Versatile 4, 6-dimethyl-2-mercaptopyrimidine based ionic liquids as high-performance corrosion inhibitors and lubricants

Article abstract of DOI:10.1016/j.molliq.2019.04.042

Ionic liquids (ILs) are extensively used in many fields owing to their attractive characteristics. Good structure designability provides ILs an opportunity to integrate multiple functions into their structures, which could promote their industrial applications as versatile materials. In this study, three 4, 6-dimethyl-2-mercaptopyrimidine (DMMP) based phosphonium ILs were firstly synthesized. The corrosion inhibition behaviors of these ILs for mild steel in 0.5 M H₂SO₄ aqueous solutions were investigated by electrochemical tests. Results indicated that all of the three ILs exhibited surprising inhibition efficiencies as high as 99%, even at an ultralow concentration of 0.1 mM. The ILs' adsorption behavior on surface obeys Langmuir isotherm, indicating that the ILs inhibit corrosion of mild steel by monolayer adsorption on the mild steel surface. Moreover, due to the superior adsorption capacity on steel surface, DMMP based ILs are expected have great application potentials as high-performance lubricating materials. As expected, DMMP based ILs showed excellent lubrication performances both at 30 °C and 100 °C as compared to PAO and 1-butyl-3-methylimidazolium tetrafluoroborate (LB104). Surface analyses indicated that tribofilms consist of ILs adsorption layer and tribochemical products, such as FeS, Fe₂O₃ and organic compounds containing C[sbnd]O bonds, are responsible for the predominant lubrication performances. Therefore, versatile ILs designed in this paper have gathered two significant abilities for industrial applications, which are high corrosion inhibiting properties and excellent lubrication performances. This work is believed to be instructive for designing multifunctional ILs and promoting the ILs' application in industry in the future.

Full text of DOI:10.1016/j.molliq.2019.04.042

Accepted Manuscript
Versatile 4, 6-dimethyl-2-mercaptopyrimidine based ionic liquids
as high-performance corrosion inhibitors and lubricants
Yi Li, Songwei Zhang, Qi Ding, Baofeng Qin, Litian Hu
PII:
S0167-7322(18)33357-9
DOI:
https://doi.org/10.1016/j.molliq.2019.04.042
MOLLIQ 10765
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
20 November 2018
23 February 2019
7 April 2019
Please cite this article as: Y. Li, S. Zhang, Q. Ding, et al., Versatile 4,
-dimethyl-2-mercaptopyrimidine based ionic liquids as high-performance corrosion
6
inhibitors and lubricants, Journal of Molecular Liquids, https://doi.org/10.1016/
j.molliq.2019.04.042
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ACCEPTED MANUSCRIPT
Versatile 4, 6-dimethyl-2-mercaptopyrimidine based ionic liquids as
high-performance corrosion inhibitors and lubricants
a, b
a, c, *
a, c
a
a, *
Yi Li , Songwei Zhang
, Qi Ding , Baofeng Qin , Litian Hu
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, P. R. China
b
University of Chinese Academy of Sciences, Beijing 100049, P. R. China
c
Qingdao Center of Resource Chemistry & New Materials, Qingdao 266071, P. R. China
*Corresponding authors: Songwei Zhang (zhangsw@licp.cas.cn); Litian Hu (lthu@licp.cas.cn)
Abstract: Ionic liquids (ILs) are extensively used in many fields owing to their attractive
characteristics. Good structure designability provides ILs an opportunity to integrate multiple
functions into their structures, which could promote their industrial applications as versatile
materials. In this study, three 4, 6-dimethyl-2-mercaptopyrimidine (DMMP) based phosphonium
ILs were firstly synthesized. The corrosion inhibition behaviors of these ILs for mild steel in 0.5
2 4
M H SO aqueous solutions were investigated by electrochemical tests. Results indicated that all
of the three ILs exhibited surprising inhibition efficiencies as high as 99%, even at an ultralow
concentration of 0.1 mM. The ILs’ adsorption behavior on surface obeys Langmuir isotherm,
indicating that the ILs inhibit corrosion of mild steel by monolayer adsorption on the mild steel
surface. Moreover, due to the superior adsorption capacity on steel surface, DMMP based ILs are
expected have great application potentials as high-performance lubricating materials. As expected,
DMMP based ILs showed excellent lubrication performances both at 30℃ and 100℃ as
compared to PAO and 1-butyl-3-methylimidazolium tetrafluoroborate (LB104). Surface analyses
indicated that tribofilms consist of ILs adsorption layer and tribochemical products, such as FeS,
2 3
Fe O and organic compounds containing C-O bonds, are responsible for the predominant
lubrication performances. Therefore, versatile ILs designed in this paper have gathered two
significant abilities for industrial applications, which are high corrosion inhibiting properties and
excellent lubrication performances. This work is believed to be instructive for designing
multifunctional ILs and promoting the ILs’ application in industry in the future.
Keywords: DMMP based ionic liquids; Adsorption capacity; Steel substrate; Corrosion inhibitors;
Lubricating performances
1

