Facile synthesis and investigation of 1,8-dioxooctahydroxanthene derivatives as corrosion inhibitors for mild steel in hydrochloric acid solution

Article abstract of DOI:10.1039/c5nj02707a

An environmentally friendly and facile method for the preparation of 1,8-dioxooctahydroxanthene between aldehydes with dimedone or cyclohexane-1,3-dione is presented in the presence of trichloromelamine (TCM) under solvent-free conditions. Also, for the f

Full text of DOI:10.1039/c5nj02707a

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New Journal of Chemistry
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DOI: 10.1039/C5NJ02707A
Facile Synthesis and Investigation of 1,8ꢀDioxooctahydroxanthene
Derivatives as Corrosion Inhibitors for Mild Steel in Hydrochloric
Acid Solution
Behrooz Maleki,*a Ali Davoodi,b Mojtaba Vakili Azghandi,c Mehdi Baghayeri,a Elahe
Akbarzadeh,d Hojat Veisi,e Samaneh Sedigh Ashrafi,a Massomeh Raeia
aDepartment of Chemistry, Hakim Sabzevari University, Sabzevar, 96179ꢀ76487, Iran
Eꢀmail:b.maleki@hsu.ac.ir, Tel: +98ꢀ571ꢀ4002643 Fax: +98ꢀ571ꢀ4410300
bMaterials and Metallurgical Engineering Department, Faculty of Engineering, Ferdowsi
University of Mashhad, Mashhad 91775ꢀ1111, Iran
cMaterials Science and Engineering Department, Faculty of Engineering, BuꢀAli Sina
University, Hamadan, Iran
dYoung Researchers and Elite Club, Islamshahr Branch, Islamic Azad University, Islamshahr,
Iran
eDepartment of Chemistry, Payame Noor University(PNU), 19395ꢀ4697 Tehran, Iran
Abstract
An environmentally friendly and facile method for preparation of 1,8ꢀdioxooctahydroxanthene
between aldehydes with dimedone or cyclohexaneꢀ1,3ꢀdione is presented in the presence of
trichloromelamine (TCM) under solventꢀfree condition. Also, for the first time the corrosion
inhibitive performance of some 1,8ꢀdioxooctahydroxanthene derivatives on mild steel in 0.1 M
1

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HCl was investigated by AC and DC electrochemical techniques and quantum chemical method.
Efficiency more than 97% was obtained with 1 mmol/L 1,8ꢀdioxooctahydroxanthene. Density
functional calculations showed that the lone pair electrons of oxygen in the structure of 1,8ꢀ
dioxooctahydroxanthene are suitable sites to interact with the metal. Finally in presence of 1,8ꢀ
dioxooctahydroxanthene, a decrease in surface roughness and corrosion attacks were
demonstrated by microscopic examinations, respectively.
Key words: Inhibitors, 1,8ꢀDioxooctahydroxanthene, Corrosion, Quantum chemistry, DFT,
Potentiodynamic polarization.
1. Introduction
Xanthene derivatives as heterocyclic compounds have been interested because of their various
pharmacological properties such as antibacterial, antiviral and antiꢀinflammatory activities.1ꢀ4
1,8ꢀdioxooctahydroxanthenes contain reactive pyran ring system and have applications in
medicinal chemistry5ꢀ7 and fluorescent materials.8 Interestingly, this structurally complex
molecule can be synthesized by a simple one pot tandem reaction between an aldehyde (1 equiv)
and dimedone or cyclohexaneꢀ1,3ꢀdione (2 equiv) in the presence of a catalyst.9ꢀ12 However, in
absence of a catalyst the reaction produce Knoevenagel type adducts, to give an open chain
intermediate, 2,2’ꢀaryl/alkyl methyleneꢀbis(3ꢀhydroxyꢀ5,5ꢀdimethylꢀ2ꢀcyclohexeneꢀ1ꢀone).13,14
Thus, value of a synthetic method for this important class of heterocycles depends mainly on
identifying a simple, cost effective, and ecoꢀfriendly catalyst and a simple workꢀup procedure.
