv, vs SCE µa/cm -2 K ohms rate, mpy 0.5 M H 2 SO Table 2: EIS data of C-Steel in 0.5 M H 2 SO 4 solution at 25 C 2 R S, C dl,µfcm N R ct, Ω cm 2 Ωcm M H 2 SO Table 3: Corrosion parameters obtained from Tafel polarization curves of C-Steel in 0.5 M H 2 SO 4 solution at 25 C -E corr mv, vs SCE i corr µa/cm -2 c a Corrosion rate, mpy 0.5 M H 2 SO Fig.3 shows four obtained corrosion test results of C-steel in 3.5% NaCl solution. Fig.3(A,B,C, & D) represents the electrical impedance spectrometry, linear polarization, Tafel polarization and potentiodynamic polarization respectively. Data analysis obtained from the curves are shown in Table 4, 5 & 6 respectively. The corrosion potential (E corr ) and corrosion current density (i corr ) from linear polarization has values of (mv, vs SCE) and (µa/cm -2 ) respectively. The slope of polarization resistance (R P ) shows value of KΩ and the corrosion rate of mpy. Fig. 3A has two values that determined charge transfer resistance and the capacitance between bulk solution and electrode surface when the frequency is varying from 100 khz to 0.1 Hz. At low frequency, the real part impedance was 200 KΩ, while the imaginary part (C dl ) was zero. As frequency raised and reached the values for R s & C dl at maximum numerical numbers of 5.81 Ω.cm 2 and 2.47 mf.cm 2 respectively are presented in Table 5. The corrosion potential (E corr ) and corrosion current density (i corr ) from Tafel polarization curves has values of -649 (mv, vs SCE) and 163 (µa/cm-2) respectively. The corrosion

6 rate from the Tafel polarization measurement shows mpy. The results of linear polarization and Tafel polarization measurement has very close values and the corrosion potential (E corr ) has that close to positive 1 from which it could be illustrate that, NaCl solution is somehow impartial for C-steel where the rate of corrosion is low which confirmed by the corrosion rate that has an average value of 70 mpy (milli-inches/year) in linear and Tafel polarization result. Fig.3: Accelerated electrochemical techniques at C-Steel immersed in 3.5% NaCl where A) AC Impedance, B) Linear polarization resistance, C) Tafel polarization and D) potentiodynamic polarization curves respectively Table 4:Corrosion parameters obtained from linear polarization curves of C-Steel in 3.5 % NaCl solution at 25 C -E corr mv, vs SCE i corr µa/cm -2 Rp, K ohms c a Corrosion rate, mpy 3.5% NaCl Table 5: EIS data of C-Steel in 3.5 % NaCl solution at 25 C 2 R S, C dl,µfcm N R ct, Ωcm 2 Ω cm 2 3.5% NaCl

7 Table 6: Corrosion parameters obtained from Tafel polarization curves of C-Steel in 3.5 % NaCl solution at 25 C -E corr mv, vs SCE i corr µa/cm -2 c a Corrosion rate, mpy 3.5% NaCl By comparing the corrosion result of C-steel in two different solutions, we found that the corrosion rate of C-steel in 3.5% NaCl solution (which was 70 mpy in average) is extremely lower than C-steel in 0.5 M H 2 SO 4 (which has corrosion rate of 260 mpy in average). This difference is probably due to the highly corrosive behavior of H 2 SO 4 compare to almost neutral behavior of ionic solution of NaCl. 4. Conclusion: It is clear that the corrosion rate in acidic solution is more than salt solution. For that reason, C-steel pipes corrodes when there is the presence of acidic medium. This could be prevent by some anticorrosion coatings.

8 References: [1] Corrosion.ksc.nasa.gov, (2014). Fundamentals of Corrosion and Corrosion Control. [online] Available at: [Accessed 17 Nov. 2014]. [2] Corrosion-doctors.org, (2014). Corrosion electrochemistry. [online] Available at: [Accessed 18 Nov. 2014]. [3] Corrosion-doctors.org, (2014). Corrosion Basics: An Introduction. [online] Available at: [Accessed 17 Nov. 2014]. [4] Arab, S.T., Abdulsalam, M., Alghamdi, H. and Emran, K. (2014). The Crevice Corrosion of 316 L SS Alloy in NaCl Solution at Different Applied Potentials. Open Journal of Physical Chemistry, 04(02), pp