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1
. Introduction
As one of the most important metallic materials in the world, steel is extensively used in
various industrial devices and machines which play a key role in our daily lives. Corrosion and
wear are the main forms of equipment failure and engineering destruction, which will further
cause the excessive consumption of energy and resource [1, 2]. Effective protection measures,
including lubrication and corrosion prevention, could remarkably prolong the service life of
mechanical equipment and conserve resources. Therefore, in fields of lubrication engineering and
industry, corrosion inhibitors and other additives are always introduced in commercial formulated
lubricating oils and greases [3-5]. Nevertheless, it may lower the efficiency or induce antagonistic
effect while packaging many additives together during the industrial application [6, 7]. The fact is
that the compatibility of functional additives with each other is always a challenge. Therefore, it is
of great practical significance to develop versatile lubricating materials with high corrosion
inhibiting efficiency and excellent lubricating performance.
Ionic liquids (ILs) are fascinating liquid materials near room temperature only composed of
organic cations and organic/inorganic anions [8]. Owing to their various attractive characteristics
such as low vapor pressure, high thermal stability, nonflammability, low melting point and wide
electrochemical window, ILs have great potentials to be used in a wide range of applications
[
9-11]. ILs were first researched as lubricants in 2001 [12]. Amounts of researches have proved
that ILs exhibited predominant friction-reducing and antiwear properties, and further
demonstrated to be good candidates to be used as high-performance lubricants or lubricant
additives [13-17]. Avariety of ILs has been used as corrosion inhibitors in acidic solutions [18-21].
Either cationic or anionic moiety of these ILs (most of them are cations) have strong adsorption
capacity on electrode, which could effectively retard the corrosion of electrode in corrosive
solutions, resulting in high corrosion inhibiting efficiency. However, few researches about
versatile ILs equipping these two significant features were reported, which restrict the industrial
application of ILs. Therefore, it is a prospective work to develop versatile ILs with both
lubricating and corrosion inhibiting properties by introducing functional groups into the structure
of ILs.
Mercaptopyrimidine and its derivatives are widely used as corrosion inhibitors in different
corrosive solutions such as HCl, H
2
SO
4
3 2 3 4
, HNO , CO and H PO [22-24]. These derivatives
2

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exhibited high inhibition efficiencies in researched systems due to their strong adsorptive abilities
with metallic materials. In this study, 4,6-dimethyl-2-mercaptopyrimidine (DMMP) was used as
precursor compound as well as corrosion inhibiting group to synthesize functionalized ILs.
Electrochemical tests are utilized to determine the corrosion inhibiting performances by adding
DMMP based ILs into the corrosive H SO solutions. What’s more, lubrication performances are
2
4
also investigated as lubricants through friction tests and worn surface analyses. This work may
make a great progress on versatile ILs with multifunctional abilities in the application of
lubricating industry.
2
. Experimental section
2
.1 Materials
The traditional IL reagent 1-butyl-3-methylimidazolium tetrafluoroborate (LB104) with a
purity of ≥98% is obtained from Lanzhou Institute of Chemical Physics, CAS. The following
chemical reagents are commercially available: PAO10 oil (Mobil), tributylphosphine (Aladdin),
1
4
-bromooctane (Aladdin), 1-bromodecane (Aladdin), 1-bromotetradecane (Aladdin),
,6-dimethyl-2-mercaptopyrimidine (Aladdin), potassium hydroxide (Tianjin Kaitong Chemicals
Regent Company).
The specimens with a dimension of Φ24.0×7.9 mm are mild steel disks in corrosion tests and
AISI 52100 steel disks in friction tests. All the specimens are abraded and polished to gain a
smooth surface with roughness about 25 nm. Then the samples were cleaned in acetone
ultrasonically and stored under N
.2 Synthesis and characterization of the ILs
The synthesis routes of novel ILs are divided into two steps as follows:
P444R][Br] (R=8, 10, 14): tributylphosphine (51.2 g, 0.253 mol) and 0.264 mol alkyl
bromide s（ 1-bromooctane 51.0 g, 1-bromodecane 58.4 g, 1-bromotetradecane 73.2 g, respectively)
were added to a 250 mL three necked flask fitted with a reflux condenser and N protection
atmosphere. Any remaining
2
atmosphere after drying at room temperature.
2
[
2
systems. The solution was stirred for 12 h at 120℃ under N
2
impurities were removed by reduced pressure distillation at 120℃.
[
P
444R][DMMP] (R=8, 10, 14): 4,6-Dimethyl-2-mercaptopyrimidine (14 g, 0.1 mol) was
mixed with potassium hydroxide (5.6 g, 0.1 mol) in methanol firstly. Afterwards, 0.1 mol
[
P
444R][Br] was also dissolved in methanol and the solution was added into the mixtures. The
3

