Tetrahydroacridines as corrosion inhibitor for X80 steel corrosion in simulated acidic oilfield water

Article abstract of DOI:10.1016/j.molliq.2019.111478

X80 steel is commonly used in pipeline, tubing, casing and other facilities in oilfield construction. Its corrosion behaviors in 15% HCl without or with tetrahydroacridines was investigated by weight-loss tests, electrochemical techniques along with scann

Full text of DOI:10.1016/j.molliq.2019.111478

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Tetrahydroacridines as corrosion inhibitor for X80 steel corrosion
in simulated acidic oilfield water
Weiwei Zhang, Hui-Jing Li, Meirong Wang, Li-Juan Wang,
Qianwen Pan, Xubiao Ji, Yaqian Qin, Yan-Chao Wu
PII:
S0167-7322(19)32354-2
DOI:
https://doi.org/10.1016/j.molliq.2019.111478
MOLLIQ 111478
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
26 April 2019
2 July 2019
30 July 2019
Please cite this article as: W. Zhang, H.-J. Li, M. Wang, et al., Tetrahydroacridines as
corrosion inhibitor for X80 steel corrosion in simulated acidic oilfield water, Journal of
Molecular Liquids(2019), https://doi.org/10.1016/j.molliq.2019.111478
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Tetrahydroacridines as corrosion inhibitor for X80 steel corrosion in
simulated acidic oilfield water
Weiwei Zhang,a Hui-Jing Li,a,* Meirong Wang,b Li-Juan Wang,b,* Qianwen Pana,
Xubiao Jia, Yaqian Qina, and Yan-Chao Wua,c,*
a School of Marine Science and Technology, Harbin Institute of Technology, Weihai 264209, P. R.
China
b School of Materials Science and Engineering, Harbin Institute of Technology, Weihai 264209, P.
R. China
c Weihai Institute of Marine Biomedical Industrial Technology, Wendeng District, Weihai 264400, P.
R. China
Abstract: X80 steel is commonly used in pipeline, tubing, casing and other facilities in oilfield
construction. Its corrosion behaviors in 15% HCl without or with tetrahydroacridines was
investigated by weight-loss tests, electrochemical techniques along with scanning electron
microscope (SEM)/energy dispersive spectroscopy (EDX), atomic force microscope (AFM) and
X-ray photoelectron spectroscopy (XPS). The experimental results indicated that
tetrahydroacridines have excellent inhibitive performance for X80 steel in 15% HCl, and the ηmax of
MPTA and EPTA reached up to 97.87% and 95.96% at 400 ppm, respectively. The properties of
tetrahydroacridines are mixed-type based on polarization studies, and the adsorption mode follows
Langmuir isotherm and physical adsorption is dominant. On the basis of surface analysis and
characterization, the formation and characteristic of an adsorption-related protective inhibitor film
on the X80 steel surface have been verified.
Keywords: X80 steel; corrosion inhibition; electrochemical; SEM; AFM; XPS
1. Introduction
In the oil and gas industry, different grades of steel are used in building materials such as
pipelines, tubing and sleeves. Steel grades such as API-5L X70, X80, X60, API-5CT J55, N80-P110,
and K55 have been widely used in line pipes, welded casings and tubing pipes, and their basic
difference is the relative amount of elements in the alloy [1-3]. In practical applications, the key
factor in selecting steel grades is the ability to withstand very harsh conditions, with corrosive
_________________
* Corresponding author. Tel.: +86-0631-5687230；
E-mail address: lihuijing@iccas.ac.cn (H.-J. Li), lijuanwanghit@163.com,
ycwu@iccas.ac.cn (Y.-C. Wu)

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erosion being one of the priorities. An important grade of steel in oil and gas production is X80
steel because of its superior mechanical quality and high cost-effective [1]. With the increase of oil
and gas field exploitation, most newly exploited oilfields are deep wells and ultra-deep wells, and
the corrosion factors in the wells are more complex and variable, which greatly affects the
efficiency of oil production. Therefore, stimulation of wells, secondary and enhanced production
operations become very essential. However, in oil production, the surface of equipment often appear
corrosion and scaling problems due to changes in pressure, temperature and composition, etc [3,4].
Especially during the stimulation treatments, acids (usually 15-28% HCl) are typically introduced
into pipelines at elevated temperatures to dissolve stratigraphic rocks, expand flow paths and
remove scale [5-7]. In doing so, pipes and other metallic structures are highly exposed to corrosive
acids, resulting in damage to component integrity, pipeline rupture and leakage, which greatly
affects the efficiency of oil production and produces other negative social effects. In addition,
corrosion or failure may require replacement or maintenance of the equipment, which could cause
the factory to close, shutdown time and undesirable damage to the company's interests. A practical,
economical and effective way to solve these problems is to use corrosion inhibitors. The inhibitors
are believed to act by adsorbing their active functionalities on metal surface and form a layer of
physical or chemical properties that prevents corrosion of the steel. Generally, excellent corrosion
inhibition performances are manifested by inhibitors bearing larger electronegativity atoms (i.e., N,
O, S) as well as conjugated-bond structures or aromatic rings, such as imidazoline, quinolones,
thioureas, pyridines, thiourea, mercaptans and quaternary salts were frequently utilized as corrosion
inhibitors during stimulation treatments [8-10]. Various corrosion inhibitors have been exploited to
narrow the gap between corrosion inhibition tests and practical steel protection. Even so, some of
them are toxic, very expensive or too harsh synthetics that cannot be overlooked, making the
exploitation of an environmental-friendly and efficient inhibitor for steel protection a high priority.
Tetrahydroacridines as an N-heterocyclic organic compound is being scrutinized significantly due
to its low toxicity, easy synthesis, excellent coordination performance and unique antimicrobial
activity. It is known that acridines are potential corrosion inhibitors for steel in acidic media [11-13].
We believe that tetrahydroacridine compounds may also be an effective corrosion inhibitor platform
for iron and steel protection as it contains the availability of N atom and pi-bonds that can form
coordination bonds with iron atoms to form a dense adsorption film on steel surface. Meanwhile,
tetrahydroacridines are cost-effective and environmentally friendly [14], so they have great
application prospects in the field of corrosion and protection. Despite extensive works performed on
heterocyclic molecules as corrosion inhibitors for steel during stimulation treatments, no research

