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Green Chemistry Letters and Reviews

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Corrosion protection of 1018 carbon steel using anavocado oil-based inhibitor

O. Sotelo-Mazon, S. Valdez, J. Porcayo-Calderon, M. Casales-Diaz, J. Henao, G.Salinas-Solano, J. L. Valenzuela-Lagarda & L. Martinez-Gomez

To cite this article: O. Sotelo-Mazon, S. Valdez, J. Porcayo-Calderon, M. Casales-Diaz, J. Henao,G. Salinas-Solano, J. L. Valenzuela-Lagarda & L. Martinez-Gomez (2019) Corrosion protection of1018 carbon steel using an avocado oil-based inhibitor, Green Chemistry Letters and Reviews,12:3, 255-270, DOI: 10.1080/17518253.2019.1629698

To link to this article: https://doi.org/10.1080/17518253.2019.1629698

© 2019 The Author(s). Published by InformaUK Limited, trading as Taylor & FrancisGroup

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Corrosion protection of 1018 carbon steel using an avocado oil-based inhibitorO. Sotelo-Mazona, S. Valdeza, J. Porcayo-Calderon b, M. Casales-Diaza, J. Henaoc, G. Salinas-Solanoa, J.L. Valenzuela-Lagardad and L. Martinez-Gomez a,e

aInstituto de Ciencias Físicas, Universidad Nacional Autónoma de México, Cuernavaca, México; bCIICAp, Universidad Autónoma del Estado deMorelos, Cuernavaca, México; cCONACYT-CIATEQ A.C., Querétaro, México; dCentro Regional de Educación Superiror de la Costa Chica,Universidad Autonoma de Guerrero, Florencio Villarreal, México; eCorrosion y Protección (CyP), México City, México

ABSTRACTThe corrosion phenomenon in oil and gas industry is a serious issue affecting metallic structuresand pipelines. The main aggressive species flowing through oil pipelines is NaCl. The presenceof gases in the corrosive medium (CO2 and H2S), increases, even more, its aggressiveness.Usually, to mitigate corrosion, liquid inhibitors are added to the corrosive medium. In thepresent study, N-hydroxyethyl-imidazoline derivatives of avocado oil was synthesized as agreen corrosion inhibitor. The test was performed on 1018 carbon steel at 5, 10, 25, 50 and100 ppm. Electrochemical techniques as polarization curves (PC), the open circuit potential(OCP), the lineal polarization resistance (LPR) and the electrochemical impedancespectroscopy (EIS) were applied. The results obtained from the OCP indicates that theinhibitor can be classified as a cathodic type one. On the one hand, PC, RPL and EISdetermined that the highest corrosion rate was obtained without the inhibitor. On the otherhand, by adding inhibitor, the corrosion resistance increased, obtaining the optimalconcentration at 10 ppm. The EIS technique in the angle phase format showed the formationof two time constants, which indicates the inhibitor presence. Finally, the SEM analysesshowed a minor attack at 5 and 10 ppm.

ARTICLE HISTORYReceived 31 January 2018Accepted 5 June 2019

KEYWORDSInhibitor; imidazolines;electrochemical techniques;corrosion

1. Introduction

Carbon steel parts in oil industry are often susceptible tosuffer corrosion damage because of the presence ofdifferent aggressive species such as carbon dioxide(CO2). In recent years, due to the trend of increasingCO2 injection into oil wells, preventing damage in thehydrocarbon transporation lines has been a serious chal-lenge. As a result, the addition of corrosion inhibitors inthe hydrocarbon production lines is currently a

common practice to prevent corrosion failures associ-ated with CO2 (g) and impurities (chlorides, cyanides,etc.) (1, 2).

There are two types of corrosion inhibitors devel-oped to prevent CO2 induced corrosion; these inhibitorsare based on inorganic and organic compounds. Inor-ganic inhibitors are mainly chromates and nitrates,which form a film on the metal surface acting as abarrier against corrosive species. The disadvantage of

© 2019 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,distribution, and reproduction in any medium, provided the original work is properly cited.

CONTACT O. Sotelo-Mazon oscarsotelo.m@hotmail.comSupplemental data for this article can be accessed https://doi.org/10.1080/17518253.2019.1629698.

GREEN CHEMISTRY LETTERS AND REVIEWS2019, VOL. 12, NO. 3, 255–270https://doi.org/10.1080/17518253.2019.1629698

inorganic inhibitors is that these must be added in largedoses and sometimes they present low effectiveness.On the other hand, organic inhibitors are added insmall doses, and as a result, the cost of investment toinhibit corrosion is reduced. Among the differenttypes of organic inhibitors, imidazoline and their precur-sors (amide) have been used successfully to preventcorrosion (2). Oleic imidazoline inhibitors have especiallyproved to be effective for protecting metallic surfacesagainst corrosion. These are compounds that can beadsorbed on the surface of the metal by the displace-ment of water molecules, forming a protective barrier/layer which is of an oil-like nature, avoiding freeaccess of corrosive agents (H2O, Cl

−, H+ and HCO−3 )

and/or blocking the active sites (3, 4). The adsorptionof organic inhibitors depends on the inhibitor’s molecu-lar structure, the metal’s surface charge and type ofelectrolyte (5).