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solution was stirred at room temperature for at least 6h. Then the insoluble precipitates were
filtered and the filtrate was concentrated by evaporating the methanol. Anhydrous acetone was
used to extract the ILs again. After filtration, the acetone was evaporated to get the products. The
obtained ILs were dried at 80℃ under vacuum over 12h. After purification and drying under
vacuum, the water contents in three ionic liquids were obtained by Karl Fisher titration method
(
831 KF Coulometer, Metrohm). Kinematic viscosities of ILs were measured at the temperatures
of 40 and 100 °C using a viscosity tester (SYP1003). The results are listed in Table 1. The
1
structures of the synthesized final ILs (Fig. 1) were characterized by H NMR and HRMS. The
results are listed as follows.
1
[
P
4448][DMMP] (P8): H NMR (400 MHz, CDCl
3
) δ 6.27 (s, 1H), 2.44 (m, 8H), 2.22 (s, 6H),
+
-
1
6
6
.22-1.44 (m, 24H), 0.94 (t, 9H), 0.88 (t, 3H). m/z: 315.3174([P4448] ), 139.0331([DMMP] ).
1
[
P
44410][DMMP] (P10): H NMR (400 MHz, CDCl
3
) δ 6.29 (s, 1H), 2.46 (m, 8H), 2.29 (s,
+
-
H), 1.21-1.46 (m, 24H), 0.94 (t, 9H), 0.88 (t, 3H). m/z: 343.3475([P44410] ), 139.0330([DMMP] ).
1
[
P
44414][DMMP] (P14): H NMR (400 MHz, CDCl
3
) δ 6.28 (s, 1H), 2.46 (m, 8H), 2.26 (s,
+
-
H), 1.25-1.47 (m, 24H), 0.95 (t, 9H), 0.88 (t, 3H). m/z: 399.4128([P44414] ), 139.0328([DMMP] ).
Fig. 1. Molecular structures of tested ILs.
The thermal stabilities are investigated by TGA test (STA449F3, NETZSCH) from 25 °C to
8
2
00 °C with a heating rate of 10 °C/min in N atmosphere. The phase transition temperatures of
ILs are also measured by DSC test (200F3, NETZSCH). The results of thermodynamic properties
are listed in Table 1. These ILs have high decomposition temperatures indicating high thermal
stabilities. At room temperature, three ILs show liquid states due to their low melting points.
Therefore, these results demonstrated that DMMP based ILs could be used at room temperature
and high temperature due to their wide liquid range and high thermal stability.
4

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Table 1 Thermodynamic properties, kinematic viscosities, the state at 30℃ and water content of
g m
the synthesized ILs: glass transition temperature (T ), melting temperature (T ), thermal
decomposition temperature (T
Kinematic
d
)
Water
content
(wt.%)
0.592
2
ILs
T (℃)
g
T (℃)
m
T (℃)
d
viscosity (mm /s) State at 30℃
4
0℃ 100℃
P8
-46.0
-46.9
-52.3
25.1
25.4
28.8
284.3
288.4
289.3
1897.3 66.7
2608.3 73.7
2655.4 73.0
Viscous liquid
Viscous liquid
Viscous liquid
P10
P14
0.606
0.483
2
.3 Electrochemical tests
Electrochemical experiments are performed on Gamry Reference 3000 advanced
electrochemical workstation in a conventional three-electrode system including reference
electrode (mercurous sulfate), counter electrode (platinum) and working electrode (mild steel)
2
with exposed surface area of 1.0 cm . The electrolytes are the test solutions of 0.5 mM H
2
SO
4
with
and without the addition of different concentrations of ILs. Before the tests, mild steel electrode
was immersed in the test solution at open circuit potential (OCP) for 1.0 h to get a steady state.
The electrochemical impedance spectroscopy (EIS) was measured at OCP in frequency range
5
-1
from 10 Hz to 10 Hz with a sinusoidal AC perturbation amplitude 5 mV. The potentiodynamic
polarization curves were collected in the potential range from -350 mV to 350 mV versus OCP
with a scan rate of 0.5 mV/s. To ensure validity of full potentiodynamic polarization scan, separate
anodic and cathodic polarization experiments of all the samples were conducted starting from the
OCP with a new specimen and a new solution. In each experiment, the separate anodic and
cathodic arms are very similar to the corresponding full scan curves. The results of some typical
samples were shown in Supporting Information, Fig. S1. The data from EIS and
potentiodynamic polarization curves were analyzed by Gamry Echem. Analyst software. Each
electrochemical measurement was performed at least three times under the same experimental
conditions to ensure a satisfactory reproducibility.
2
.4 Tribological tests
5