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has been conducted on the inhibitive effect of tetrahydroacridines as corrosion inhibitors in acidic
oilfield water.
In this study, 2-methyl-9-phenyl-1,2,3,4-tetrahydroacridine (MPTA) and ethyl
9-phenyl-1,2,3,4-tetrahydroacridine-2-carboxylate (EPTA) were designed as corrosion inhibitors for
X80 steel in 15% HCl, which simulate real field acidizing conditions. The corrosion inhibition
properties of MPTA and EPTA were studied by utilizing weight-loss tests and electrochemical
techniques. The morphological characteristics and elemental compositions of adsorption film on
steel surface were investigated by scanning electron microscopy (SEM)/energy dispersive
spectroscopy (EDX), atomic force microscope (AFM) and X-ray photoelectron spectroscopy (XPS).
Moreover, the inhibition mechanism of inhibitors with X80 steel was evaluated by adsorption,
thermodynamic. Our finding offers a new light for designing eco-friendly tetrahydroacridine-type
inhibitors for X80 steel under oil well acidizing conditions.
2. Experiments
2.1. Specimens and solutions
The X80 steel sample composition used for the experiments was C (0.064), Si (0.025), Mn
(1.56), P (0.013), S (0.004), Cu (0.01), Cr (0.021), Nb (0.056), V (0.005), Ti (0.025), B (0.0006)
and balance Fe. The dimensions of X80 steel materials used for weight loss were cut into 2 cm × 2
cm × 0.02 cm (sectional area = 4 cm2), 0.5 cm × 0.5 cm × 2 cm for electrochemical experiments and
1 cm × 1 cm × 0.02 cm for surface analysis. For electrochemical experiments, the working surface
(0.25 cm2) of the electrode was polished with sandpaper of different grades (400-1200) before using
them for experiments. The analytical grade 37% HCl was diluted with double distilled water to
prepare a 15% HCl solution to simulate the real field acidizing concentration. The concentration of
MPTA and EPTA inhibitors in corrosive solution ranged from 100 ppm (100 mg/L) to 400 ppm.
MPTA and EPTA were synthesized from 2-aminobenzophenone and cyclohexanones in our
laboratory, of which citric acid was the only catalyst, and the reaction process shown in Fig. 1. The
synthetic procedure has the advantages of using harmless non-metallic catalyst, easy availability of
raw materials, high regioselectivity and so on. The mixture of 2-aminobenzophenone (197.2 mg, 1.0
mmol), cyclohexanones (1.5 mmol) and citric acid (50 mmol%) in toluene (1.5 mL) was stirred at
120 °C under the atmosphere of air for 24 h. After the reaction, the residue was purified by silica
gel column chromatography (PE/AcOEt = 1:5), and the molecular structures of compounds were
further corroborated by 1H NMR, 13C NMR, and IR, shown below:
1
MPTA: H NMR (400 MHz, CDCl3) : 8.02 (d, 1H, J = 8.4 Hz), 7.61-7.57 (m, 1H), 7.54-7.44 (m,
3H), 7.32-7.30 (m, 2H), 7.25-7.21 (m, 2H), 3.33-3.14 (m, 2H), 2.69-2.63 (m, 1H), 2.23 (dd, J = 10.8,