The molecular structure of imidazoline consists on afive-membered ring, which contains two atoms of nitro-gen (head group), a hydrocarbon chain (hydrophobictail) and a pendant side chain attached to one of thenitrogen atoms. (6–8). It is well known that the adsorp-tion of this type of inhibitors on a metal surface canoccur by either physical or chemical adsorption pro-cesses. The chemical adsorption can be conducted byformation of an iron–nitrogen coordinate-type bondand by a pi-electron interaction between the pi-electronin the head group and iron. Alternatively, the physicaladsorption is given by coulombic attraction, whichinvolves electrostatic attraction between the chargedmetal surface and the charged molecules of the inhibi-tor (9, 10). Some authors have mentioned that thelength of the hydrocarbon chain in the imidazolinestructure is also known for their ability to form protec-tive films (8, 11, 12), due to the unsaturation of oil-liketails (hydrophobic tail). This feature favors the adsorp-tion on the metal surface by means of a flat-adsorptionprocess, blocking active sites and reducing the cor-rosion process (5).

According to the above-mentioned, the aim of thisstudy was the synthesis of an inhibitor corrosion (N-hydroxyethyl-imidazoline derivatives of avocado oil)and the evaluation of its electrochemical performance.The avocado oil is unsaturated and the predominantfatty acid is oleic, whose amount can be up to 80% ofthe total fatty acids (13). In recent years, green inhibitorsbased on oleic sources have turned out to be a viablealternative, not only for their performance, but also fortheir environmental friendly nature. In the presentstudy, the oleic source was obtained from avocadowaste, which led to reduce the cost of inhibitor synthesis.

The fact that this inhibitor can be prepared from waste isimportant because it does not affect the avocadofarming as a source of food for humans, being a goodway of developing sustainable inhibitors. In addition,the high oil content (15–30%) that is present in theavocado fruit (13), and the high production of this fruitin Mexico are advantages that favor the usage ofavocado oil as an alternative to oleic sources.

2. Experimental procedure

2.1. Preparation of the inhibitor

An N-hydroxyethyl-imidazoline as inhibitor, based onavocado oil, was synthesized in the present study. Forthe synthesis of the inhibitors, avocado oil was extractedby mechanical method (pressing) from avocado waste.The synthesis of the amide was made by mixing N-2(hydroxyethyl) aminoethylamine (11.99 g) and avocadooil (30 g) in a 250 ml flask in a molar ratio 3:1. Then,this solution was stirred during 2 h at 140°C. The com-plete formation of amide was observed by chromato-graphy (14). Finally, fatty amide was heated at 160°Cduring 12 h in vacuum, obtaining hydroxyethyl-imidazo-line. The complete transformation was observed bychromatography method. The inhibitors were dissolvedin 2-propanol (99.9% purity). The concentrations ofinhibitor used in this study were 5, 10, 25, 50 and100 ppm.

2.2. Preparation of the electrochemical cell

The electrochemical cell employed in this study con-sisted of a conventional three electrodes glass cell (stan-dard ASTM G5). An electrode made of platinum was usedas reference electrode and a graphite rod was used ascounter electrode. The material used as working elec-trode (WE) was 1018 carbon steel. Before the tests,cylindrical specimens (4.55 cm length, 0.63 cm diameter)were ground to 600 grade emery paper, then these werefirst rinsed with distilled water and later by ethanol in anultrasonic bath for 10 min. The glass cell was filled with a3 wt% NaCl + 10% diesel solution, and saturated withCO2 gas bubbling prior (during 2 h) and during the test(24 h). The temperature of the test was 50°C. For eachelectrochemical test, a volume of 400 ml of fresh solutionwas used.

2.3. Electrochemical measurements

The electrochemical characterization was carried out bymeans of polarization curves (PC), open circuit potential

256 O. SOTELO-MAZON ET AL.

(OCP), linear polarization resistance (LPR) and electro-chemical impedance spectroscopy (EIS). For PCmeasurements, it was applied a sweep rate of 1 mV/sin a potential range of −400 to +700 mV versus theopen circuit potential. LPR measurements wereobtained by polarizing the specimens from −20 to+20 mV versus open-circuit potential at a sweep rateof 1.0 mV/s, meanwhile the polarization resistance (Rp)was measured from the slope of the potential–currentcurve in the vicinity of corrosion potential (Ecorr). Theresistance (Rp) value obtained from the measurementswas used to determine the inhibition efficiency. Theopen-circuit potential (OCP) of the working electrodewas also studied. EIS was measured in an interval of fre-quency from 100,000 Hz to 0.01 Hz, applying an ampli-tude of −10 to +10 mV. LPR and OCP measurementswere conducted every 60 min during 24 h. EIS measure-ments were made at different times (1, 3, 6, 9, 12, 18and 24 h) until 24 h. A model PC4 300 Gamry potentio-stat was used to perform the tests. After testing, cor-roded specimens were analyzed in an opticalmicroscope.

In addition, to determine the interaction between theinhibitor and the 1018 carbon steel, a study of adsorptionisotherms was carried out using the Langmiur model. Tocalculate the value of the surface coverage(θ), the LPR

results were used, assuming a uniform corrosionprocess in the working electrode surface.

