Test material and solution

AISI 1018 carbon steel samples were employed in this study. Standard three identical electrodes with a triangular configuration were used in the Linear Polarization Resistance and Harmonic Distortion analysis techniques. The electrical resistance method used Microcor® probes in the cylindrical element form. Gravimetric coupons by Metal Samples® were used for the weight loss test. The surface samples were polished with 600 grit SiC paper and finally rinsed with distilled water, acetone and ethanol.

Experiments were performed in a 3 wt% NaCl solution, with a pH value of 4.7, in tri-distilled water and a mixture which consisted of 90 vol.% of this solution plus 10 vol.% diesel. The CO₂ gas was bubbled during 2 hours prior to testing in order to remove the dissolved oxygen from the test solution. Through the experiment, the electrolyte was kept deareated and saturated with CO₂ by continuous bubbling with CO₂. For all experiments, the temperature was kept constant at 50 ± 2 °C. Inhibitor was added 2 hours after pre-corroding the specimen. The weight loss experiments lasted 336 h in order to follow the formation of any iron carbonate film. Additionally, the use of field measurement techniques was to evaluate the corrosion control by using inhibitors.

Weight loss and Electrochemical measurements

All the weight loss tests were obtained in a glass cell with coupons, which were removed every certain period of time; the time exposition total was 336 hours. Electrical Resistance probes were obtained by CORRDATA® Corrosion Monitoring System. Electrochemical techniques employed included, Linear Polarization Resistance, LPR, and Harmonic Distortion Analysis (HDA), which were obtained by using a SmartCET™ System. Linear Polarization Resistance and Harmonic Analysis were conducted with three electrodes mode. Linear Polarization Resistance technique involves measurements of the polarization resistance of a corroding electrode using a small amplitude polarization of the electrodes. The slope of the response at the corrosion potential, the polarization response, is then inversely proportional to the corrosion current. Harmonic Analysis technique is a measurement of the non-linear current distortion arising during the LPR measurement. The data is analyzed with the using Fast Fourier Transform analysis to provide a measure of the corrosion current, and to provide an on-line estimate of the corrosion rate calculation about the Stern-Geary constant.

Among the used inhibitors, commercial hidroxietil-imidazoline type inhibitors were added at a 25 ppm concentration which was found to be an optimum one to inhibit the corrosion phenomenon ^7,8,9,10. Similar to the weight loss experiments, the working electrode was allowed to pre-corrode during 2 hours before the corrosion inhibitor was added. The inhibitors used were: HEIO, HEIE and HEIC. HEIO is an inhibitor whose chemical name is 1H-imidazole-1-ethanol, 4,5-dihydro,-2-C15-17 unsaturated alkyl derivatives, the inhibitor HEIE is an inhibitor whose chemical name is 1H-imidazole-1-ethanol, 4,5-dihydro, -2-nor coco alkyl derivatives and HEIC is an inhibitor whose chemical name is 1H-imidazole-1-ethanol, 2- Heptadecyl4, 5- Dihydro.

Coupons Weight loss

The corrosion rate by coupons weight loss was calculated through the equation (1):

Where CR is the corrosion rate (mpy), K is a constant (3.45*10⁶), W is weight loss during exposition time (g), A is exposure area (cm²), T is the exposition time (hours) and D is the density (g/cm³).

Table 1 summarizes the weight loss results after exposing 1018 carbon steel during 336 hours to a CO₂-saturated 3% NaCl solution with and without 10 vol.% diesel at 50 °C with and without 25 ppmof hidroxyethyl-imidazoline. The high efficiency of these inhibitors is evident, since the corrosion rate is decreased from 57 mpy for the uninhibited 3% NaCl solution to 1.44 mpy, 0.41 mpy and 1.43 mpy respectively, obtaining an efficiency value of 97,47%, 99.28% and 97.49% for each one of the inhibitors respectively. The corrosion products morphology of specimen in the uninhibited solution after 192 and 336 hours of exposure, consists of a porous layer of FeCO₃, Figures 1-2, mainly siderite with equiaxial grains between 10-15 μm long.

Electrical resistance (ER)

Corrosion rate was calculated by the electrical resistance method too. The electrical resistance of a metal or alloy element is given through the equation (2):

Where L is the probe element length (cm), A is the cross-sectional area (cm²), and r is the specific resistance of the probe metal (Ω*cm). Reduction or metal loss in the element cross section, A, due to corrosion will be accompanied by an increase in the element electrical resistance (R). To obtain the corrosion rate, a series of measurements are made over a period of time, and the results are plotted as a function of exposure time. The corrosion rate can be determined from the slope of the resulting plot ^{(5, 6)}.