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17.0 Hz, 1H), 2.08-2.01 (m, 1H), 1.92-1.84 (m, 1H), 1.63-1.53 (m, 1H), 1.00 (d, J = 6.6 Hz, 3H);
13C NMR (100 MHz, CDCl3) : 158.7, 146.3, 146.2, 137.0, 129.1, 128.9, 128.5, 128.4, 128.2, 127.8,
127.6, 126.5, 125.7, 125.2, 36.4, 33.7, 31.0, 29.2, 21.7; IR (film): νmax = 2922, 2864, 2361, 1572,
1484, 1454, 1355, 1260, 1153, 1073, 927, 800, 758, 703 cm-1.
EPTA: 1H NMR (400 MHz, CDCl3) δ: 8.01 (d, 1H, J = 8.4 Hz), 7.63-7.58 (m, 1H), 7.54-7.44 (m,
3H), 7.32 (d, 2H, J = 3.9 Hz), 7.26-7.21 (m, 2H), 4.16-4.10 (m, 2H), 3.36-3.17 (m, 2H), 2.92-2.83
(m, 2H), 2.49-2.44 (m, 1H), 2.15-1.97 (m, 2H), 1.22 (t, 3H, J = 7.1 Hz)；13C NMR (100 MHz,
CDCl3): δ/ppm = 174.9, 157.5, 147.0, 146.4, 136.6, 129.2, 128.9, 128.7, 128.4, 128.0, 126.7, 126.2,
125.9, 125.6, 60.6, 39.8, 32.9, 30.0, 28.5, 25.8, 14.2; IR (film): νmax = 2939, 2359, 2254, 1723, 1572,
1373, 1260, 1075, 1029, 906, 827 cm-1.
2.2. Weight loss experiments
The measurement was carried out in a thermostatic water bath containing 100 mL of uninhibited
solution and the solution containing MPTA and EPTA at 30 °C for 24 h. Before weighing, the
surface samples were carefully washed with ultra-pure water and dried with nitrogen. The procedure
was carried out on triplicate samples, and the average corrosion rate was calculated. The corrosion
rate (v) was obtained from Eq. (1):
푊 − 푊
1
2
푣 =
(1)
푠 × 푡
where W1 and W2 (mg) are the weight loss of specimen before and after immersion, s (cm2) is the
area of steel samples, and t (h) is the soaking time. The inhibition efficiency (ηw) was obtained from
Eq. (2):
푣표 − 푣
휂푤 =
× 100%
(2)
푣표
where vo and v are the corrosion rate without and with inhibitors, respectively. Besides, the effect of
temperature like 50 °C, 70 °C and 90 °C was also studied by weight loss method.
2.3. Electrochemical measurements
The CHI 760E workstation of the conventional three−electrode system is used for
electrochemical measurement, where X80 steel is a working electrode, saturated calomel electrode
is a reference, and the platinum is an auxiliary electrode. The polarization curves were scanned
from −750 to −250 mV (vs. SCE) at a scan rate of 1 mV s-1, and linear polarization resistance
experiments were done from -20 to +20 mV vs. Ecorr at the scan rate of 0.125 mV s-1. The
electrochemistry impedance tests were conducted under EOCP in the frequency range of 100
kHz−100 mHz with an AC voltage amplitude of 5 mV. Before electrochemical measurements, keep
the working electrode in contact with the solution under test for 30 minutes to reach a stable state.

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2.4. Surface analysis
The surface morphological of X80 steel in non-inhibiting and inhibiting acid solution at 30 °C
for 48 h were studied by SEM (SUPR55, Zeiss, Germany, 2k magnification), and AFM
(MFP-3D-BIO, Asylum Research, America) using tapping mode in the Air, RTESPA probe (k = 40
N/m and fo = 302 kHz). The associated EDX was used to provide qualitative information on the
composition of surface elements. X-ray photoelectron spectroscopy (XPS) analysis was carried out
using a Thermo VG Scientific ESCALAB 250 spectrometer with a monochromatic Mg Ka X-ray
source. The XPS spectra were fitted by XPS PEAK 4.1 software with a reference value of C 1s peak
at 284.6 eV [15].
3. Results and discussion
3.1. Weight loss measurements
Weight loss is a commonly used method to evaluate the corrosion inhibition performance of
inhibitor, and it has high reliability. The corrosion behavior of X80 steel in 15% HCl with different
concentrations of MPTA and EPTA was studied by weight loss technique. The corrosion rate (v)
and inhibition efficiency (ηw) are shown in Fig. 2. As can be seen from Fig. 2, the v value is
obviously reduced and ηw is increased after adding inhibitors. The maximum ηw values of MPTA
and EPTA are 96.14% and 94.43%, respectively, and the order of v value is v (MPTA) < v (EPTA).
The inhibition behavior of MPTA and EPTA can be explained by the adsorption of their molecules
on the steel surface. Moreover, it is noteworthy that the values of ηw followed the order: ηw
(MPTA) > ηw (EPTA), showing that the substituent has a certain influence on the corrosion
inhibition effect of the inhibitor. Recent studies have confirmed that the contribution of a
substituent to the inhibition potential is directly related to the Hammett substituent constant (σ), and
an inhibitor with negative σ value is related to the stronger adsorption tendency compared with the
positive or smaller negative σ value [16]. The σp value for –CH3 (-0.17) is more negative than that
of –CO2C2H5 (+0.45), indicating that –CH3 substituent has a higher electron donating ability than
the –CO2C2H5 substituent that favors the bonding of inhibitors with metals, exhibiting higher
inhibition efficiency.
3.2. Effect of temperature
Surface temperature is usually different from downhole temperature, so understanding the effect
of temperature on inhibitor is critical, and temperature studies can be used to understand the kinetics
and thermodynamic processes of metal corrosion. Table 1 shows the corrosion rate (v) of X80 steel
with or without MPTA and EPTA in 15% HCl measured by weight loss at different temperatures
(30–90 °C). As illustrated in Table 1, the v value improved and the ηw reduced after increasing