3. Results and discussion

3.1. Synthesis of the inhibitor

Fourier transform infrared spectroscopy (FTIR) is acharacterization technique that is ideal for studying tri-glycerides and their reaction products as a whole (14).Figure 1 shows the FTIR spectra of avocado oil, fattyamide and hydroxyethyl-imidazoline. The band centeredat 3005.84 cm−1 is assigned to symmetrical vibrationswith extension = C–H of alkenes located in the largechains of the oil. The bands centered at 2924.27 cm−1

and 2854.34 cm−1 are assigned to asymmetric and sym-metrical vibrations, respectively, mainly with CH2 exten-sions. The band centered at 1744.83 cm−1 reflects theabsorbance of the C = O carbonyl ester group of the tri-glycerides or phospholipids. The rest of the bands in theFTIR spectrum of avocado oil are located at 1461.93 cm−1

and 1377 cm−1, and are attributed to C–H and CH3

flexions of the methylene group, while bands locatedat 1278 cm−1, 1237.97 cm−1, 1168.34 cm−1 and1095 cm−1 are attributed to the C–O extension of theester group in the triglyceride molecule (15). The FTIR

Figure 1. Infrared spectra of avocado oil, fatty N-[2-[(2-hydroxyethyl) amino] ethyl]-amide (fatty amide) and hydroxyethyl-imidazoline.

GREEN CHEMISTRY LETTERS AND REVIEWS 257

spectra of the Fatty N-[2-[(2-hydroxyethyl)amino]ethyl]-amide derived from avocado oil (Figure 1) has a bandat 3296.81 cm−1, which is assigned to the N-H ofamine. A band located at 1644.36 cm−1 corresponds tothe C = O extension of secondary amide. The band at1554.35 cm−1 corresponds to the N-H group, and theband at 1461 cm−1 corresponds to the C–N extensionof the amide (14).

Unlike the amide case, that shows two bands at1644.36 y 1554.35 cm−1 (16), the FTIR spectrum of imida-zoline shows a band at 1603.07 cm−1, which correspondsto the C = N bond in the ring of imidazoline; interestingly,this band indicates the successful transformation of theamide to imidazoline (6, 17). The bands at2922.62 cm−1 and at 2853.01 cm−1 in the amidespectra and the band at 2923.34 cm−1 and at2853.67 cm−1 in the imidazoline spectra correspond tothe asymmetric and symmetric vibrations of CH exten-sion in the lipids (mainly CH2). The band at723.36 cm−1 also matches the three FTIR spectra (oil,amide, and imidazoline) that corresponds to the CH2 ofmethylene chains.

In addition, Figure 2 shows the 1H NMR spectrum(300 mHz, CD3OD) used for characterize the N-hydro-xyethyl-imidazoline derivatives of avocado oil. This spec-trum show one peaks at δ 0.88 ppm, which is associatedwith the (CH3) group. Also, it is possible to observe twopeaks at δ 2.01 ppm and δ 1.58 ppm, which is relatedto the protons of group (m, −CH2, 4H), while the peaksat δ 5.31 ppm and δ 1.26 ppm show the protons of thealkyl chain group of fatty acids (m, −C = CH-, 2H). Onthe other hand, the peaks at δ 3.65 and δ 3.25 are associ-ated with the protons of the imidazoline ring (m, −CH2,

4H). Finally, the two peaks at δ 3.43 ppm and δ

3.55 ppm are related to the protons in the pendantgroup (m, −CH2, 4H) (18, 19).

3.2. Polarization curves

The polarization curves for the 1018 carbon steel elec-trode are presented in Figure 3. In the absence ofinhibitor, the anodic branch presents an active behav-ior, however, when reaching a potential of −291 mV,the icorr decreased. The deceleration of the anodiccurrent density is due to the formation of Fe carbon-ates films (FeCO3) on the steel surface, which is likelypartially protective since there is no passive zone.Regarding this result, previous studies have reportedthat, for prolonged exposure times, this film is not pro-tective (20–22).

Once the inhibitor was added, the corrosion potentialof the 1018 steel electrode turned to more positivevalues with respect to the same system without inhibi-tor. On the other hand, the current density valueobtained for the 1018 steel without inhibitor(0.06300 mA/cm2) was higher than that obtained withinhibitor for all concentrations (see Table 1). Thelowest current density value was found after adding10 ppm of inhibitor (0.000246 mA/cm2). The short dis-placement of the corrosion potential (Ecorr) towardsmore noble potentials and the reduction in the currentdensity are evidence of the inhibitory effect attributedto the adsorption of the hydroxyethyl-imidazoline onthe metal surface, forming a protective film and blockingactive sites (12, 20, 23). The addition of 10 and 100 ppmpresented the formation of passive zones at the

Figure 2. H1 NMR (300 mHz, CD3OD) spectrum for N- hydroxyethyl-imidazoline derivates of avocado oil.

258 O. SOTELO-MAZON ET AL.

potentials of −79 mV and −185 mV, respectively. Theimprovement in the corrosion behavior of the steel inthe present study can be associated with the enhancedproperties of the passive film due to the interactionbetween the molecules of inhibitor and corrosion pro-ducts, decreasing the anodic dissolution rate of thesteel surface (9, 24).