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The friction and wear tests were carried out to evaluate the tribological properties on an
Optimol SRV-IV oscillating reciprocating friction and wear tester at 30℃ and 100℃. The contact
between tribopairs was achieved by pressing the upper running ball (AISI 52100 steel with a
diameter of 10 mm) against the lower stationary disc (AISI 52100 steel). Tribological tests were
conducted at a load of 100N, an amplitude of 1 mm and a frequency of 25 Hz. The relative
humidity was 10-30%. The friction coefficient was monitored automatically by the SRV test
system. The wear volume of lower steel disc was measured after friction test by a noncontact
surface mapping microscope profilometer MicroXAM-3D. The tests at each condition were
repeated at least three times.
2
.5 Surface analysis
The morphology and chemical composition of worn surfaces were analyzed by a scanning
electron microscope (SEM, JSM-5600LV) equipped with an energy-dispersive X-ray
Spectrometer attachment (EDS, KEVEX). In addition, the X-ray photoelectron spectrometer (XPS)
was carried out to investigate tribochemical reaction products generated on the worn surfaces at
high temperature. The most reactive elements related to ILs and tribopairs were detected.
Previously, the specimens were ultrasonically cleaned in ethanol. Al Kα radiation was the exciting
source of XPS. The binding energy of contaminated carbon (C 1s at 284.8 eV) was used as the
reference.
3
. Results and discussion
3
.1 Potentiodynamic polarization curves
2 4
Potentiodynamic polarization curves for mild steel in 0.5 M H SO solution with and without
different concentrations of P8 at 25℃ are displayed in Fig. 2. The electrochemical parameters
calculated from the related potentiodynamic polarization curves are listed in Table 2, such as
corrosion potential (Ecorr), corrosion current density (icorr), corrosion inhibition efficiency (η),
anodic and cathodic Tafel slope (β
a c
and β ). The corrosion inhibition efficiency (ηTafel) and surface
coverage (θTafel) are defined as follows:

i



corr
0
Tafel  1-
100%

i

corr
(
1)
Tafel Tafel /100%
(
2)
6

ACCEPTED MANUSCRIPT
where 푖푐표푟푟 and 푖0
are the corrosion current densities with and without inhibitor, respectively
푐표푟푟
[
20, 25].
It can be seen from Fig. 2, the addition of P8 causes a remarkable decrease in current density
from both anodic and cathodic branch of polarization curves. The cathodic polarization curves are
parallel with each other, which suggest that cathodic hydrogen ions reduction mechanisms are not
changed by varying the concentrations of the inhibitors. In the anodic branch of curves, a
significant inhibiting effect can be observed at low overpotential, especially at higher
concentrations. This suggests that the corrosion inhibiting effect is generally related to the
formation of protective film of adsorbed ILs on the metal surface [25, 26]. However, the corrosion
current density increases sharply at more positive potentials. This phenomenon is ascribed to the
significant dissolution of mild steel, resulting in the desorption of IL inhibitor from the electrode
surface [25].
2 4
Fig. 2. Potentiodynamic polarization curves for mild steel in 0.5 M H SO with and without
different concentrations of P8.
As is shown in Table 2, compared to the blank, the Ecorr in solution with ILs shifts towards
negative, and all the displacements of different concentrations are lower than 85 mV. The results
reveal that P8 act as mixed type inhibitors on mild steel surface. And the inhibiting mechanism
a c
may be mainly due to geometric blocking effect [25]. The variations of two Tafel slopes (β and β )
suggest that both anodic and cathodic reactions are suppressed by adsorption of the ILs on mild
steel surface [27]. Further, icorr values tend to decrease, while ηTafel enhances with the increase in
7

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IL inhibitor concentration. This is ascribed to the blocking effect on the cathodic and anodic
reaction sites at higher concentration and thereby hindering the electrochemical reactions on
electrode surface [25]. Therefore, higher concentration of P8 inhibitor tends to form completer and
more stable adsorption layer on electrode surface. When the concentration of P8 inhibitor is only
0
.03 mM, ηTafel is as high as 91.9 %. For other concentrations, the values are higher than 99%,
which indicates that the addition of P8 could perfectly solve the corrosion problem of mild steel
caused by H SO solution. Thus, P8 is an effective corrosion inhibitor for mild steel in H SO
solution.
2
4
2
4
Table 2 Fitted potentiodynamic polarization curves parameters for mild steel in 0.5 M H
2 4
SO with
different concentrations of P8.
-
2
C (mM)
Blank
Ecorr(mV)
i
corr(uAcm )
-β
c
(mV/dec)
β
a
(mV/dec)
η
Tafel
%
θ
Tafel
-890.0±3.2 3090.0±80.9 262.2±8.1
17.02±3.2
18.21±3.5
22.03±4.2
24.16±2.8
33.04±3.9
41.84±4.6
/
/
0
0
0
0
0
.03
.05
.1
-939.4±2.8 251.5±9.8
-925.6±2.0 31.4±6.9
-906.3±2.2 25.4±3.4
-897.2±1.8 8.9±2.1
-895.0±2.1 6.5±1.3
143.4±5.2
123.0±3.3
123.3±2.5
115.1±3.1
126.6±3.7
91.9
99.0
99.2
99.7
99.8
0.919
0.990
0.992
0.997
0.998
.3
.5
3
.2 Electrochemical impedance spectroscopy
To get more information about the surface properties of the mild steel as well as kinetics of
corrosion reaction processes, electrochemical impedance spectroscopy was also obtained in test
solutions. The Nyquist plots are presented in Fig. 3a. A depressed capacitive loop can be seen in
the Nyquist diagram, indicating that the corrosion of mild steel is mainly controlled by charge
transfer process at electrode/solution interface [22, 25]. Roughness and inhomogeneity of mild
steel surface may be the reason why the capacitive loop is not a perfect shape[25]. The diameter of
2 4
capacitive loop is enlarged significantly when P8 was added into 0.5 M H SO solution. And it
enhances with the increase of the concentration of P8, which confirms that the corrosion of mild
steel is retarded effectively. The low frequency inductive loop appears in Nyquist plot of low
8