Nꢀhalo reagents have the potential to promote important reactions such as halogenation,
oxidation, and protection as well as formation of CꢀX, CꢀO, and C=O bonds. Chloramines such
as trichloromelamine (TCM) are used as bleaching agents, disinfectants and bactericides, due to
their function as chlorinating agents and oxidants. It was registered by EPA as an active
2

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ingredient for food contact surface sanitizer, 3rd sink sanitizer and hard surface disinfectant. It
was first used by the US Military as a fruit and vegetable disinfectant.15ꢀ18
On the other hand, many industrial sections such as acid cleaning bath, water cooling system,
various refinery units, pipelines, chemical operations, steam generators, ballast tanks, oil and gas
production units are involved with inhibitors due to high corrosion rates in these parts.19ꢀ24
Recently attention was made to study more environmental friendly chemical compounds as green
corrosion inhibitors. The adsorption of organic molecules at the metal/solution interface is a great
interest in surface science and can remarkably change the corrosion resistance of the metal.25 It is
generally accepted that the first step in the adsorption of an organic inhibitor on a metal surface
usually involves replacement of one or more water molecules adsorbed at the metal surface.26
inhibitor +xH2O(ads) → inhibitor +xH2O(sol)
(sol)
(ads)
The inhibitor may combine with freshly generated Fe2+ ions on steel surface, forming metal
inhibitor complexes.27,28
Fe2+ +inhibitor →[Fe*inhibitor]2+
(ads)
(ads)
Most of the well known acid inhibitors are organic compounds containing nitrogen, sulfur and/or
oxygen atoms.29,30
Our studies mainly focuses on facile chemical synthesis of 1,8ꢀdioxooctahydroxanthene
derivatives. Moreover, its corrosion inhibiting as a green compound on mild steel in hydrochloric
acid solution is investigated by electrochemical techniques. The advantages of this compound as
corrosion inhibition are the less solubility and large molecule size. Less soluble molecules have a
greater tendency to be adsorbed and have more stability than soluble molecules. Because 3,3,6,6ꢀ
tetramethylꢀ9ꢀ)4ꢀnitrophenyl(ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione is a less soluble
3

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molecule, so has greater stability and better inhibitive behavior than completely soluble
molecule. In general, the molecules having the larger area create the better the inhibitive
behavior. However, with very large molecules (such as 3,3,6,6ꢀtetramethylꢀ9ꢀ)4ꢀnitrophenyl(ꢀ
3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione), there may be steric hindrance problems when a
large molecule attempts to fit onto a surface already partially occupied with previously adsorbed
molecules.
2. Experimental
2.1. Materials
Chemicals were obtained from Merck, SigmaꢀAldrich and Fluka. IR spectra were recorded on a
Shimadzu 435ꢀUꢀ04 spectrophotometer (KBr pellets). 1H NMR spectra were obtained using Jeol
FT NMR 90 MHz and spectrometer in CDCl3 using TMS as an internal reference. Melting points
were determined in open capillary tubes in a Stuart BI Branstead Electrothermal Cat No:IA9200
apparatus and uncorrected. Trichloromelamine (TCM) was commercially available and used as
received. Electrochemical behavior was investigated by IVIUM Potentiostat. Corrosion attack
morphology investigations were conducted by OLYMPUS optical microscopy. Quantum
chemical chemical studies were performed by Gaussian 98 software.
2.2. General Procedure for Synthesis of 1,8ꢀDioxooctahydroxanthene
To a mixture of aldehydes (1 mmol) and cyclic 1,3ꢀdicarbonyl compounds (2 mmol),
trichloromelamine (TCM, 20 mol%) was added and the mixture was heated in an oil bath at
110˚C with good stirring for the appropriate time as indicated in Table 2. The progress of the
reaction was monitored by TLC (nꢀhexane:ethyl acetate, 8:2). After completion of the reaction,
hot EtOH (1 mL, 96%) was added and the mixture stirred for 5 min. Then, the catalyst (TCM)
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separated by filtration. The residue was washed with iceꢀwater. A solid precipitated which
collected by filtration and washed with H2O and then dried to give the corresponding 1,8ꢀ
dioxooctahydroxanthenes (3aꢀo). Finally, the obtained product was recrystallized from ethanol.
2.2.1. Spectral Data for 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione (3e): Yellow solid; mp: 224ꢀ226 ºC (lit. 222ꢀ223 ºC); IR (KBr, cmꢀ1): 3035,
2980, 1685, 1670, 1620, 1533, 1360, 1213, 1173, 1010, 860, 780, 745, 703; 1H NMR (400 MHz,
CDCl3): δ 0.998 (s, 6H), 1.12 (s, 6H), 2.15ꢀ2.24 (m, 4H), 2.54 (s, 4H), 4.83 (s, 1H), 7.47 (d, 2H),
8.11 (d, 2H); 13C NMR (100 MHz, CDCl3): 196.26, 162.93, 151.49, 146.48, 129.35, 123.44,
114.54, 50.59, 40.84, 32.35, 32.23, 29.25, 27.27.