It is worth noting that, based on the calculation of theinhibition efficiency (IE(%)), it is observed that theoptimal concentration of inhibitor is 10 ppm, turningout an efficiency value of 99.6% (see Table 1). The inhi-bition efficiency (IE(%)) was determined from thecurrent density, calculated by the Tafel extrapolationmethod, according to the following equation:

IE(%) = ib − iiib

[ ]∗100 (1)

where ib is the current density of corrosion for the system

without inhibitor, and ii is the current density of corrosionwith the presence of inhibitor in the solution (25).

3.3. Open circuit potential (Eocp)

Figure 4 presents the open circuit potential (Eocp) plotversus time for the 1018 steel electrode. In theabsence of inhibitor, the corrosion potential presentsan increase towards more positive values. Previousstudies have reported that an increase in the corrosionpotential values, towards positive ones, indicates theformation of a passive layer (26). This fact is attributedto the formation of a Fe carbonate layer (FeCO3) thathelps to mitigate the corrosion process (21). It isworth mentioning that the minimum temperaturerequired to obtain FeCO3 layer is 50°C (27). However,after 17 h, there is a decrease in Eocp, meaning thatprobably the FeCO3 layer increases its thickness until17 h, and subsequently, cracks down from the steelsurface (22).

The present results reveal that the addition of theinhibitor into the aqueous medium produces a changein the Eocp curve. Particularly, once the inhibitor wasadded, the potential values decreased sharply towardsmore negative potentials. This behavior indicates thatthe inhibitor can be classified as a cathodic type one. Itis well known that the change in the potential valuesof more than 85 mV with respect to potential that ofthe system without inhibitor is associated with thoseinhibitors that can be classified as anodic or cathodic

Table 1. Electrochemical parameters and efficiency obtained forthe 1018 steel electrode using different concentrations ofinhibitor at 50°C.Inhibitor(ppm)

Ecorr(mV/Pt)

Icorr(mA/cm2)

Ba(mV/decade)

Bc(mV/decade)

IE(%)

0 −460 0.0630 70 358 –5 −361 0.0288 65 212 54.210 −321 0.00024 58 231 99.625 −233 0.0058 70 182 90.750 −177 0.0062 56 273 90.1100 −302 0.0060 40 212 90.4

Figure 3. Evaluation of the inhibitory effect at different concentrations (0–100 ppm) by applying polarization curves for the 1018 steelelectrode in a 3% NaCl-10% diesel solution bubbled with CO2 at 50°C.

GREEN CHEMISTRY LETTERS AND REVIEWS 259

type. If the difference between the potential values isbelow 85 mV, the inhibitor is classified as a mixed-typeinhibitor (27, 28).

It is known that the dissolution of CO2 in water pro-duces carbonic acid (Equation (2)).

CO2(aq) + H2O ↔ H2CO3(aq) (2)

The H2CO3 allows to conduct the cathodic reactions thatcontrol the corrosion process (10).

2H2CO3 + 2e− � H2 + 2HCO−3 (3)

2HCO−3 + 2e− � 2H2 + 2CO−

3 (4)

2H+ + 2e− � H2 (5)

Inhibitors of cathodic type are able to suppress thecathodic corrosion reactions (Equations (3)–(5)), such asthe hydrogen evolution reaction. That is, the inhibitorcan be adsorbed on cathodic sites and competes withthe hydrogen ions, therefore, it reduces the evolutionof the H2 gas (28).

3.4. Linear polarization resistance (LPR)

Figure 5 shows the linear polarization curve for the 1018steel electrode in the presence and in the absence ofinhibitor. One can observe that the lowest polarizationresistance was obtained without inhibitor. The corrosionprocess of the carbon steel in the presence of CO2

involves the anodic dissolution of iron and the cathodicevolution of hydrogen. The main anodic reaction, in theabsence of inhibitor, is the dissolution of iron. This

reaction is presented as follows:

Fe � Fe+2 + 2e− (6)

During the corrosion process, an iron-carbonate layercan be formed on the steel surface according to the fol-lowing reaction:

Fe+2 + CO(−2)3 � FeCO3 (7)

When the inhibitor is added, using different concen-trations, the corrosion resistance increased. This factcan be linked to the formation of corrosion products(FeCO3) and the adsorption of the inhibitor film on thesurface of the material, both layers increased the cor-rosion resistance of the material. In addition, the satur-ation of CO2 in the electrolyte favored the adsorptionof imidazoline type inhibitors on the surface of thematerial (8). After 3 h of tests, it was observed a quasi-stable behavoir of the LPR curves of the 1018 steel elec-trode using different concentrations of inhibitor, exe-pecting for 10 ppm, which reached its stability after16 h. The steady state of the corrosion process revealedthe protective nature of the inhibitor, forming a film thatcan modify the properties of the metallic surface. Firstly,the inhibitor can block active sites in short periods oftime, and subsequently, at longer periods of time, thiscan produce a geometric obstruction, that is, the effectof inhibition is originated from the reduction of the reac-tion area of the metallic surface (20, 24). On the otherhand, the test performed without inhibitor presentedan unstable behavoir. Its worth mentioning that thehighest polarization resistance was observed at a

Figure 4. Open circuit potential of a 1018 steel electrode in the absence and in the presence of inhibitor in a 3% NaCl-10% dieselsolution bubbled with CO2 at 50°C.