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concentration and gradually disappears with the increase of the IL concentration, which is
regarded as a “degradation” phenomenon of impedance spectroscopy [25].
Fig. 3 EIS and corresponding equivalent circuits models for mild steel in 0.5 M H SO with and
2
4
without different concentrations of P8: (a) Nyquist plots; (b) and (c): Bode plots; (d): Equivalent
circuits.
As for Bode plots shown in Fig. 3b and 3c, the absolute impedance after adding P8 is higher
than blank solution and increases with increasing the concentration. This suggests that the
adsorption film on electrode surface tends to be completer and more stable with increasing the
concentration of P8. In phase angle plots, one time constant is confirmed according to the only one
phase peak. The phase angles are all lower than 90 degree, which may be ascribed to the
frequency dispersion due to the geometrical factors [22].
To obtain the impedance parameters, the equivalent electric circuits in Fig. 3d are employed
s
to fit the EIS results. In the circuits, R is the solution resistance, Rct is charge transfer resistance,
R and L are the inductive elements. CPE is the constant phase element which is often used to give
L
9

ACCEPTED MANUSCRIPT
a more accurate fitting impedance result instead of double layer capacitance (Cdl). The impedance
of CPE which can be given as:
1
ZCPE

n
Y0

j

(
3)
where Y is the CPE constant, j is the imaginary unit, 휔 is the angular frequency and n is the
0
phase shift related to surface inhomogeneous [22, 25]. The values of Cdl arecalculated for a circuit
based on the equation:
1
1
-1
n
n
ct
C  Y R
dl
0
(
4)
The fitted impedance parameters using the equivalent electric circuits are calculated and listed in
Table 3. The corrosion inhibition efficiency (ηEIS) and surface coverage (θEIS) are calculated by
charge transfer resistance according to the equation:
0
ct
Rct - R
EIS

100%
Rct
(
5)
EIS  /100%
EIS
(
6)
where Rct, 0 and Rct are the charge transfer resistances without and with P8 [19, 25, 27].
Table 3 Fitted EIS parameters for mild steel in 0.5 M H
2
SO
4
with different concentrations of P8.
-5
2
C
R
s
Y
0
×10
n
R
ct (Ωcm )
R
L
L
C
dl
η
θ
EIS
2
n
-2
2
2
-2
(
mM)
(Ωcm )
Ss cm
(Ωcm )
(Hcm )
(μFcm )
(%)
/
Blank
4.8±0.6
3.9±0.5
4.0±0.7
3.8±0.4
3.6±0.3
3.8±0.5
61.60±4.31
6.67±1.56
2.58±1.23
1.82±0.92
1.81±0.48
1.73±0.75
0.859±0.022
0.863±0.031
0.870±0.032
0.918±0.015
0.887±0.051
0.865±0.012
9.1±2.7
/
/
263.1±20.8
28.0±3.5
13.7±1.7
12.7±1.1
11.6±0.8
10.9±0.6
/
0
0
0
0
0
.03
.05
.1
64.3±8.4
6.6±1.1
23.2±3.4
2.542±0.012
0.659±0.005
85.9
98.3
99.1
99.5
99.7
0.859
0.983
0.991
0.995
0.997
543.8±17.9
982.1±30.6
1738.0±57.4
3074.0±109.6
-
6
938.8±58.7
386.7±46.9
453.5±41.4
(5.10±0.02)×10
-4
.3
(1.74±0.03)×10
-5
.5
(1.45±0.14)×10
It can be seen from Table 3, R increases remarkably after adding different concentrations of
ct
2 4
P8 into 0.5 M H SO solution. Consequently, ηEIS enhances with the increase of the values of Rct.
The high values of Rct indicate that the corrosion of mild steel is retarded significantly by P8.
1
0