2.3. Specimen and Corrosive Media
The specimen (working electrode) was selected from a mild steel rod with following chemical
composition (in wt%): 0.17% C, 0.3% Si, 0.4% Mn and Fe balance. The sample was then
mounted in a selfꢀcure epoxy resin resulting in 0.785 cm2 exposed area. Before each test, the
specimen was ground down to 1200 grit emery paper, then washed with ethanol and dried with
warm air. The electrolyte solution in all experiments was 0.1 M HCl prepared by Merck reagent
and deionised water. Also, the inhibited solutions were prepared with direct adding of various
concentrations of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀ
dione which was in emulsion form in 0.1 M HCl. To control the cell temperature at constant
condition while running the experiments, a water bath with accuracy within ±1 ºC was
employed.
2.4. Electrochemical Measurements
The electrochemical measurements were carried out in a conventional threeꢀelectrode cell in
which the working electrode was the described mild steel sample and the saturated calomel
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electrode (SCE) and a platinum wire were as reference and counter electrode, respectively. An
IVIUM Potentiostat was employed to perform all electrochemical measurements. In order to
reach a steady state, the working electrode was immersed in the prepared solution which was
open to air in stagnant condition for 30 min. To measure linear polarization resistance (LPR), the
potential of the electrode was perturbed from −20 to +20 mV around corrosion potential with 0.5
mV/s scan rate. Also, potentiodynamic polarization was carried out with a constant sweep rate of
1 mV/s in the range of –300 to +300 mV with respect to corrosion potential. The impedance
measurement was performed using AC signals with 10 mV amplitude in the frequency range
from 100 KHz to 0.1 Hz at corrosion potential. EIS Analyzer software was used to fit the
experimental results of EIS measurements using appropriate equivalent circuit.
2.5. Corrosion Attack Morphology Investigation
Optical microscopy was accomplished in order to investigate corrosion attack morphology in
absence and presence of inhibitor. The surface of specimens was mechanically polished with
down to 0.05 µm alumina slurry until a mirrorꢀlike surface was acquired. Subsequently, they
were immersed in 0.1 M HCl in absence and presence of 0.8 ppm (mmol/L) inhibitor for 150
min at room temperature. After bringing out of each specimen, it was washed and cleaned by
ethanol and immediately dried at warm air.
2.6. Quantum chemical study
In order to investigate the effect of molecular structures of monomers, quantum chemical studies
were carried out. The optimized structure and the highest occupied molecular orbital (HOMO) of
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three monomers were geometrically calculated by DFT method using B3LYP level and 3–21G**
basis set with Gaussian 98 software.
3. Result and discussion
3.1. Chemistry and Synthesis
In recent years, trichloromelamine (TCM) has been used as an effective catalyst in organic
synthesis due to its safety, lowꢀprice, easy availability and handling with a considerable low
amounts. According to our recent studies on the development of efficient and environmentally
benign procedures using reagents and catalysts,31ꢀ45 it has been decided to examine the possibility
of synthesizing 1,8ꢀdioxooctahydroxanthenes derivatives via various aldehydes, dimedone or
cyclohexaneꢀ1,3ꢀdione using trichloromelamine (TCM) as a highly efficient reagent under
solventꢀfree conditions (Scheme 1).
R
O
CHO
O
O
TCM (10 mol%)
2
R1
+
R1
R1
Solvent-Free, 1100C
R1
O
O
R
R1
R1
R1 = H, CH3
1a-o
2
3a-o
NHCl
N
N
TCM =
ClHN
NHCl
N
Scheme 1. Facile method for preparation of 1,8ꢀdioxooctahydroxanthenes.