260 O. SOTELO-MAZON ET AL.

concentration of 10 ppm. This result is in agreement withthe results obtained above in the polarization curves,therefore, it can be deduced that 10 ppm is theoptimal concentration of inhibitor in the medium at50°C. The inhibitory ability of the system is a functionof the adsorption of the inhibitor on the metal surface,and this is a function of the chemical structure of theinhibitor, the type of metal and type of medium (29). Pre-vious studies reported that when the absorbed mol-ecules of inhibitor exceed a certain number of atomson the metallic surface, being these molecules remakablyclose, the electrostatic repulsion between the negativecharge of the penadant group promotes the desorptionof the inhibitor, leading to expose active sites on metalsurface and, as a result, presenting low corrosion resist-ance (20).

Figure 6 shows the inhibition efficiency (IE(%)) calcu-lated from the experimental polarization resistance (Rp)data according to the following equation (5, 12):

IE(%) = LPRi − LPRbLPRi

∗100 (8)

Where LPRi is the polarization resistance with the pres-ence of inhibitor and LPRb is the polarization resistancewithout inhibitor. The highest inhibition efficiency(99.6%) (Table 2) was obtained at 10 ppm. It is a goodscenario to obtain high efficiencies at low concentrationsof inhibitor since the medium is not substantiallymodified and the expenses associated with the additionof inhibitor are kept low (30). These results reveal that theinhibitor has a high affinity for the metal surface since itsadsorption on the electrode surface was relatively fast. Itis also inferred that there is a formation of a protectivefilm with excellent barrier properties.

3.5. Adsorption isotherm

The adsorption isotherms provide information about theinteraction between the inhibitor and the carbon steelsurface. The type of interaction is influenced by theadsorption of the organic compounds, such as physicaladsorption and chemisorption. The type interactiondepends on the chemical structure of the inhibitor, the

type of electrolyte, the state of charge and nature ofthe metal. There are different adsorption isothermmodels, such as the isotherm of Temkin, Langmuir,Frumkin and Flory-Huggins (31). In the present study,the Langmuir adsorption isotherm model was employed.This model is based on assumption that each site ofmetal surface have an adsorbed specie (molecule) (10).To obtain the Langmuir isotherm, the LPR experimentaldata were fitted. The degree of surface coverage (θ) onthe metal by the adsorbed inhibitor can be estimatedfrom Equation (9).

u = %IE100

(9)

The degree of surface coverage (θ) is related to theinhibitor concentration (Ci) by Equation (10) (32). Thevalue of Kads can be calculated from the interpretationof the line on the Ci/θ-axis (Figure 6).

Kads.Ci = u

(1− u)(10)

Where Kads is the equilibrium constant of adsorption andis related to the standard free energy of adsorption-(DG◦

ads) according to the following equation (33, 34):

DG◦ads = −RT ln(1× 10−6 Kads) (11)

Where “R” is the universal constant of ideal gases, “T” isabsolute temperature and 1 × 10−6 is the concentrationof water molecules expressed in mg/L (30). When plot-ting Ci/θ versus Ci, a straight line with a slope valueclose to 1 was obtained (Figure 7); on the other hand,the correlation coefficient (R2), calculated from linearregression analysis, had a value greater than 0.9997.Both the slope and the correlation coefficient valuessuggest that the inhibitor’s behavior obeys to the Lang-muir model (10, 23, 30, 32). Interestingly, the DG◦

ads valuewas of −40 kJ/mol; this negative value suggests that theinhibitor is spontaneously adsorbed on metal surface.The magnitude of this value also indicates an adsorptionprocess carried out by chemisorption. Previous studiesassociated free energy values equal or above −20 kJ/mol with a physical adsorption process, that is, electro-static interactions occur between the inhibitor and themetallic surface. On the other hand, free energy valuesaround −40 kJ/mol or even more negative are associatedwith chemisorption process, that is, the charge is sharedor transferred from organic species (corrosion inhibitor)to the metal surface to form a coordinated type ofmetal bond (30, 32). However, DG◦

ads valules less negativethan −40 kJ/mol are also associated to physical adsorp-tion, commonly interpreted as the formation of a filmadsorbed with an electrostatic character (35).

Table 2. Electrochemical parameters of OCP, LPR and %IE for the1018 steel electrode at different concentrations of inhibitor andat 50°C for 24 h.Inhibitor (ppm) Eocp (mV/Pt) LPR (Ω-cm2) %IE

0 −571 342.6 –5 −845.1 6719.0 97.810 −845.1 45970.0 99.625 −763.3 2147.0 94.350 −883.6 2694.0 95.6100 −1030.7 5577.0 97.4

GREEN CHEMISTRY LETTERS AND REVIEWS 261

3.6. Electrochemical impedance spectroscopy (EIS)

Figure 8 shows Bode plots in both the impedancemodule (Figure 8(A)) and phase angle (Figure 8(B))formats without the addition of inhibitor and fordifferent times. Figure 8(A) shows that the highest resist-ance was obtained at 9 h and 12 h. Afterwards, adecrease is observed, presenting the lowest resistanceat 24 h. The angle phase shows the formation of a timeconstant at intermediates frequencies. Moreover, a

change in the angle phase is observed towards low fre-quencies from 0 to 24 h. This change is associated withthe thinning and/or detachment of the protective layer(FeCO3) formed on the steel surface (14), It is possiblethat the iron carbonate layer remains on the steelsurface by short periods of time, but it can detach asthe time increases, presenting a higher corrosion rateat longer times. This behavior is in agreement with theresult obtained in Eocp plot (Figure 4) where, at 17 h,the potential changed to active values. However,

Figure 5. Polarization resistance of a 1018 steel electrode in the absence and in the presence of inhibitor in a 3% NaCl-10% dieselsolution bubbled with CO2 at 50°C.