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temperature, which is due to the adsorption equilibrium tends to desorption direction with
increasing temperature, resulting in a decrease in metal surface coverage. This means MPTA and
EPTA are temperature dependent corrosion inhibitors that can be used for corrosion protection of
medium and high temperature media. The activation energy (Ea) of corrosion process is calculated
from Arrhenius Eq. (3):
퐸푎
( )
3
ln푣corr = ln퐴 −
푅푇
where vcorr is the rate of corrosion, R is gas constant, T is absolute temperature, and A is
pre-exponential factor. The Ea values in the absence and presence of MPTA and EPTA (only
studied 400 ppm) obtained from the slope of the plots of lnvcorr vs. 1/T (Fig. 3) are shown in Table 2.
As depicted in Table 2, the values of Ea with additives are higher than the uninhibited solution,
supporting the physical adsorption mechanism, which is probably associated with the electrostatic
adsorption film formed on the X80 steel surface [17,18]. Furthermore, the activation enthalpy (ΔH*)
and entropy (ΔS*) of the corrosion process may be assessed by the influence of temperature, and the
values were calculated from Eq. (4):
푣corr
푇
푅
Δ푆∗
푅
−훥퐻∗
푅푇
=
exp (
) exp (
)
(4)
푁ℎ
where h is Planck’s constant and N is Avogadro’s number. The plots of ln(vcorr /T) vs. 1/T for blank,
MPTA and EPTA are illustrated in Fig. 4. The values of ΔH* and ΔS* were obtained from the slope
(ΔH*/R) and intercept [ln (R/Nh) + (ΔS*/R)] of the linear plots, and the results are presented in Table
2. The positive values of ΔH* indicate the endothermic nature of the steel dissolution process and
was retarded by the inhibitor-containing solution. Compared with free acid solution, the increase of
ΔH* value in the presence of additives indicates the existence of physical adsorption. The higher
ΔS* value with additives can be regarded as a quasi-substitution process between the inhibitor
molecule and the water molecule at the solution-metal interface [19,20].
3.3 Adsorption isotherms
Adsorption isotherms are the most advisable way to learn the adsorption mechanism of
inhibitors. Fig. 5 shows the plots of C/θ versus C for MPTA and EPTA at 30–90 °C, and the
adsorption parameters are shown in Table 3. In this study, it was found that the Langmuir isotherm
showed the optimal fitting results because the linear regression coefficient was almost 1 (Table 3),
showing a single layer adsorption characteristics. The Langmuir isotherm is shown in Eq. (5):
퐶
휃
1
=
+ 퐶
(5)
퐾푎푑푠

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In the formula, C represents the inhibitor concentration, θ is the surface coverage, and Kads is the
adsorption equilibrium constant associated with the standard adsorption free energy (ΔG0ads), as per
following relationship:
0
6
(
)
∆퐺푎푑푠 = −푅푇ln 1 × 10 퐾푎푑푠
(6)
where 1×106 is the amount of water molecules expressed in ppm, T is the thermodynamic
temperature and R is the universal gas constant. As seen in Table 3, the ΔG0 values are ranged
ads
between –25.41 kJ·mol-1 and –30.56 kJ·mol-1, which indicates the adsorption of two
tetrahydroacridines on the X80 steel surface is a mixed from physical and chemical adsorption
[21-25]. In addition, the value of Kads follows: MPTA > EPTA, and MPTA shows larger negative
ΔG0 value than EPTA, further confirming that MPTA exhibits stronger adsorption stability than
ads
EPTA.
From the listed values of Kads and ΔG0 in Table 3, ΔH0 (standard enthalpy) and ΔS0
ads
ads
ads
(entropy) of adsorption can be calculated using Eq. (7):
∆퐻푎0푑푠 ∆푆푎0푑푠
+
6
(
)
ln퐾푎푑푠 = −
− ln 1 × 10
(7)
푅푇
푅푇
The values of ΔH0ads and ΔS0ads can be determined from the slope and intercept of the plot of lnKads
vs. 1/T (Fig. 6) and were listed in Table 3. As seen in Table 3, the negative and small values of
ΔH0 indicate that the adsorption of MPTA and EPTA is an exothermic process, leading to
ads
physical adsorption [21,22]. The positive values of ΔS0ads arise from the substitution process, which
can be attributed to the increase in the solvent entropy and more positive water desorption entropy
[22].
3.4. Open circuit potential (EOCP) curves
The variation of open circuit potential (EOCP) of X80 steel electrode with time in 15% HCl
solution in absence and presence of inhibitor was shown in Fig. 7. It could be observed from OCP
curves that the EOCP shifted toward negative potentials with addition of different concentration of
inhibitor, which can be explained by the adsorption of inhibitor on the X80 steel surface [11]. By
increasing the concentration of the inhibitor, the steady state potentials (EOCP) moves in a more
negative direction, indicating that the cathodic reactions is more affected than the anodic reaction. It
takes about 20 min to reach a stable state, so we chose a 30-minute immersion time for
electrochemical measurements.
3.5. Potentiodynamic polarization and linear polarization resistance (LPR)
The polarization curve of X80 steel in 15% HCl solution in the absence or presence of MPTA
and EPTA are presented in Fig. 8. Table 4 displays the corrosion potential (Ecorr), corrosion current