Initially, in the presence of a catalytic amount of TCM (20 mol%) under solventꢀfree
conditions at 110°C for 30 minute was produced 3,3,6,6ꢀtetramethylꢀ9ꢀ(phenyl)ꢀ3,4,5,6,7,9ꢀ
hexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione (3a) in the twoꢀcomponent reaction of benzaldehyde (1a),
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dimedone (2) and a catalytic amounts of TCM (20 mol%) under solventꢀfree conditions at
110°C, 3,3,6,6ꢀtetramethylꢀ9ꢀ(phenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione (3a) was
produced in 30 minutes ( Table 1, entry 1). The effect of temperature on the rate of the reaction
was studied (entries 2ꢀ3). Decreasing in in temperature leads to decreasing product yields (entry
2). Next, the optimum amount of TCM was evaluated (entries 4ꢀ5). The highest yield was
obtained with 20 mol% of the catalyst at 110 °C (entry 1). It was observed that the reaction did
not proceed at 110 °C in absence of TCM (entry 6).
Table 1. Optimizing the Reaction Conditions
O
CHO
O
O
TCM
2
+
Me
Me
Me
Me
Solvent-Free
O
Me
O
Me
1a
2
3a
Entry
Catalyst (mol %)
Temperature (0C)
Time (min)
Yield (%)a
1
2
3
4
5
6
20
20
20
30
10
ꢀ
110
100
120
110
110
110
30
30
30
30
30
30
82
76
80
78
74
ꢀb
aIsolated yields. bNo reaction.
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Based on the optimized reaction conditions, the reactions of dimedone or cyclohexaneꢀ1,3ꢀ
dione, and various aromatic aldehydes were investigated as well. As shown in Table 2, aromatic
aldehydes carrying either electronꢀwithdrawing or electronꢀdonating substituents afforded good
yields of products with high purity at 110 oC under solventꢀfree conditions.
Table 2. Preparation of 1,8ꢀDioxooctahydroxanthene using TCM (20 mol%)
Product
(3)
R
R1
Time
(min)
Yield (%)a
Mp (°C)
Found
Reported
C6H5
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
CH3
H
30
30
40
30
20
20
50
30
30
40
30
40
82
84
80
80
86
88
80
84
79
80
82
78
203ꢀ205
233ꢀ235
225ꢀ227
251ꢀ253
224ꢀ226
168ꢀ170
218ꢀ219
237ꢀ239
226ꢀ227
240ꢀ242
263ꢀ265
272ꢀ273
203ꢀ204
230ꢀ232
225ꢀ227
252ꢀ254
222ꢀ223
170ꢀ172
217ꢀ218
239ꢀ238
226ꢀ227
245ꢀ247
263ꢀ265
270ꢀ271
a
b
c
d
e
f
4ꢀClC6H4
2ꢀClC6H4
2ꢀNO2C6H4
4ꢀNO2C6H4
3ꢀNO2C6H4
4ꢀCH3C6H4
4ꢀBrC6H4
4ꢀFC6H4
g
h
i
2ꢀNO2C6H4
4ꢀNO2C6H4
C6H5
j
H
k
l
H
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4ꢀClC6H4
4ꢀCH3C6H4
4ꢀBrC6H4
H
H
H
30
60
30
76
74
80
289ꢀ291
262ꢀ263
229ꢀ231
288ꢀ290
262ꢀ263
228ꢀ231
m
n
o
aIsolated yields.
The formation of 1,8ꢀdioxooctahydroxanthene derivatives proposed as follow (Scheme 2):
NHCl
N
NH2
In-Situ
N
N
N
Cl+
+
ClHN
H2N
NHCl
NH2
N
N
Trichloromelamine
Melamine
Cl
R
O
O
Cl+
Cl
+
R1
R1
R1
R1
O
O
O
H
H
O
O
Cl+
OCl
H
H
R
R
R1
OCl
R1
R
H
R
R
O
O
O
H
-HOCl
OCl
OCl
H
OCl
R1
O
R1
R1
O
OCl
R1
R1
R1
Cl
O
Cl+
R1
R1
R1
O
O
R1
R
R
R
O
O
O
O
O
O
H
-HOCl
R1
R1
R1
R1
R1
R1
R1
R1
O
3a-o
O
R1
R1
R1
OCl
R1
OH
OH
Cl
Scheme 2. Proposed mechanism.
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3.2. Investigation of 1,8ꢀDioxooctahydroxanthene Derivatives as Green Corrosion
Inhibitors
3.2.1. Potentiodynamic Polarization
The effect of the concentration of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ
2Hꢀxantheneꢀ1,8ꢀdione on the mild steel corrosion rate in 0.1 M HC1 solution at 25 °C has been
studied by potentiodynamic polarization. The polarization curves have been depicted in Fig. 1
and also the electrochemical parameters including corrosion potential (Ecorr), corrosion current
density (icorr), cathodic and anodic Tafel slopes (βa and βc) obtained by the extrapolation of Tafel
slopes have been presented in Table 3.