Figure 6. Inhibition efficiency (IE) as a function of time obtained from LPR measurements.

262 O. SOTELO-MAZON ET AL.

another fact that can explain the present results is thedevelopment of cracks in the oxide layer since, at 50°C,this layer is porous and non-homogenuos, allowingfree access of corrosive species (H2O, H+, Cl− andHCO−

3 ) (29).Figure 9 shows the impedance plots with and without

inhibitor after 24 h of test. Interestingly, the Nyquist plotfor the system in the absence of inhibitor (Figure 9(A))presents the formation of a semicircle, indicating thatthe corrosion process occurred by charge transfer resist-ance. In addition, at low frequencies, the formation ofone inductive loop was observed. This fact is associatedwith intermediates adsorption species, such as FeOH(21). The Bode plot, in the phase angle format, showsthe formation of one time constant, reaching an anglearound 60°; this fact indicates that FeCO3 layer presentedpoor protective characteristics. In fact, it is well knownthat a phase angle of 90°, or closer, will have better pro-tective characteristics. As a result, Bode plots, particularly,the impedance module (|Z|) shows that the lowest resist-ance was obtained for the system without inhibitor.

When the inhibitor is added, an increase in the cor-rosion resistance of the carbon steel is presented; thisbehavior is clearly observed in the impedance module |Z| (Figure 9(B)), where the highest resistance wasobtained with the addition of 10 ppm of inhibitor. Thisvalue is higher with respect to the value obtainedusing the system without inhibitor. The highest resist-ance can also be inferred by the increase in the

semicircle diameter of the Nyquist plot. The Nyquistplot presented a charge transfer process, perceivingthe formation of one capacitive semicircle for all the con-centrations evaluated. Moreover, the formation of induc-tive loops at low frequencies was observed. This fact isassociated with an adsorption process of intermediatesand inhibitor molecules that cover the metal surface (21).

The angle phase plot shows the formation of two timeconstants for all the concentrations evaluated. The pres-ence of one time constant at high frequency (100000 Hz)is attributed to formation of an inhibitor layer self-assembled on the carbon steel surface (3). However,these time constants were displaced towards theregion of intermediate frequencies with the increase ofconcentration. For example, the lowest concentration(5 and 10 ppm) of inhibitor presented the formation ofone time constant (not well defined) in the high fre-quency region, displaying the highest phase anglevalue (73°) at 10 ppm. This fact is attributed with anincrease in the thickness of the inhibitor layer formedon carbon steel surface, resulting in an increase in theirprotective ability (14). Concentrations ranging from 25to 100 ppm presented phase angle values thatdecreased and experienced a displacement towardsintermediate frequencies. Likely, this displacement isassociated with the desorption of the inhibitor layerdue to an increase in concentration. This fact was pro-moted by electrostatic interactions among the moleculesof inhibitor. Consequently, the best performance of

Figure 7. Adsorption isotherm of the inhibitor on the 1018 carbon steel surface in 3% NaCl-10% diesel solution, bubbled with CO2 at50°C for different concentrations.

GREEN CHEMISTRY LETTERS AND REVIEWS 263

inhibitor was obtained using 10 ppm. Previous studieshave reported that the optimal concentration is a func-tion of the packing efficiency of the molecules on themetal surface. When the inhibitor is not concentratedenough, there are unprotected sites, but when this is pre-sented in excess is possible that electrostatic repulsionforce (due to interaction between the surfactant tails)will occur, which prevent an efficient packing leavingunprotected sites on the metal surface (3, 5). Otherauthors have reported that electrostatic repulsion canoccur among the negative charges of the pendantgroup, which is part of the molecular structure of the imi-dazoline, resulting in the desorption of the molecules ofinhibitor. This fact results in the promotion of

unprotected sites, manifesting high corrosion rates (22).Previous studies have also reported that the moleculestend to adsorb at right angle (perpendicularly adsorb)on the metal surface when adding high concentrationsof inhibitor because of a repellent function between mol-ecules. As a result, unprotected sites are present on themetal surface since the adsorption in the parallel direc-tion can cover a larger area on the metal surface (21).

On the other hand, the phase angle plot (Figure 9(C))shows a second time constant in the intermediate fre-quency region. This time constant experiences a displa-cement towards the low frequency direction when theconcentration ranges from 25 to 100 ppm. The presenceof this time constant indicates the formation and

Figure 8. Bode plots for the 1018 carbon steel electrode in 3% NaCl + 10% diesel with CO2 at 50°C without inhibitor and for differenttimes.