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When the concentration is 0.05 mM, a quite low concentration, the ηEIS reaches a satisfying value
of 98.3 %, which indicates that P8 functions as a very high effective corrosion inhibitor for mild
2 4
steel in 0.5 M H SO solution. Comparatively, the values of Cdl decreases with the increase of P8
concentration. This phenomenon may be related to decreasing in local dielectric constant and/or
increasing in the thickness of the electrical double layer, which can be explained by the formation
of protective film through the adsorption of P8 at mild steel/solution interface [25]. And the
protective film tends to be completer and more stable with the increase of P8 concentration. The
values of n are close to unity, suggesting that the interface behaves like capacitance and the
corrosion mechanism of mild steel is mainly controlled by charge transfer process [25]. What’s
more, the corrosion inhibition efficiencies obtained from EIS results are good agreement with
those from potentiodynamic polarization curves.
3
.3 Effect of alkyl chain length on corrosion inhibiting properties
To be a comparison, another two ILs P10 and P14 are also used as corrosion inhibitors in 0.5
2 4
M H SO solution at a concentration of 0.1 mM. The electrochemical measurement results
together with those of P8 are shown in Fig. 4. For the three potentiodynamic polarization curves,
they are similar with each other, indicating that these ILs with different length of alkyl chain have
the same inhibition mechanism for mild steel in 0.5 M H SO solution. In Nyquist plots, three ILs
2
4
show similar shape and the diameter of loop increases with the increase of alkyl chain. The
absolute impedance of solution is also increased when increasing the length of alkyl chain, which
indicates that P10 and P14 with longer alkyl chain have better corrosion inhibiting performances
2 4
than P8 in 0.5 M H SO solution. The parameters concluded from potentiodynamic polarization
curves and EIS plots are listed in Table 4. It can be seen that the values of Rct increase while icorr
decrease with the increase of the length of alkyl chain, consequently, corresponding corrosion
inhibition efficiencies are increased. All the corrosion inhibition efficiencies calculated from two
methods are higher than 99%, which indicates that all the three ILs are high effective corrosion
2 4
inhibitors for the corrosion of mild steel in 0.5 M H SO solution. Moreover, this kind of inhibitor
displays more effective protection for mild steel corrosion with increasing length of alkyl chain in
ILs.
1
1

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Fig. 4 The comparison of EIS and potentiodynamic polarization curves between three ILs: (a)
Nyquist plots; (b) and (c): Bode plots; (d): Potentiodynamic polarization curves.
Table 4 Comparison of fitted parameters for three ILs.
2
-2
ILs
R
ct (Ωcm )
η
EIS
%
i
corr (uAcm )
3090±80.9
25.41±3.4
15.16±2.1
11.32±0.7
η
Tafel
%
Blank
9.1±2.7
/
/
0
0
0
.1mM P8
.1mM P10
.1mM P14
982.1±30.6
1598.0±51.4
2311.0±58.9
99.1
99.4
99.6
99.2
99.5
99.6
3
.4 Adsorption isotherm
As discussed above, P8 exhibited high corrosion inhibiting properties due to its adsorption at
mild steel/solution interface. Thus, to better understand the adsorption mechanism of studied ILs
at mild steel/solution interface, several types of adsorption isotherms were examined for the
electrochemical test results. However, only the Langmuir adsorption isotherm is the most suitable
equation, which could be presented as follows:
1
2

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C
1

+C

Kads
(
7)
where C is the concentration of ILs; θ is the surface coverage, which has been calculated from
potentiodynamic polarization curves and EIS results; Kads is the adsorptive equilibrium constant
[
23, 25].
The fitted results are presented in Fig. 5. Both linear regression coefficients (R ) are
2
approximately 1, which suggesting the results could be fitted well by Langmuir adsorption
isotherm. Therefore, a monolayer of adsorbed P8 is formed at the mild steel/solution interface.
Kads could be calculated from the fitted line, which is also used to calculate the stand
0
adsorption Gibbs free energy (ΔGads ) [19, 25, 28].
0
ads

G  -RT ln

55.5Kads

(
8)
where the value 55.5 is the concentration of water in the solution with a unit of mol L ; R is the
-
1
-
1
-1
0
gas constant (8.314 Jmol K ); T is the absolute temperature. The values of ΔGads are calculated
-
1
as -43.33 and -41.95 kJ mol , which may deduce that the adsorption behavior of P8 is a
combination of both physisorption and chemisorption [28]. What’s more, the negative values
indicate that adsorption process of P8 at mild steel/solution interface occurred spontaneously.
2 4
Fig. 5 Langmuir isotherm plots for mild steel in 0.5 M H SO with different concentrations of P8:
(
a) Tafel data; (b) EIS data.
3
.5 Tribological properties
To probe the potentials of DMMP based ILs to be used as neat lubricants, the tribological
properties on steel/steel pairs are researched at 30℃ and 100℃. Comparatively, nonpolar oil
PAO10 and conventional IL LB104 are also utilized as lubricants at the same test conditions.
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3

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Fig. 6 shows the friction coefficient curves and wear volumes of different oil samples at 30℃.
As shown in Fig. 6a, the curve of PAO is much higher than those of ILs and fluctuates sharply
during the friction process. The friction coefficient of LB104 is obviously lower than that of PAO
but slightly higher than those of DMMP based ILs. All the curves of four IL samples remain stable
during the whole friction process. P8 and P10 show similar lubricating behavior while the two
curves almost overlap. P14 shows the lowest friction coefficient due to its longest alkyl chain. In
Fig. 6b, ILs exhibit lower wear volumes than PAO obviously. DMMP based ILs give slightly
lower values than LB104. The values of three DMMP based ILs have minor difference. This
indicates that DMMP based ILs present better tribological properties than PAO oil and LB104 at
3
0℃.
Fig. 6 (a) Evolution of friction coefficient with time at 100 N for different oil samples at 30℃; (b)
Wear volumes of steel discs lubricated by different oil samples at 30℃.
Fig. 7 shows the SEM morphologies of worn surfaces lubricated by different oil samples and
elements distribution map corresponding to worn surfaces lubricated by DMMP based ILs. PAO
results in the largest wear scar compared to those of ILs. The worn surface is very rough with
many deep friction scratches. For traditional IL LB104, there are only some shallow scratches.
The wear scars of DMMP based ILs are obviously smaller than those of PAO and LB104. All the
worn surfaces are quite smooth and almost no rubbing scratches. In the element maps, there is
little difference between the inside and the outside of wear scar. This indicates that little
tribochemical products exist on the worn surface and tribochemical reactions rarely occurred at
1
4