Figure. 1. Potentiodynamic polarisation curves of inhibitor in different concentrations at 25°C.
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Table 3. Potentiodynamic polarization parameters of mild steel in 0.1 M HCl solution in absence and presence of
various concentration of inhibitor.
Ecorr
icorr
Concentration
(ppm)
βa
βc
Ƞ
θ
ꢀ
(mA/cm2)
(mV/decade)
(mV/decade)
(%)
(mV vs. SCE)
Blank
0.001
0.01
0.1
ꢀ562
ꢀ561
ꢀ555
ꢀ538
ꢀ509
1.24
0.94
0.53
0.09
0.05
420
335
288
120
80
392
377
316
186
157
ꢀ
0.24 24.2
0.57 56.9
0.93 92.9
0.96 96.4
1
Also, the values of surface coverage (θ) and inhibition efficiency have been calculated using
the following equations.46
ic0orr − icorr
θ
=
(1)
(2)
ic0orr
η
=
θ
×100
Where (i°corr) and (icorr) represents the corrosion current densities in absence and presence of
inhibitor, respectively.
Figure 1 indicates that 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione is capable to significantly decrease both anodic and cathodic current
densities. Table 3 shows that the inhibitor concentration has a positive effect on inhibitive
behaviour of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀ
dione so that the efficiency values considerably increase with increasing in 3,3,6,6ꢀtetramethylꢀ9ꢀ
(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione concentration (i.e. 24.2% in 0.001
mmol/L to 96.4% in 1 mmol/L). It is believed that 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ
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3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione molecules adsorb onto metal surface and
consequently slow down metal dissolution and hydrogen evolution by blocking many available
active sites. Although, the presence of defects on organic layer created by inhibitor adsorption
cannot be ruled out, therefore hydrogen evolution and metal dissolution may occur from these
regions where there is no inhibitor coverage (i.e. 1ꢀ θ).20 In addition, The polarization curves
shown in Figure 1 reveal a fact that, in the case of lower concentration (<0.01 ppm), the inhibitor
decreased the cathodic current density more strongly than the anodic current density that in lower
concentrations of inhibitor, 3,3,6,6ꢀtetramethylꢀ9ꢀ)4ꢀnitrophenyl(ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione has a tendency to more block the cathodic sites which act as suitable sites for
hydrogen evolution. In contrast, in the case of higher concentration (>0.01 ppm), the inhibitor
decreased the anodic current density more strongly than the cathodic current density that in
longer concentrations of inhibitor the cothodic site has covered by inhibitor molecules and
3,3,6,6ꢀtetramethylꢀ9ꢀ)4ꢀnitrophenyl(ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione
has
a
tendency to more block the anodic sites which act as suitable sites for anodic dissolution.47ꢀ49
Also, Table 3 reveals that in presence of inhibitor, the values of anodic and cathodic Tafel
slopes (βa and βc) have been significantly diminished and also the corrosion potential slightly
shifted toward more positive values. The variation of Tafel slopes refers to change in reaction
kinetics. This can be explained that with anodic and cathodic polarizing of the sample, the
reaction kinetics speed up in inhibited solution in comparison with uninhibited one. This is a
negative effect of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀ
dione on corrosion rate of mild steel. However, the considerable decrease in active area is the
main reason for diminishing net corrosion current density in inhibited solution.50 Considering
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that 1,8ꢀdioxooctahydroxanthene decreases both anodic and cathodic current densities while it
has no effect on corrosion potential, this compound can be classified as mixedꢀtype inhibitors.
3.2.2. Electrochemical Impedance Spectroscopy (EIS)
Electrochemical impedance spectroscopy (EIS) is a valuable method which has been widely
used to study the inhibitive behavior of many compounds.20 It provides information on both the
resistive and capacitive behavior at interface and makes possible to assess the performance of the
tested compounds as inhibitors for metals.51 In this part, the effect of inhibitor concentration on
inhibitive behavior of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ
1,8ꢀdione on mild steel in 0.1 M HCl has been investigated using EIS measurement after 1 hr
immersion time at 25 °C. Figure 2 depicts Nyquist plots in absence and presence of different
inhibitor concentrations.
Figure. 2. Nyquist plots in absence and presence of different inhibitor concentrations at 25 °C.
Inserted image shows the equivalent circiut model.