The results from the Electrical Resistance curves for 1018 carbon steel exposed to the CO₂-saturated 3% NaCl solution at 50 °C with and without 25 ppm of inhibitors are shown on Figure 3. In this system the average corrosion rate value for the uninhibited solution was 72.6 mpy, with inhibitor addition the average corrosion rate, and after the inhibitors addition the corrosion rates were 0.65 mpy inhibitor, 0.3 mpy and 1.24 mpy for HEIO, HEIE and HEIC inhibitors respectively. With this technique, the higher efficiency of corrosion inhibitors were clear, obtaining efficiencies values of 98.95, 99.61 and 98.39% for HEIO, HEIC and HEIE inhibitors respectively.

The electrical resistance curves for 1018 carbon steel exposed to the CO₂-saturated in mixture of 3%NaCl + 10 vol.% diesel at 50 °C with and without hidroxyethyl imidazoline-type inhibitors are shown on Figure 4. The average corrosion rate value in the uninhibited solution was 77.3 mpy, but once the inhibitor was added the average corrosion rate was decreased down to 0.7 mpy with HEIO, 0.3 mpy with HEIE and 1.5 mpy for HEIC-type inhibitor. Similar to the obtained results in 3% NaCl without diesel, solution, the higher efficiency of these corrosion inhibitors were clear, obtaining efficiencies values of 99.07% for HEIO, 99.62% fory HEIE and 98.02% by HEIC-type inhibitor.

The industrial use of ER monitoring has been documented for various authors, G.R. Cameron and L.G. Coker ¹¹ showed that sulfide environments present difficulties for the ER technique; electrically conducting iron sulfide corrosion products cause underestimates of metal loss. Gareau ¹² illustrated the use of a computerized system for data transmission from several remote sites to a central control facility.

At this central facility, the ER data were monitored in real time, manipulated, and analyzed to obtain information on corrosion at the remote sites Thierry, Thoren and Leygraf, ¹³ compared the weight-loss results, ER measurements, and LPR measurements in cooling water systems. They concluded that the ER measurements indicated corrosion rates greater than those observed on the coupons when there was significant localized corrosion of the coupons, concluding that ER technique is sensitive to pitting attack and provides a rate that is some intermediate value between the general corrosion rate and the penetration rate.

Electrochemical techniques

The electrochemical nature of corrosion, allows that measurements of the electrical properties of the metal solution interface. In linear polarization resistance technique, a small potential perturbation is applied to the working electrode and the resulting current is measured. The ratio between the potential and current perturbations is known as the polarization resistance (Rp). That can be it related to the corrosion current through following Stern-Geary equation (3):

Where R _p is the polarization resistance; i_corr is the uniform corrosion current density and B is an empirical polarization resistance constant that is related to the anodic (ba) and cathodic (bc) Taffel slopes through the equation (4);

The corrosion rate obtained by r by linear polarization resistance are for 1018 carbon steel exposed to the CO₂-saturated 3% NaCl solution at 50 °C with and without 25 ppm inhibitor is shown in Figure 5, where it can be seen that the average corrosion rate without inhibitors was 150.87 mpy, but once the corrosion inhibitor was added the corrosion rate obtained were 0.81, 0.14 and 1.89 mpy for HEIO, HEIE and HEIC inhibitors respectively.

The corrosion rate obtained by linear polarization resistance are for 1018 carbon steel in the system with CO₂-saturated 3%NaCl + 10 vol.% diesel at 50 °C, is shown in Figure 6, obtaining an average corrosion rate without inhibitor of 144.47 mpy, whereas the average corrosion rates were 0.04 0.062 and 0.16 mpy for HEIO, HEIE and HEIC-type inhibitors. It can be seen that in presence of diesel, the corrosion rates with inhibitors were much lower than those in absence of it, and, in all systems, the efficiency of inhibitor was evident, obtaining efficiency values higher than 99%. It should be noted that all tests were performed by triplicated. Arrow indicated addition of inhibitor of corrosion and your effect immediately.

In the Harmonic Distortion Analysis technique, a low frequency sinusoidal potential is applied to a three electrode measurement system, and the resulting current is measured three different frequencies is used to verify the repeatability of the technique. The amplitude is in the range of 10-30-mV peak to peak. The frequency used is typically 0.1-10 Hz. Analyzing the primary frequency and the harmonics allow extraction of the required kinetic parameters of the corrosion process. Based on Stern-Geary model of the electrical double layer, a polarized electrode near the corrosion potential by a sinusoidal voltage of angular frequency oo and voltage amplitude Uo, over a period of time; t, gives a faradaic current density I ^6,14,15, given through the equation (5);

The current density of the Faradaic process will have a distorted sinusoidal form due to the non linear nature of the anodic and cathodic partial processes and will include higher harmonics having kro angular frequency (k = 2, 3, ...). This phenomenon is termed Faradaic distortion ^{14,15,16,17,18,19}. The amplitudes of the harmonic components are obtained by Fourier's series expansion of the exponential terms, equation (6):

The first term of the series is the i_o D.C component (Faradaic rectification), the coefficients of the other terms are the amplitudes of the harmonic components. For the first three harmonics components are given by equations (7), (8), and (9):