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density (icorr) and Tafel slopes (βa and βc) obtained by Tafel extrapolation method. The ηi was
calculated according to Eq. (8):
푖c0orr − 푖corr
( )
휂푖 % =
( )
8
× 100
푖c0orr
where, i0
and icorr are the current density values of unprotected and protected steel in acidic
corr
solution, respectively. The icorr values in Table 4 reduced as the inhibitor concentration increased
indicating the inhibitive effects of MPTA and EPTA. Simultaneously, the potential obtained for all
studied inhibitors shifted to relatively more negative and less than 58 mV with respect to the blank,
showing that MPTA and EPTA could be mixed type inhibitors but predominantly restrained the
cathode process. Similar results were obtained for the corrosion inhibition of other nitrogen
heterocyclic compounds as a steel corrosion inhibitor in acidic solution [26-28]. Besides, the values
of βa and βc were the absence of significant change after adding inhibitors also revealed that the
anodic dissolution of steel and hydrogenation reaction are slowed down by the surface blocking
effect of inhibitors [29,30]. The shape of the curves is similar, which also demonstrates that the
addition of MPTA and EPTA do not lead to changes in the electrochemical corrosion reaction
mechanism of electrodes. Moreover, the calculated ηi increased with the increase of MPTA and
EPTA concentration, with the maximum ηi of 97.87% and 95.96%, respectively, similar to weight
loss results.
The LPR experiments were carried out by recording the electrode potential ±20 mV around
corrosion potential (Stern plots). The polarization resistance (Rp) obtained by LPR method is used to
calculate the corrosion current (icorr) by using the Stern–Geary kinetics equation [30]:
훽푎훽푐
1
푖푐표푟푟
=
×
(9)
2.303(훽푎 + 훽푐) 푅p
Inhibition efficiency (ηLPR) is calculated using the following equation:
푅p − 푅p0
( )
휂LPR % =
( )
10
× 100
푅p
where R0 and Rp are polarization resistances of uninhibited and inhibited solutions, respectively.
P
The Rp and ηLPR values (Table 4) increased considerably with the increase of MPTA and EPTA
concentrations. Besides, the ηLPR values calculated by LPR and potentiodynamic polarization curves
are comparable. The coincidence between these two methods can suggest that no significant surface
changes occur during the polarization measurements [30].
3.6. Electrochemical impedance spectroscopy (EIS)
EIS were carried out to investigate the corrosion behavior of protected and unprotected X80
steel. The Nyquist plots of X80 steel for MPTA and EPTA are displayed in Fig. 9. As shown in Fig.

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9, all Nyquist plots show similar capacitive loops over the entire frequency range, and the
capacitive loop diameter with inhibitors is greater than that of non-inhibitors, and it increases
further with the increase of concentration. Besides, the imperfect semicircular nature with the
addition of inhibitors was caused by frequency dispersion and inhomogeneity of electrode surface
[24,29,31]. Corresponding bode plots (Fig. 10), the frequency range of maximum phase increases
with rising inhibitors' concentration, indicating MPTA and EPTA are effectively adsorbed inhibitor
molecules on steel surface. The values of absolute impedance were increased significantly at low
frequencies with the increase of inhibitor concentration, and have the same trend as the change of
capacitive diameter, which indicates higher protection of X80 steel in inhibited 15% HCl solution.
The impedance data are determined according to the equivalent circuit shown in Fig. 11. The
chi-squared (χ2) was used to evaluate the precision of the fitted data, the low χ2 values (Table 5)
obtained for all the results indicate that the fitted data have good agreement with the experimental
data. The fitted dates are listed in Table 5 includes Rs (electrolyte resistance), Rct (charge transfer
resistance), Rf (film resistance), Qf and Qdl represent constant phase elements (CPE), representing
film capacitance (Cf) and double layer capacitance (Cdl), respectively. The impedance of CPE was
evaluated from Eq. (11):
n −1
(
)
푍CPE = [푌 푗푤 ]
(11)
0
where Y0 is proportionality coefficient, j is imaginary number, ω is angular frequency, and n is
phase shift. Also, the ηz was calculated by polarization resistance Rp in Eq. (12):
(푅p − 푅p0)
( )
휂푧 % =
× 100
(12)
푅p
where R0 and Rp are the sum of Rf and Rct in the absence or presence of inhibitor, respectively. As
p
presented in Table 5, the values of Rf and Rct increased significantly with increasing inhibitors’
concentration leading to enhanced inhibitor efficiency. This behavior was assigned to the formation
of protective layer at metal/solution interface and strongly hinders local corrosion caused by
chloride-induced. While, Cf and Cdl values showed a decreasing trend with the presence of MPTA
and EPTA inhibitor, which is due to a decrease in local dielectric constant and/or an increase in the
thickness of electric double layer capacitor [11,17,32]. The reduction of capacitance and increase of
resistance indicate that the absorbed organic molecule displaces the water molecule on metal
surface and reduce the acid solubility of X80 steel, thus hindering further charge and mass transfer.
Besides, another finding in Table 5 that there was no significant change in the phase shift n value,
indicating charge transfer controls the dissolution mechanism of steel. Lastly, it can be concluded
that MPTA possesses the higher resistance. Hence the ηz of X80 steel in test solution with MPTA is