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3
0℃. The excellent lubrication performances of DMMP based ILs might be ascribed to the high
viscosities of ILs and/or IL layers adsorbed on the frictional surfaces.
Fig. 7 SEM and elements distribution maps of worn surfaces lubricated by different oil samples at
3
0℃: (a) PAO; (b) LB104; (c) P8; (d) P10; (e) P14.
As for 100℃, the evolution of friction coefficients and wear volumes are shown in Fig. 8.
The friction coefficient of PAO shows a rising trend and fluctuates during the whole friction
process in Fig. 8a. Four ILs exhibit lower friction coefficients compared to that of PAO. And
steady friction could be observed during the test. Three DMMP based ILs exhibit lower friction
than traditional IL LB104. After the running time about 100s, the curves turn to be stable. With the
increasing of alkyl chain length of cation, the stable friction coefficient decreases slightly. As is
shown in Fig. 8b, PAO exhibits the largest wear volume. And four ILs still exhibit lower values.
What’s more, DMMP based ILs show better antiwear behavior than LB104 due to their lower
wear volumes.
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5

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Fig. 8 (a) Evolution of friction coefficient with time at 100 N for different oil samples at 100℃; (b)
Wear volumes of steel discs lubricated by different oil samples at 100℃.
Fig. 9 shows the SEM morphologies of worn surfaces lubricated by different oil samples and
elements distribution map corresponding to worn surfaces lubricated by DMMP based ILs at
1
00℃. Regarding to the wear scar lubricated by PAO, the surface shows severe plastic
deformation with adhesive wear. Combining the friction coefficient curve in Fig. 8a, this situation
implies lubricating failure in sliding pairs. In the wear scar lubricated by LB104, corrosion sites
-
are observed, which might be caused by decomposition products from BF
4
anion. Many deep
scratches are also observed on the surface. In the wear scars lubricated by DMMP based ILs, the
worn surfaces are very smooth with only shadow scratches and no corrosion phenomenon is
observed. In the element distribution maps, S is apparently rich in wear scar while other elements
are distributed uniformly on the surface. It is believed that tribochemical reactions were occurred
between DMMP based ILs and sliding pairs during friction test. Tribochemical products
containing S element was produced and spread out on the worn surface, which helps to reduce
friction and wear.
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6

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Fig. 9 SEM and elements distribution maps of worn surfaces lubricated by different oil samples at
1
00℃: (a) PAO; (b) LB104; (c) P8; (d) P10; (e) P14.
To get further insight into the composition of the lubricating film, XPS spectra of worn
surfaces lubricated by three DMMP based ILs at high temperature are collected and shown in Fig.
1
0. Three DMMP based ILs show similar spectra, which demonstrates that similar tribochemical
reactions with sliding pairs have occurred. After dealing with the peaks of characteristic elements,
results indicate that the same products were generated on the worn surfaces for three DMMP
2 3
based ILs. The binding energy of O 1s peaks at 529.9 eV and 531.5 eV are belong to Fe O and
C-O bonds respectively [14, 29, 30]. The signal of C-O bonds may be detected from organic
compounds, which may be the products generated from ILs’ oxidation during friction test [14].
There are no noticeable peaks in the spectra of P 2p indicating no tribochemical products
containing P formed on the worn surface. This is coincident with the results of element
distribution map in Fig. 9. The peaks of S 2p could be separated to two peaks at 160.9 eV and
1
62.1 eV, which match closely to S 2p3/2 and S 2p1/2 of FeS [31]. Therefore, FeS is the main
product containing S element generated from tribochemical reactions during friction test.
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7