Where β_a = b_a/2.303, β_c = b_c/2.303, the first order modified Bessel functions are noted by I_n (n = 1, 2, 3...). This formula can be simplified if Uo amplitude is limited to the extent (Uo < β_{a and} Uo < β_c) that the Besel functions could be approximated with the small arguments. Thus, the equations derivate of above formula where simplified by Devay and Mészáros as follows by equations (10, 11, 12):

Solving the equations system, Knowing amplitudes i ₁, i ₂, and i ₃, Devay and Mészáros proposed the following equations for the determination of corrosion current for the determination of i _corr, β _a and β _c equations (17, 18, 19)

Figures 7 and 8 show the corrosion rate obtained by using the harmonic distortion analysis. In Figure 7, the average corrosion rate without inhibitor was 53.21 mpy, but once the corrosion inhibitors were added the average corrosion rate obtained were 0.78, 0.29 and 0.86 mpy for HEIE, HEIO and HEIC-type inhibitors respectively. Figure 8 shows that, when diesel is added, the average corrosion rate without inhibitor was 54 mpy, whereas with inhibitor the average corrosion rates were 0.42, 0.088 and 0.46 mpy for HEIO, HEIE and HEIC-type inhibitors respectively. Once again, with or without inhibitors, the corrosion rates in presence of diesel were lower than those obtained without diesel, and in all systems, the inhibitor efficiency was more than 99%.

The corrosion rate obtained by using the LPR method in the uninhibited solution overestimates corrosion rates as compared to the results obtained by harmonic distortion analysis, but similar results were obtained by Durnie, Jefferson, and Kinsella ¹⁶ They demonstrated that Harmonic Distortion Analysis yields comparable corrosion rate data to LPR under a variety of carbon dioxide corrosion conditions, but a regular error of approximately 100% was evident in comparison to the HAD corrosion rates against comparable data obtained by using Linear Polarization Resistance, which was attributed to the assumed Tafel slopes in the LPR, while HAD is capable of determined Tafel Slopes in each measurement cycle.

Thus, it can be concluded from all the obtained results with the different techniques, the differences in the obtained results with the SmartCET equipment id sue to the fact that this is a "Black box" which does not allow us to know the used scan rate, Tafel slopes, etc. It can be said that by using these equipments the use of corrosion inhibitors can be monitored to detect its action on the metal surface and their persistency on top of the metal surface.

Finally, some polarization curves were performed for 1018 carbon steel exposed to the CO2-saturated 3% NaCl solution and CO2 saturated 3% NaCl-Diesel at 50 °C with and without 25 ppm of hydroxyl ethyl type imidazolines are shown on Figures 9 and 10.

In the solution without diesel, Figure 9, it can be seen that in both cases the data describes a distinctly active behavior, without demonstrating the existence of a passive layer. The Ecorr value for the uninhibited solution was -740 mV, which was nobler than for the inhibited solution (-640 mV) ^20,21. For the uninhibited solution, the existence of a cathodic limiting current density value can be seen, give carbonic acid as follows (1):

Subsequently that the solution is de-aerated, the dominant cathodic reactions are the reduction of H⁺ ions, dissociation of carbonic acid (2, 3, 4):

And water reduction as follows (5):

The main anodic reaction, in absence of inhibitor, is iron dissolution according to (6):

Though it may be through several steps. During this corrosion process, a corrosion scale of iron carbonate, FeCO₃, would form on the surface of carbon steels according to (7):

In absence of oxygen, the primary cathodic reactions are dependent on the pH solution, and it could be the reduction of bicarbonate ions, water and hydrogen ions, equations (3), (4) and (5). At a low pH value, H+ reduction is the governing cathodic reaction because of the high H⁺ concentration. When pH increases to 4-6, as in the present case, the direct reduction of and H₂CO₃ become important. At a high overpotential, the governing cathodic reaction changes to direct reduction of water. Thus, in the present case, the diffusion of and H₂CO₃ ions to from iron carbonate, FeCO₃, is the rate controlling step, which is independent of the inhibitor concentration, as shown in the cathodic branch of polarization curves in Figure 2. On top of this iron carbonate film, the inhibitor-formed film is insulating that protect the metal from the electrolyte.

The change in the corrosion rate with time, obtained from the LPR results, is shown in Figure 6, where it can be seen that for the uninhibited solution, the corrosion rate remained constant with time. However, as soon as 25 ppm of corrosion inhibitor was added, the corrosion rate decreased for more than four orders of magnitude.

In presence of Diesel, Figure 10, the E_corr value for the uninhibited solution was -740 mV, which was made noble for the inhibited solution and was -610 mV approximately. This shows the effect of the inhibitor, where the data exhibited that the E_corr was -610 mV approximately. This can be due to the fact that the inhibitor is soluble in diesel and dispersible in water, thus, a better transport o is expected, and, thus improve its corrosion performance.