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higher than that of the EPTA, which is consistent with the results of weightlessness and polarization
curves.
A comparison with other acidizing oilfield inhibitors by EIS measurement for steel was shown
in Table 6. MPTA and EPTA, possessing a unique aromatic ring system, are among the excellent
corrosion inhibitors.
3.7. SEM and EDX analysis
Representative surface micrographs of X80 steel after immersion in corrosion solution for 48 h
without or with 400 ppm MPTA and EPTA were examined using SEM (Fig. 12) and EDX (Fig. 13)
so as to determine the formation of protective film on the steel surface. As shown in Fig. 12a, the
X80 steel surface was severely corroded and badly damaged in 15% HCl solution. In contrast, the
corrosion degree of steel surface was significantly reduced and clearly a smooth surface with a little
shallow pitting corrosion in the solution containing MPTA and EPTA (Fig. 12b and c), showing that
the corrosion of X80 steel was well safeguarded by the investigated inhibitor. The surface of the
MPTA-protected steel was flatter than the EPTA protection and was close to the polished surface of
the sample.
EDX energy spectrum analysis provides a clear understanding of the elemental composition of
steel surface. The EDX spectrum of X80 steel without and with 400 ppm MPTA was shown in Fig.
13. In blank acidic solution, the EDX spectrum illustrated in Fig. 13(a) show the characteristic
peaks of iron and the small peaks that characterize chloride and oxygen, which represent corrosive
products. After adding MPTA inhibitor (Fig. 13b), the peaks of Fe were largely suppressed
compared to that of blank acid, and the characteristic peak of N appeared around 0.4 keV, which is
derived from the N elements in the MPTA inhibitor. This result clearly indicates that MPTA
molecule was adsorbed on steel surface.
3.8. AFM observation
AFM is a powerful tool for observing nano- to microscale surface topography and has become a
superior choice for discussing the corrosion processes at metal/solution interfacial [33]. Analysis of
the photographs allowed quantification of surface roughness over an area of 5 μm × 5 μm. The
two-dimensional (2D) and three-dimensional (3D) AFM images of uninhibited and inhibited X80
steel surfaces with 400 ppm MPTA and EPTA in 15% HCl solution for 24 h are shown in Fig. 14. It
is clearly seen from Fig. 14a that the X80 steel sample shows a relatively uneven and rough
structure due to erosion by the hydrochloric acid medium. In presence of MPTA (Fig. 14b) and
EPTA (Fig. 14c), the sample surface becomes relatively flat, while MPTA provide a more uniform
surface. The maximum height scale of X80 steel in blank solution was 712 nm, while the
corresponding values in the presence of MPTA and EPTA were 106 nm and 134 nm respectively.

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These images and parameters suggest that the inhibitive ability of the studied compounds follows
the order of MPTA > EPTA, which is in good agreement with the electrochemical measurements
and SEM studies. The AFM results further confirm that the formation of adsorbed protective film of
MPTA and EPTA on the X80 steel surface.
3.9. XPS analysis
In order to confirm the nature of the organic film adsorbed on the X80 steel surface and the
adsorption mode of the inhibitor, XPS test was used to study in the presence of the best inhibitor
(MPTA). As shown in Fig. 15a, the XPS spectra with three elements (C, N and Fe) were detected,
which provides evidence for the adsorption of the inhibitor on the X80 steel surface. Fig. 15(b-d)
illustrates the high-resolution XPS spectra of specific elements (C1s, N1s and Fe2p3/2) protected
layers. According to Fig. 15b, the C1s spectrum was fitted to three peaks. The small peak at 284.6
eV was ascribed to C–C, C=C and C–H aromatic bonds, and another peak located at 286.3 eV was
related to C-N and/or C=N bonds in tetrahydroacridine ring. The peak at 288.7 eV, which has
higher binding energy was attributed to C=N+ structures may be caused by the proton of the =N–
structure in the tetrahydroacridine ring [34-36]. The N 1s peaks (Fig. 15c) at 399.2 eV, 400.7 eV
and 401.6 eV were observed, the first peak was assigned to C–N and the unprotonated N atoms
(=N– structure) [17,20], the peak at 400.7 eV, representing N–Fe groups which due to the
coordination of nitrogen in the MPTA ring with iron atom. Indeed, the N–Fe bond complex can
make the peak shift to a higher binding energy compared to the uncoordinated =N– structure [17].
The last peak at 401.6 eV was ascribed to the protonated nitrogen, which gives rise to positive
polarization nitrogen atoms, and therefore the binding energy is increased [36]. The Fe 2p3/2 (Fig.
15d) showed three peaks around 710.6 eV, 713.4 eV and 715.2 eV. The first peak at 710.6 eV was
attributed to ferric compounds, such as Fe2O3 (i.e., Fe3+ oxide) and FeOOH (i.e., oxyhydroxide).
The second peak appeared at 713.4 eV was due to the presence of a small concentration of FeCl3 on
X80 steel surface in the hydrochloric acid solution, while that appeared at 715.2 eV may be
assigned to the satellite of Fe(III) [20].
The results obtained by XPS analysis support the adsorption of inhibitor on the X80 steel
surface. Especially, the presence of the nitrogen species on the X80 steel surface indicates that
MPTA can be adsorbed on the X80 steel surface by physisorption simultaneously accompanied by
chemisorption in 15% HCl, which corroborates the thermodynamic study. In addition, the Fe 2p3/2
analysis showed that the formation of a stable and insoluble layer (Fe2O3, FeOOH) makes the
protected layer become more complete and possess excellent anti-corrosion ability.
4. Conclusions