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Fig. 10 XPS spectra of Fe 2p, O 1s, P 2p and S 2p on the worn surface lubricated by different
DMMP based ILs at 100℃: P8 (top); P10 (middle); P14 (bottom).
3
.6 Mechanism discussion
As discussed above, DMMP based ILs could effectively protect mild steel from the corrosion
2 4
of 0.5 M H SO aqueous solution, even at an ultralow concentration of 0.1 mM. This kind of ILs
can be used as high-efficient corrosion inhibitor in corrosive acidic systems. According to the data
extracted from the Langmuir adsorption isotherm, the ILs inhibited corrosion of mild steel by
monolayer adsorption on the mild steel surface and the adsorption type involved physisorption and
2 4
chemisorption. According to the references, in the corrosive acidic solutions (HCl or H SO ),
2
+
corrosion inhibitors could form complex compounds with Fe generated from the dissolution of
steel during the corrosion test [32, 33]. The Fe(Ⅱ)-inhibitor complex compounds adsorbed on the
+
surface, protecting the electrode from the corrosive attack by H [22, 32]. Regarding the DMMP
based ILs, the anionic moiety containing S atom and pyrimidine ring have strong coordination
2
+
ability with metal ions, for example, Fe in this work. The cationic moiety of ILs could also
adsorb on the steel surface through electrostatic attraction. Consequently, Fe(Ⅱ)-IL complex
compounds could be formed at the working electrode/solution interface and effectively retarded
2 4
the corrosion of mild steel by H SO solution.
When used as lubricants, DMMP based ILs showed lower friction coefficient and less wear
volumes compared with PAO and LB104 at 30℃ and 100℃. It is known that tribo-stress could
induce positive charge on the steel surface during the friction process [34], where the anionic
moiety of ILs, DMMP ions, could be adsorbed on the surface. The counter cationic moiety of ILs,
tributyl(alkyl)phosphonium ions, could also be adsorbed through electrostatic attraction.
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However, without precise experimental evidence, it is very difficult to accurately
prove the existence of an adsorption layer between sliding pairs. As proved previous,
DMMP based ILs have strong absorbability with steel. Thus, it can be deduced that
they might also adsorb on steel surface when used as lubricants. At 30℃, tribochemical
reactions are rarely observed due to the scarcity of thermomechanical excitations during friction
process [35]. DMMP based ILs have high viscosities as is shown in Table 1. The lubrication
regime maybe the mixed lubrication in Stribeck curve. For oil samples tested in this work, under
the same conditions, DMMP based ILs with higher viscosities exhibited better tribological
performances. Accordingly, high viscosities, little tribochemical products and adsorbed ILs are
responsible for the excellent tribological performances at 30℃. Regarding high temperature 100℃,
some tribochemical products such as FeS, Fe O and organic compounds containing C-O bonds
2
3
generated through the reactions induced by the harsh sliding conditions. This boundary tribofilm
drastically reduced the shear strength during sliding process. It can be concluded that the
formation of tribofilm on the worn surface is believed to be responsible for the excellent
friction-reducing and antiwear performances of DMMP based ILs at 100℃.
In conclusion, DMMP based ILs have strong adsorption capacity on steel substrate, which
induced the formation of adsorption film on the surface. The adsorption film could protect steel
surface from the corrosion of acidic solutions and providing lubrication protection. Furthermore,
tribochemical products further offered the excellent lubrication performances at harsh tribological
conditions. Thus, the superior adsorption capacity on steel surface gives DMMP based ILs great
potentials to be used as corrosion inhibitors and lubricants.
4
. Conclusion
A series of new DMMP based ILs were firstly synthesized. This family of ILs has high
thermal stability and wide liquid range, indicating their potential applications in many scientific
and technical fields. When being used as corrosion inhibitor in 0.5 M H SO aqueous solutions,
2
4
DMMP based ILs exhibited predominant corrosion inhibiting effect for mild steel, even at an
ultralow concentration of 0.1 mM. The adsorption behavior of ILs obeys Langmuir adsorption
isotherm, which proves that the ILs inhibit corrosion of mild steel by monolayer adsorption on the
0
mild steel surface. The values of calculated ΔGads suggest that both chemisorption and
physisorption existed in the interactions between ILs and the steel surface. When being used as
1
9

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lubricants, DMMP based ILs showed excellent lubrication performances with lower friction
coefficient and less wear volumes than PAO and traditional IL LB104 in different conditions.
Surface characterization techniques such as SEM, EDS and XPS were utilized to probe the
composition of tribofilm. Results indicated that the tribochemical reactions preferred to occur at
high temperature. Tribochemical products such as FeS, Fe O , and organic compounds containing
2
3
C-O bonds were generated during the friction process. The tribofilm formed at 100℃ is
composed of adsorption layer and tribochemical products, which protected the frictional pairs and
reduced the friction and wear. While being conducted at 30℃, high viscosities, little tribochemical
products and adsorbed ILs are responsible for the excellent lubrication performances. In summary,
DMMP based ILs showed superior adsorption capacity with steel, which bring out two
outstanding abilities, corrosion inhibiting and lubricating properties. Therefore, this work will be
instructive for designing multifunctional ILs and promoting the ILs’ application in industry
definitely.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (51575506,
5
1505462 and 51705508).
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[
[
[
1
[
[
[
[
[
[
[
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[
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Graphical Abstract
2
2

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Highlights


Three new DMMP based phosphonium ILs were firstly synthesized.
This kind of versatile ILs has gathered two significant abilities including
corrosion inhibiting and lubricating properties.

Three ILs showed high corrosion inhibition efficiencies even at 0.1 mM for mild
steel in 0.5 M H2SO4.


The adsorption on mild steel obeys Langmuir isotherm.
Three ILs as lubricants for steel/steel contact exhibited excellent tribological
properties at 30℃ and 100℃.
2
3