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As seen, Nyquist plots consist of a depressed semicircle in inhibited and uninhibited
solutions. This capacitive loop is attributed to the doubleꢀlayer capacitance in parallel with the
charge transfer resistance. In addition, Figure 2 shows that a rise in 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀ
nitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione concentration results in increasing
the semicircle diameter, indicating the inhibition of the corrosion process.52ꢀ54
The impedance parameters have been obtained by EIS analyser software using appropriate
equivalent circuit (see inserted image in Fig. 2) for metalꢀelectrolyte interface and presented in
Table 4. Rs, Rct, and CPE represent solution resistance, charge transfer resistance and constant
phase element, respectively. A small inductive or quasi inductive loop at very low frequency was
observed. Several reasons have been proposed for this observation such proton
adsorption/desorption process, etc. Here this has been neglected since its contribution to the total
impedance was negligible.
Table 4. EIS parameters of mild steel electrode in 0.1 M HCl solution in absence and presence of various
concentrations of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione at 25 °C.
Concentration
(ppm)
EIS
LPR
̘
Rct
P
Cdl
Ƞ%
Rp
Ƞ
n
̳
(Ω.cm2) (µF.cmꢀ2)
(µF.cmꢀ2)
(Ω.cm2)
(%)
(Ω.cm2)
13.8
14.1
16
19.6
30.1
73.3
217.9
355
540
430
8.2
6.3
6.1
0.86
0.88
0.89
0.89
0.89
257
238
41
ꢀ
41.24
52.41
104
ꢀ
Blank
0.001
0.01
0.1
35
73
91
95
21
60
84
90
20
35
270
1
15.7
37
417
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Also, the percentage of inhibitor efficiency
resistance by means of following formula:49
η
has been calculated by the charge transfer
R − R0
%η
=
×100
(3)
R
Where, R° and R are the charge transfer resistance of mild steel in absence and presence of
inhibitor, respectively. In addition, the values of doubleꢀlayer capacitance have been calculated
by Eq. 4:47
1
1−n
n
n
Cdl = P Rct
(4)
In above expressions, P, n and Rct are the constant phase element (CPE) parameter, CPE
exponent and charge transfer resistance of mild steel, respectively. As known, the CPE element
is a nonꢀideal capacitance which is generally used to simulate experimental data.20,29 Deviations
from the ideal semiꢀcircle can partly be due to the surface inhomogeneities and mass transport
processes. Thus, in these situations, pure double layer capacitors are better described by a
transfer function with CPE.21,55,56
Evaluation of Table 4 revealed that with increasing in 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ
3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione concentration, the values of Rct increase while the
double layer capacitance values decrease. An increase in Rct refers to more block of the active
area at the metal surface as a result of increasing inhibitor concentration. Also, the decrease in
doubleꢀlayer capacitance indicates that in presence of various inhibitor concentrations, the
double layer reinforced onto the surface. Since, the doubleꢀlayer between the charged metal
surface and the solution is considered as an electrical capacitor. In addition, the values of n show
an increase with increasing inhibitor concentration. This can be attributed to the decrease in
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surface roughness and the slightly improvement of surface homogeneity due to inhibitor
adsorption on the most active sites.20,29,57
The
3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione
adsorption on the iron surface decreases its electrical capacity due to displacing the water
molecules and other ions which are originally adsorbed on the surface with inhibitors molecules.
According to Helmholtz model, double layer capacitance is related to structural parameters as
follows:57
ε ε
d
0
Cdl =
S
(5)
Where d is the thickness of the film, S is the surface area of the electrode, ε° is the
permittivity of the air and ε is the local dielectric constant. The decrease in Cdl is probably due to
a decrease in local dielectric constant and/or an increase in the thickness of a adsorb layer at
electrode surface in 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ
1,8ꢀdione.58 Although, by increasing in inhibitor concentration, a decrease in the surface area
which act as a place for surface charging may also be as other reason for Cdl decreasing.59
It should be mentioned that the efficiency calculated by EIS results in absence and presence
of different concentration of investigated 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀ
hexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione are in close correlation with those of polarization in. In
addition, EIS measurement confirms that this compound has a good inhibitive behaviour for mild
steel in 0.1 M HCl media.
3.2.3. Linear Polarization Resistance (LPR)
The effect of inhibitors concentration on corrosion behavior of mild steel in 0.1 M HCl solution
at 25 °C has been studied by LPR method which is a useful and fast method. The LPR values
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and also efficiency calculated by Equation 3 have been presented in Table 4. LPR results confirm
the inhibitive properties of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione, also conclusions obtained from potentiodynamic polarization and EIS
results.
