Abstract. Steel is an important material and has been widely used in today's industrial production. Using organic corrosion inhibitors is an effective means to prevent steel from corrosion. Generally, the molecular structures of inhibitors can have a major impact on their corrosion inhibition efficiency. In this work, the influence of alkyl chain length of three alkyltriazoles on the corrosion inhibition of iron has been investigated by density functional based tight binding (DFTB) approach. Several typical descriptors such as frontier molecular orbital, adsorption energy, density of states have been discussed in detail. Our findings will contribute to the understanding of the inhibition mechanism and the designing of novel corrosion inhibitors.

1. Influence of the alkyl chain length of alkyltriazoles on the corrosion inhibition of iron: A
DFTB study
Lei Guo, Min Wu, Savaş Kaya, Meihang Chen, and Loutfy H. Madkour
Citation: AIP Conference Proceedings 1995, 020015 (2018); doi: 10.1063/1.5048746
View online: https://doi.org/10.1063/1.5048746
View Table of Contents: http://aip.scitation.org/toc/apc/1995/1
Published by the American Institute of Physics

2. Influence of the Alkyl Chain Length of Alkyltriazoles on the
Corrosion Inhibition of Iron: A DFTB Study
Lei Guo1, a)
, Min Wu1, b)
, Savaş Kaya2, c)
, Meihang Chen1, d)
and
Loutfy H. Madkour3, e)
1
School of Material and Chemical Engieering, Tongren University, Tongren 554300, China.
2
Department of Chemistry, Faculty of Science, Cumhuriyet University, Sivas 58140, Turkey.
3
Chemistry Department, Faculty of Science and Arts, Baljarashi, Al Baha University, Al Baha 65635, Saudi Arabia.
a)
Corresponding author: cqglei@163.com
b)
crfyibin@163.com
c)
savaskaya@cumhuriyet.edu.tr
d)
chenmeihang0123@126.com
e)
loutfy_madkour@yahoo.com
Abstract. Steel is an important material and has been widely used in today's industrial production. Using organic
corrosion inhibitors is an effective means to prevent steel from corrosion. Generally, the molecular structures of
inhibitors can have a major impact on their corrosion inhibition efficiency. In this work, the influence of alkyl chain
length of three alkyltriazoles on the corrosion inhibition of iron has been investigated by density functional based tight
binding (DFTB) approach. Several typical descriptors such as frontier molecular orbital, adsorption energy, density of
states have been discussed in detail. Our findings will contribute to the understanding of the inhibition mechanism and
the designing of novel corrosion inhibitors.
Key words: Steel; Corrosion; Adsorption; Density functional based tight binding (DFTB).
INTRODUCTION
Steel is the most common industrial material, which has been utilized in a wide variety of applications such as
construction, pipe lines and petroleum industries. However, the use of this invaluable material can be limited in
many industries due to corrosion attack [1, 2]. The use of inhibitors is the most convenient and economic method for
corrosion protection of mild steel [3]. According to the literatures, organic compounds containing N, O or S groups
or having π bonds in their structures are found to be effective inhibitors in most corrosive media due to their
chelating action and the formation of an insoluble physical diffusion barrier on the substrate surface [4]. Besides,
compounds with high molecular mass have also proved to be effective inhibitors. The existing data reveal that most
organic inhibitors act by adsorption on the metal surface [5, 6]. This adsorption is influenced by the nature of
substrate surface, the type of aggressive electrolyte, as well as the molecular structures of inhibitors. Recently,
Zhang et al.[7] have synthesized a series of benzotriazole derivatives with various carbon chains (n =1,3,4,6,7,
9,10,12,16), and they found that the derivative can have best inhibition performance for copper corrosion in 3.5 wt%
NaCl solution when n =7.
In recent years, different kinds of experimental approaches (such as weight loss, electrochemical, XPS, AFM,
SEM methods) were adopted to assess the inhibition efficiency of many inhibitors. However, they are usually time
consuming, expensive, and flawed in understanding the anti-corrosion mechanism at the molecular and even atomic
level. Fortunately, molecular simulation techniques such as density functional theory (DFT) calculation, molecular
2018 International Symposium on Mechanics, Structures and Materials Science (MSMS 2018)
AIP Conf. Proc. 1995, 020015-1–020015-5; https://doi.org/10.1063/1.5048746
Published by AIP Publishing. 978-0-7354-1711-3/$30.00
020015-1