264 O. SOTELO-MAZON ET AL.

evolution of the corrosion products layer, while the dis-placement is associated with its thinning (3). On the con-trary, the time constant observed for 5 and 10 ppmmoves towards a high frequency direction, which isattributed to the adsorbed inhibitor, protecting comple-tely the carbon steel surface.

The evolution over time of the EIS plots to theoptimal concentration (10 ppm) is presented in Figure10. Figure 10(A) indicates a corrosion process domi-nated by charge transfer resistance. This result showsan increase in the semicircle diameter as elapsed time.The Bode plot in impedance module |Z| (Figure 10(B))shows that the highest resistance was presented at18 h. The phase angle plot (Figure 10(C)) displays theformation of two time constants for the different con-centrations of inhibitor. These time constants arelocated in the high and intermediate frequencyregion. Interestingly, the presence of the second timeconstant is notable at 3 h due to the adsorption ofinhibitor on the metal surface. The imidazoline-basedinhibitors have higher adsorption energy than watermolecules, that is, molecules of inhibitor are absorbedpreferently on the metal surface, resulting in a high cor-rosion resistance (23).

It is worth mentioning that previous studies using imi-dazoline-based inhibitors have observed that theaddition of diesel (oil-like part) into the corrosive solutionpromotes the rapid adsorption of the inhibitors. This factpromotes the reduction in the corrosion rate and helpsto increase the inhibition efficiency (29). The time con-stant established at high frequencies is associated witha film formed of inhibitor on the metallic surface, mean-while that time constant located at low frequencies isrelated to the corrosion products layer. The maximumvalue of the phase angle (73°) was presented at 24 h inthe high frequency region. One can note that whenthe phase angle is increased as a function of time, inthe high frequency region, it evolves towards a higherfrequency direction. The time constant formed at inter-mediates frequencies also moves towards higher fre-quencies. This fact indicates the effect of inhibition inthe dissolution process of iron, which is influenced bythe adsorption of the molecules and indicates theunion and self-assembly of the inhibitor film on thesurface of the metal (12, 14). The reactive sites of imida-zoline molecules located in the imidazoline ring (headgroup) favor the preferential adsorption of the imidazo-line ring on the metal surface by the formation of coor-dinated bonds with atoms on the metal surface (21).

The inhibition efficiency percentages were calculatedby using the resistance values of bode plot in the impe-dance module format |Z| at 24 h (see Table 3). The results

were obtained by using the Equation (12), where Ri is theresistance with the inhibitor and Rb is the resistancewithout the inhibitor. As shown in the results of EI%obtained by LPR. The results of EI% by EIS indicatedthat 10 ppm is the optimal concentration.

IE(%) = Ri − RbRi

∗100 (12)

3.7. Analysis by SEM

Analysis by SEM (Figure 11) show the degradation experi-enced by the 1018 steel surface in the presence and inthe absence of inhibitor. The enhanced corrosion protec-tion provided to the 1018 steel electrode is attributed tothe FeCO3 layer formed on its surface. The formation ofthis layer is a function of the limit of solubility of Fe2+

and CO2−3 ions, that is, when the concentration of ions

is exceeded, they combine to form solid iron carbonatefilms (Equation 3). These films are formed due to a pre-cipitation process; firstly, nucleation occurs on themetal surface, followed by the growth of the crystallinefilm (36). However, there is also evidence that theselayers do not cover completely the metallic surface,resulting in some unprotected areas and/or porousareas between the metal and the FeCO3 crystals (37). Inthis study, in the absence of the inhibitor, the formationof a layer of FeCO3 was observed (see Figure 11). Thislayer presented cracks, which likely provided freeaccess to the corrosive species (H2O, Cl

−, H+, HCO−3 ),

resulting in its detachment, and therefore a high cor-rosion rate.

On the other hand, the addition of the inhibitorimproved the corrosion resistance of the material. Amore homogeneous surface on the metal is observedat 5 and 10 ppm. This is because a dense and homo-geneous corrosion product layer was formed at theseconcentrations. This fact suggests that defects on themetallic surface were blocked (9). It is worth mentioningthat there is a good inhibitor protection on the metalsurface, because the stripes produced by the roughingduring the metallographic preparation were observed.However, a heterogeneous metal surface at 25, 50 and100 ppm was observed. This fact is related to the adsorp-tion of inhibitor, which depends on number of moleculesadsorbed; that is, when molecules are too close, electro-static repulsion can occur among them, leading to theirdesorption (3). This phenomenon was explained morein detail in Section 3.6 (EIS). Due to the presence ofunprotected sites, there will be interaction between theaggressive species on the metal surface, leading to the

GREEN CHEMISTRY LETTERS AND REVIEWS 265

Figure 9. EIS plots for 1018 carbon steel in 3% NaCl + 10% diesel solution with CO2 at 50°C without and with different concentrations ofinhibitor after 24 h.

266 O. SOTELO-MAZON ET AL.

Figure 10. EIS plot for 1018 carbon steel in CO2 saturated-3% NaCl + 10 diesel at 50°C using the optimal concentration (10 ppm) ofinhibitor and for different times.

GREEN CHEMISTRY LETTERS AND REVIEWS 267

formation of carbonates and/or Fe chlorides. It is clearthat a uniform corrosion process was carried out at allinhibitor concentrations, observing a major attack to25, 50 and 100 ppm.