3.2.4. Thermodynamic Calculations of Inhibitor Adsorption
The primary step in the action of inhibitors in acid solutions is adsorption onto the metal surface,
which is usually an oxideꢀfree surface. It has been reported that the inhibition mechanism of an
inhibitor may vary with factors such as concentration, pH, nature of the anion of the acid, and
nature of the metal.60 Thus, basic information on the interaction between the inhibitor and the
mild steel surface can be provided by the adsorption isotherm. In order to obtain the
thermodynamic parameters, the relation between surface coverage (θ) values and inhibitor
concentration (C) must be considered. So far, several attempts were made to find the relation
between θ and C such as Langmuir, Temkin, Frumkin and Flory–Huggins.61,62 Langmuir
adsorption isotherm was generally found to be the appreciable description of the adsorption
behavior of the inhibitor. According to Langmuir isotherm, the relation between θ values and
inhibitor concentration, C, is considered as follows.63
C
1
=
+C
(6)
θ
Kads
Where Kads is the equilibrium constant of the adsorption process and C is the inhibitor
concentration. Figure 3 shows C/θ variations as a function of C. The obtained plot of the
inhibitor is perfectly linear with a correlation coefficient higher than 0.999 and it shows that the
3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione
obeys
Langmuir isotherm (Fig. 3). Since we have used the percentage of surface coverage values, the
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slope is 0.01 which in turn gives unit value by multiplying at 100 as unit slope for Langmuir
isotherm in Eq. 6.
Figure. 3. Langmuir adsorption isotherm of inhibitor in 0.1 M HCl at 25 °C.
The equilibrium constant of adsorption Kads is related to the standard free energy of
adsorption (∆G0ads) with the following equation.20
ꢀGa0ds = ꢀRTln(55.5Kads )
(7)
In this expression, R is gas constant, T is absolute temperature and 55.5 is concentration of
water in solution in mol/l. ∆G0ads value corresponding to 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ
3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione has been calculated (∆G0ads= ꢀ38.74 kJ/mol). The
negative sign of ∆G0ads refers to the adsorption of inhibitors onto the metal surface.64 In general,
the magnitude of ∆G0 approximately ꢀ20 kJ/mol or less negative is assumed for existing
ads
electrostatic interactions between inhibitor and the charged metal surface (i.e., physisorption).
Those ∆G0ads around ꢀ40 kJ/mol or more negative are an indication of charge sharing or charge
transferring from an organic species to the metal surface to form a coordinate type of metallic
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bond (i.e., chemisorption).25 Considering above notifications and the ∆G0 value of 3,3,6,6ꢀ
ads
tetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione (∆G0
=
ꢀ38.74
ads
kJ/mol), it can be claimed that 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione has a nearly chemical adsorption onto mild steel surface.
2.2.5. Corrosion Attack Morphology
The corrosion attack morphology of the mild steel samples immersed in inhibited and
uninhibited (blank) solutions for 150 min, has been studied by optical microscopy and the
metallographic result have been presented in Fig. 4. As seen from Fig. 3, the polished surface of
mild steel immersed in blank solution has been rough and severely corroded so that several pits
are dispersedly observed at the surface (Fig. 4 (a)). While in the case of inhibited solution, a less
corrosion attack is detected and the specimen surface is nearly unchanged. In this micrograph
(Fig. 4 (b)), pearlite phase has been partially etched and the grain boundaries of ferrite phase is
not observed.
A
B
Figure. 4. Corrosion attack morphology obtained by optical microscope; A) blank solution and
B) in presence of 1 mmol/L 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione.
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2.2.6. Effect of Molecular Structure
First estimation for the ability of inhibitive behavior of a compound is to act as an electron
donor. The adsorption of the inhibitor on the metal surface can be considered on the basis of
donorꢀacceptor interaction between inhibitor molecules and the metal surface. Organic
compounds are protonated in acidic solutions but, as the one molecule approaches the metal
surface, the molecule loses its proton so that the lone pair of electrons on the heteroatoms can
interact with the metal surface.65,66 In order to determine this effect, quantum chemical study
method was employed and the results were presented in Fig. 5.