3. dynamic simulation, and Monte Carlo simulation methods were often considered as fast and effective tools to assist
in the interpretations of experimental findings [8].
In this work, the adsorption behavior of three alkyltriazoles, i.e., N-butyl-1,2,4-triazole, N-heptyl-1,2,4-triazole,
and N-decyl-1,2,4-triazole, on iron surface was studied using the density functional based tight binding (DFTB)
calculations. Using DFTB we can carry out quantum mechanical simulations similar to DFT but in an approximate
way, typically gaining around two orders of magnitude in speed. The objective of this investigation is to evaluate the
effect of alkyl chain length on the inhibition efficiency of mentioned alkyltriazoles.
COMPUTATIONAL DETAILS
In the DFTB calculations, the interaction between alkyltriazoles and Fe(110) surface was carried out in a
simulation box with periodic boundary conditions using the DFTB+ software package (from BIOVIA Inc.). The
Fe(110) surface was modelled with a four-layer slab model. In this model, there were 49 copper atoms in each layer
representing a (7×7) unit cell, and a 30 Å vacuum region between two adjacent layers to ensure the repeated slabs
decoupled. The DFTB method was used with self-consistent-charge (SCC) corrections. This method uses a minimal
set of atomic basis functions and an approximation of the Hamiltonian of a tight binding type. SCC-DFTB has been
parametrized for organic molecules containing O, N, C, and H, for which extensive tests have been performed, as
well for molecules containing S, Zn, P, and a few other elements. The Trans3d Slater-Koster parameters was used to
describe the interaction of atoms. The convergence tolerances for energy, force, and displacement were 0.02
kcal·mol−1
, 0.1 kcal·mol−1
·Å −1
, and 0.001 Å, respectively. SCC tolerance is 10−8
and k-point mesh is 2 × 2 × 1. A
smearing of 0.005 Ha was used to count the orbital occupancy.
RESULTS AND DISCUSSION
Firstly, the optimized structures and highest occupied molecular orbitals (HOMO) of three alkyltriazoles are
represented in Figure 1. It is generally acknowledged that the corrosion inhibiting effect of inhibitor molecules can
be discussed on the basis of donor–acceptor interactions [9]. Obviously, HOMO is distributed on the triazole ring,
implying that the preferred sites for electrophilic reactions of the alkyltriazoles would most likely be the five-
membered ring.
FIGURE 1. The optimized structures and highest occupied molecular orbitals (HOMO) for the alkyltriazoles of interest.
The most stable low energy adsorption configurations of three alkyltriazoles on Fe(110) by DFTB calculations
are given in Figure 2. For all the three alkyltriazoles with different alkyl chain lengths, we find that the entire
molecule adsorbed on Fe(110) surface in nearly parallel orientation. The adsorption distance between the center
mass of alkyltriazole and Fe(110) surface is about 3.10 Å. To quantitatively appraise the interaction between each
alkyltriazole and the iron surface, the adsorption energy (Eads) was calculated using following equation [10]:
ads mol surf surf molE E E E   (1)
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4. Where Emol, Esurf, and Emol/surf are the total energies of isolated alkyltriazole, Fe(110) slab, and
alkyltriazole/Fe(110) system, respectively. The calculated Eads between alkyltriazole and Fe(110) are −1.12 Ha,
−1.21 Ha, and −1.22 Ha when carbon chain lengths n =4, 7, 12, respectively, which suggests there exist fairly strong
interaction strength. It is understandable that the absolute value of adsorption energy increase with the increase of
the alkyl chains, and the corresponding inhibition efficiency follows the same rule. Our results are similar to that
reported by Chebabe et al.[11], in which their experimental studies of the surfactant molecules in acidic medium
show that N-decyl-1,2,4-triazole, N-undecyl-1,2,4-triazole, and N-dodecyl-1,2,4-triazole have good corrosion-
inhibition efficiency and that the inhibition effect increases with the tail length of the saturated hydrocarbon.
However, this does not mean that the longer of alkyl chain, the better of inhibitive efficiency. Since some
researchers have reported that the imidazolines with too long alkyl chain showed very poor inhibition efficiency,
which can be attributed to their decreased solubility in experimental condition [12].
FIGURE 2. The most stable adsorption configurations for the studied alkyltriazoles/Fe(110) systems.
As an example, the electronic density of states (DOS) for N-decyl-1,2,4-triazole@Fe(110) adsorption system are
concerned and evident from Figure 3. As given in Figure 3a, the DOS of clean Fe(110) substrate consists of a very
broad and flat s-band and much narrower d-band characterized by much higher DOS. The most relevant states for
chemisorption are those located around the Fermi energy (Ef) [13]. We can see that Fe has a large density of d states
at the Ef, thus it displays a strong chemisorption with the inhibitor molecule. A closer examination of Figure 3b, it
indicates that there are no obvious difference for the DOS of Fe 3d after the N-decyl-1,2,4-triazole adsorption. While
the intensities of DOS for s and p states increase significantly, which can be derived from the carbon and nitrogen
atoms of investigated inhibitors.
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5. FIGURE 3. The electronic density of states (DOS) for (a) clean and (b) N-decyl-1,2,4-triazole modified Fe(110) surface.
Figure 4 shows the charge density difference (Δρ) plots for the N-decyl-1,2,4-triazole@Fe(110) adsorption
system. Δρ is defined as [14]:
 total inh Fe=      (2)
where ρtotal is the electron density of the entire inhibitor/Fe(110) system, ρinh and ρFe are the electronic densities of
the isolated inhibitor molecule and iron atom, respectively. We can find that there are electron charge accumulation
and deficit between the inhibitor-metal interface, which suggests the formation of molecule-surface chemical bonds
due to the charge redistribution.
FIGURE 4. (a) Side and (b) top views of the charge density difference for N-decyl-1,2,4-triazole@Fe(110) system.
CONCLUSION
Understanding the inhibition mechanism is very important in corrosion issues. Many molecular simulation
technologies can help us to find the most stable adsorption sites for a broad range of materials. This information can
help to gain further insight about the corrosion system, such as the most likely point of attack for corrosion on a
surface, the most stable site for inhibitor adsorption, and the adsorption energy of the adsorbed layer. The theoretical
modeling in this work suggests that the alkyltriazoles molecule can adsorb on Fe(110) surface in a planar manner
with a strong adsorption intensity, and the adsorption strength increase with the increase of the alkyl chains. These
results are beneficial for us to design and develop high efficient corrosion inhibitors.
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6. ACKNOWLEDGEMENTS
This work was sponsored by the National Natural Science Foundation of China (21706195), the Science and
Technology Program of Guizhou Province (QKHJC2016-1149), the Guizhou Provincial Department of Education
Fundation (QJHKYZ2016-105), the Research Fund for the Doctoral Program of Tongren University (trxyDH1510),
and the Provincial Key Disciplines of Chemical Engineering and Technology in Guizhou Province (ZDXK2017-8).
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