4. Conclusions

In the present study, imidazoline inhibitors based onavocado oil had a better performance compared to theperformance presented in the absence of inhibitor. Inter-estingly, the optimal concentration was obtained at10 ppm, resulting an efficiency of 99.6%. At higher con-centrations, an electrostatic repulsion can affect theadsorption of the molecules, leaving the material unpro-tected. On the other hand, the optical analysis revealedthat saturation of CO2 in the solution helped to the for-mation of corrosion products layers, such as iron carbon-ates (FeCO3). These layers contribute to mitigate thecorrosion process. The formation of the FeCO3 and theadsorption of the inhibitor on the metallic surface

Table 3. Electrochemical parameters of EIS resistance and %IE forthe 1018 steel electrode at different concentrations of inhibitorand at 50°C for 24 h.Inhibitor (ppm) EIS resistance (Ω-cm2) %IE

0 321.4 –5 6820.5 95.210 46113.1 99.325 2063.3 84.450 3765.7 91.4100 6400.5 94.9

Figure 11. Surface analysis of 1018 steel after being subjected to corrosion in the absence and presence of an inhibitor in a solution of3% NaCl-10% diesel saturated with CO2 at a temperature of 50°C.

268 O. SOTELO-MAZON ET AL.

helped to improve the protection against corrosion ofthe carbon steel. Therefore, one can deduce that thetested inhibitor is suitable to protect the carbon steelin presence of CO2, NaCl and H2O.

Acknowledgements

O. Sotelo-Mazon is thankful to the postdoctoral fellowshipDGAPA-UNAM. In addition, coauthors are gratefully withJ. J. Ramos-Hernandez by SEM analyses realized.

Disclosure statement

No potential conflict of interest was reported by the authors.

Funding

This work was supported by Universidad Nacional Autónomade México.

Notes on contributors

O. Sotelo-Mazon is currently posdoctoral associate at theCIATEQin Queretaro, Mexico. He obtained his M.Sc. and Ph.D. inMaterials Science from the Universidad Autonoma del Estadode Morelos in 2011 and 2015, respectively. He carried out hisfirst postdoctoral studies at the Universidad Nacional Autonomade Mexico unit Instituto de Ciencias Fisicas in 2018. He has beenworking on the field of corrosion at high temperature, corrosioninhibitors, thermal spray and biomaterials.

S. Valdez-Rodriguez obtained a Ph.D. in Materials Science in2005 from the National Autonomous University of Mexico. Fol-lowing this Valdez worked as a post-doctoral, research associ-ate and junior research fellow. Valdez is driven by an interestin developing an understanding of how processing and chem-istries can deliver unique microstructures and properties forscientific studies and specific applications. The goal ofValdez's research is to combine knowledge about the fabrica-tion, characterization with the functionality of solid-statematerials to generate structural materials.

J. Porcayo-Calderon is a researcher at the Universidad NacionalAutonoma de Mexico. He obtained his Ph.D. in ChemicalSciences from the same University in 1998. He works in Cor-rosion Science and Engineering.

M. Casales-Díaz obtained a Doctorate in Material Sciences in2001 at the graduate of the Advance Materials ResearchCenter, Chihuahua, Mexico. Casales carried out postdoctoralstudies at the UNAM Physical Sciences Center, and is currentlyworking at the Institute of Physical Science of the UNAM,Casales performs electrochemical and corrosion inhibitorsand preparation of synthesis of rare earth materials and incor-porate polymeric materials to dope them and fabricate electro-wound membranes.

J. Henao is currently professor at the National Council ofScience and Technology (CONACYT) unit-CIATEQ A.C. in Queré-taro, Mexico, and is member of the mexican system of research-ers (SNI) level 1. He is materials engineer from the University ofAntioquia, Colombia, and obtained his master degree in high

performance materials engineering from the University ofLimoges, France. He also obtained his Ph.D. Degree inAdvanced Technologies from the University of Barcelona,Spain. During the last years, he has been working on the fieldof thermal spray, biomaterials, and materials for energy andcorrosion applications.

G. Salinas-Solano obtained his M.Sc. and Ph.D. in MaterialsScience from the Universidad Autonoma del Estado deMorelos in 2011 and 2015, respectively. He carried out posdoc-toral studies at the Universidad Nacional Autonoma de Mexicoin 2018. He has been working on the field of corrosion at hightemperature, corrosion inhibitors and biomaterials.

J. L. Valenzuela-Lagarda is a research professor at the Universi-dad Autónoma de Guerrero, México. He obtained his M.Sc. inthe area of Food Engineering at the Universidad Autonómade Sinaloa, and his Ph.D. in Biotechnology at the Centro deInvestigación en Alimentación y Desarrollo A.C, México. Hisresearch is in the area of biotechnology and engineering.

L. Martinez-Gomez is researcher at the Universidad NacionalAutonoma de Mexico. He obtained his Ph.D. in Metallurgyfrom the Stanford University, USA, in 1983. He works inMaterials Science and Engineering.

ORCID

J. Porcayo-Calderon http://orcid.org/0000-0002-6943-3926L. Martinez-Gomez http://orcid.org/0000-0003-3227-0067

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