A
B
Figure. 5. A) Molecular structure and highest occupied molecular orbital (HOMO), B)
Molecular structure and lowest unoccuped molecular orbital
HOMO and LUMO of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione optimized by DFT method using B3LYP level and 3–21G** basis set with
21

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Gaussian 98 reveals that oxygen atoms existing in Xanthene have a large electron density. Lone
pair electrons of oxygen can be suitable sites to adsorb onto metal surface. It is reported that
these electrons are an agent for charge sharing or charge transferring, resulting in
chemisorptions. In addition, surface chelates are formed involving bonding between a surface
cation and heteroatom atom in the chelating molecule (i.e. chelates between heteroatom such as
oxygen, sulphur, nitrogen and the metal cation such as Fe+2).61,67
A second factor in determining the effectiveness of a chemisorbed organic the inhibitor is the
molecular size. In general, the molecules having the larger area create the better inhibitive
behavior. However, with very large molecules (such as 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ
3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione), there may be steric hindrance problems when a
large molecule attempts to fit onto a surface already partially occupied with previously adsorbed
molecules. Thus, complete coverage of the surface by large molecules may sometimes be
difficult to achieve.68 Competitive adsorption, in inhibited solution, is assumed to occur at low
carbon steel surface between the aggressive Cl¯ ions and the inhibitor molecules in this study. As
known, the predominant reason for corrosion process of mild steel in HCl solution is due to the
adsorption of Cl¯ ions onto its bare metallic surface sites. The mobility of Cl¯ is higher than
3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione, therefore
the 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione cannot
compete with Cl¯ ions, but 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀ
xantheneꢀ1,8ꢀdione adsorption can prevent Cl¯ ions to reach the steel surface sites by steric
effect. Also, in relation to corrosion inhibition, when an inhibited solution contains adsorbable
anions, such as halide ions, these adsorb on the metal surface by creating oriented dipoles and
consequently increase the adsorption of the organic cations on the dipoles. In these cases, a
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positive synergistic effect arises; therefore, the degree of inhibition in the presence of both
adsorbable anions and inhibitor cations is higher than the sum of the individual effects.69
Therefore, this process could explain the higher inhibition efficiency of various organic
inhibitors in hydrochloric acid solutions compared to sulfuric acid solutions. In this work, in
order to examine this phenomenon, several electrochemical tests including potentiodynamic
polarization and EIS measurement were carried out in H2SO4 media (the results were not shown
here). It was observed that the inhibitor efficiency of 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ
3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione unexceptionally decreased and approached to 40%
values.
The third factor affecting chemisorption, and thus inhibition, is the solubility of the organic
soluble molecule. Less soluble molecules have a greater tendency to be adsorbed and has more
stability than soluble molecules.57,58 Because 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀnitrophenyl)ꢀ3,4,5,6,7,9ꢀ
hexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione is a less soluble molecule, so has greater stability and better
inhibitive behavior than completely soluble molecule.
4. Conclusion
In conclusion, we have successfully developed a practical, operationally simple, economical, and
environmentally friendly procedure for the synthesis and Investigation of 1,8ꢀ
dioxooctahydroxanthene derivatives as corrosion inhibitors. Moreover, the final results of this
research including: Electrochemical measurements showed that the 3,3,6,6ꢀtetramethylꢀ9ꢀ(4ꢀ
nitrophenyl)ꢀ3,4,5,6,7,9ꢀhexahydroꢀ2Hꢀxantheneꢀ1,8ꢀdione increases charge transfer resistance
of double layer on the surface and functions as a mixedꢀtype inhibitor. Thermodynamic data
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extracted by Longmuir isotherm is showing the chemisorption adsorption and adsorption process
happen by lone pair electrons of oxygen in xanthenes.
Acknowledgements
Partial financial support from the Research Council of Hakim Sabzevari University is greatly
appreciated.
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New Journal of Chemistry
View Article Online
DOI: 10.1039/C5NJ02707A
Graphical Abstract
Facile Synthesis and Investigation of 1,8-Dioxooctahydroxanthene Derivatives as
Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Solution
Behrooz Maleki, Ali Davoodi, Mojtaba Vakili Azghandi, Mehdi Baghayeri, Elahe
Akbarzadeh, Hojat Veisi, Samaneh Sedigh Ashrafi, Massomeh Raei
NO2
O
O
Me
Me
Me
Me
O
For the first time, the corrosion inhibitive performance some of 1,8-dioxooctahydroxanthene
derivatives is proposed.