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The inhibitive ability and mechanism of MPTA and EPTA for X80 steel corrosion in 15% HCl
was studied by systematic methods and characterizations. The main conclusions are as follows:
(1) MPTA and EPTA exhibited good inhibition performance for X80 steel corrosion in
simulated acidic oilfield water, and the inhibition efficiency is related to the concentration
of inhibitors and temperature.
(2) Electrochemical results showed that MPTA and EPTA performed as mixed-type inhibitors,
which predominantly restrained cathode process.
(3) The adsorption of investigated inhibitors on the X80 steel surface is predominantly
physisorption and obeys Langmuir adsorption isotherm.
(4) Surface analysis (SEM, EDX, AFM and XPS) confirms the adsorption of MPTA and
EPTA molecules on X80 steel surface has played a good inhibition performance.
Acknowledgments
This work was supported by the Natural Science Foundation of Shandong (ZR2019MB009), the
Fundamental Research Funds for the Central Universities (HIT.NSRIF.201701), the National
Natural Science Foundation of China (21672046, 21372054), and the Found from the Huancui
District of Weihai City
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Figure captions
Fig 1. Synthetic route of the studied tetrahydroacridines.
Fig 2. Variation of (a) corrosion rate and (b) inhibition efficiency with different concentrations of
additives by weight loss. Error bars (┳) represent the standard deviations (%).
Fig 3. Arrhenius plots of lnvcorr vs. 1/T for X80 steel in 15% HCl in the absence and presence of
inhibitors.
Fig 4. Arrhenius plots of ln(vcorr /T) vs. 1/T for X80 steel in 15% HCl in the absence and presence of
inhibitors.
Fig 5. Langmuir adsorption plots for X80 steel in 15% HCl solution containing different
concentrations of (a) MPTA and (b) EPTA at 30–90 °C.
Fig 6. Plot of ln Kads vs. 1/T for X80 steel in 15% HCl solution containing different concentrations
of MPTA and EPTA.
Fig 7. EOCP – t curves for X80 steel in 15% HCl solutions with different concentrations of inhibitors:
(a) MPTA and (b) EPTA.
Fig 8. Potentiodynamic polarization curves for X80 steel in 15% HCl (a) MPTA and (b) EPTA.
Fig 9. Nyquist plots for X80 steel in 15% HCl solution (a) MPTA and (b) EPTA.
Fig 10. Bode plots for X80 steel in 15% HCl solution (a) MPTA and (b) EPTA.
Fig 11. Equivalent circuit used to fit the EIS experiment data: (a) without inhibitor and (b) the
presence of inhibitor.
Fig 12. SEM images of X80 steel surfaces: (a) 15% HCl, (b) MPTA, (C) EPTA.
Fig 13. EDX spectrum of X80 steel after immersion in 1 M HCl solution: (a) 15% HCl, (b) 15%
HCl with 400 ppm MPTA.
Fig 14. AFM images of X80 steel surfaces: (a) 15% HCl, (b) MPTA, (C) EPTA.
Fig 15. XPS spectra for X80 steel treated with 400 ppm MPTA in 15% HCl: (a) survey scan spectra
and narrow scan spectra of (b) C, (c) N, and (d) Fe.

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Table 1 Weight loss results of X80 steel in 15% HCl with MPTA and EPTA at different
temperatures; the values are the mean of three replicates and (±) corresponds to the standard
deviations.
Inhibitor/
C (ppm)
30 °C
v (mg
cm-2 h-1)
Blank 0.00 57.32±2.7
50 °C
v (mg
cm-2 h-1)
70 °C
v (mg
cm-2 h-1)
90 °C
v (mg
cm-2 h-1)
ηw
ηw
ηw
ηw
(%)
(%)
(%)
(%)
-
77.73±4.6
-
105.47±4.3
-
156.15±6.1
-
MPTA 100 12.41±1.5 78.35
200 6.67±1.1 88.36
300 3.29±0.4 94.26
400 2.21±0.4 96.14
EPTA 100 14.73±1.8 74.32
200 9.35±1.3 83.69
300 4.26±0.7 92.57
400 3.19±0.3 94.43
19.23±2.1 75.26
10.78±1.4 86.13
6.19±1.3 92.04
4.95±0.8 93.63
21.62±2.6 72.18
14.75±1.7 81.02
8.26±1.2 89.37
6.71±1.3 91.37
30.08±2.9 71.48
18.53±2.2 82.43
13.49±1.9 87.21
11.44±1.3 89.15
33.42±2.7 68.31
24.44±2.3 76.83
16.99±1.8 83.89
14.15±1.2 86.58
53.31±3.2 65.86
37.10±2.8 76.24
28.37±1.9 81.83
27.65±2.1 82.29
58.81±3.4 62.34
44.88±2.3 71.26
34.70±2.5 77.78
32.77±1.8 79.01

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Table 2 Activation parameters for X80 steel dissolution in 15% HCl without and with inhibitors.
Inhibitors
Blank
Ea (kJ mol-1)
ΔH* (kJ mol-1)
ΔS* (J mol-1 K-1)
15.06
12.31
-171.50
MPTA
EPTA
38.55
35.80
-121.28
35.11
32.36
-129.45

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Table 3 Thermodynamic parameters of inhibitors on the X80 steel surface in 15% HCl solution.
Compounds T (°C) Kads (ppm/mol) ΔG0ads (kJ mol-1) ΔH0ads (kJ mol-1) ΔS0ads (J mol-1)
MPTA
EPTA
30
50
70
90
30
50
70
90
0.0305
0.0283
0.0271
0.0250
0.0237
0.0232
0.0220
0.0210
-26.05
-27.53
-29.11
-30.56
-25.41
-26.99
-28.51
-30.04
-29.17
-18.84
76.24
EPTA
77.62

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Table 4 Polarization curve parameters for the corrosion of X80 steel in 15% HCl at 30 °C.
Inhibitors
C
Ecorr
icorr
-βc
βa
ηi
(%)
Rp
ηLPR
(%)
(ppm) (mV vs.SCE) (µA cm-2) (mV dec-1) (mV dec-1)
(Ω cm2)
-
Blank
100
300
400
100
300
400
-449
-472
-496
-507
-458
-491
-506
993.8
219.5
52.4
140.3
135.9
136.1
144.8
137.7
133.6
138.1
95.1
92.6
-
54.5
256.1
1068.3
1891.4
229.7
750.7
1384.9
-
MPTA
79.62
94.73
97.87
75.73
92.26
95.96
78.72
94.90
97.12
76.27
92.74
96.06
91.9
21.2
107.1
100.3
101.9
100.4
EPTA
241.2
76.9
40.1

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Table 5 EIS parameters for the corrosion of X80 steel in 15% HCl at 30 °C.
χ2
Inhibitors
C
Rf
Rct
Cf (n1)
Cd (n2)
Rp
ηz (%)
(ppm) (Ω cm2) (Ω cm2) (μF cm-2)
(μF cm-2)
(Ω cm2)
(10-3)
-
Blank
100
300
400
100
300
400
-
51.4
260.7
1100.0
1813.0
225.4
784.3
1385.0
-
526.5 (0.92)
51.4
2.19
-
MPTA
14.5
32.1
39.9
9.9
59.3 (1) 138.6 (0.58) 275.2
1.58 81.32
20.2 (1)
13.4 (1)
45.0 (0.61) 1132.1 2.16 95.46
37.1 (0.65) 1852.9 3.90 97.23
EPTA
77.5 (1) 182.7 (0.59) 235.3
2.95 78.16
4.06 93.68
29.0
35.6
33.4 (1)
14.5 (1)
58.3 (0.64)
813.3
40.8 (0.61) 1420.6 1.37 96.38

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Table 6 Comparison of the inhibition efficiency of MPTA and EPTA with the literature data as
acidizing oilfield inhibitor for steel.
Inhibitors
Medium of testing Concentration η (%)
Ref.
2-methyl-9-phenyl-1,2,3,4-tetrahydroacridine
15% HCl
15% HCl
400 ppm
400 ppm
97.23 This paper
96.38 This paper
Ethyl 9-phenyl-1,2,3,4-tetrahydroacridine-2-
carboxylate
Griffonia simplicifolia
Sophorolipids
1 M HCl
500 ppm
70 mg/L
91.77
89.05
[2]
[3]
oilfield water
saturated with CO2
2,3-pyrazine dicarboxylic acid
pyrazine carboxamide
15% HCl
15% HCl
0.2 wt%
0.2 wt%
0.2 wt%
400 ppm
400 ppm
400 ppm
400 ppm
0.1 mM
41.7
54.9
[6]
[6]
2-methoxy-3-(1-methylpropyl) pyrazine
Lemongrass extract
15% HCl
48.9
[6]
oilfield water
oilfield water
oilfield water
oilfield water
15% HCl
60.12
67.47
59.11
67.24
75.2
[7]
Gemini surfactants GS2
[27]
[27]
[27]
[31]
Gemini surfactants GS6
Gemini surfactants GS10
5-hydroxytryptophan

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Highlights
 MPTA and EPTA exhibited good inhibition performance for X80 steel corrosion in simulated
acidic oilfield water.
 MPTA and EPTA performed as mixed-type inhibitors.
 Adsorption of these inhibitors on steel surface obeys Langmuir adsorption isotherm.
 Inhibition efficiency correlates with molecular structure of inhibitors.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7