Volume 20 Preprint 14
Inhibition Corrosion by Liquorices
M. Koolivand_Salooki, M. Esfandyari*, M.Koulivand
Keywords: Corrosion, Liquorices, Electrochemical, Inhibitor
The use of inhibitors for the control of corrosion of metals and alloys which are in contact with aggressive environment is an accepted practice. Large numbers of organic compounds were studied and are being studied to investigate their corrosion inhibition potential. All these studies reveal that organic compounds especially those with N, S and O showed significant inhibition efficiency. But, unfortunately most of these compounds are not only expensive but also toxic to living beings. It is needless to point out the importance of cheap, safe inhibitors of corrosion. Plant extracts have become important as an environmentally acceptable, readily available and renewable source for wide range of inhibitors. They are the rich sources of ingredients which have very high inhibition efficiency The title compound Liquorices (Glycyrrhiza glabra) was synthesized and its inhibiting action on the corrosion of mild steel in 4% hydrochloric acid solutions was investigated by means of weight loss and electrochemical experiment. Results obtained revealed that Liquorices treated excellently as a corrosion inhibitor for mild steel in 4% hydrochloric acid media and its efficiency attains more than 62.4% at 0.5% of inhibitor in weight loss experiment.
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Inhibition Corrosion by Liquorices
M. Koolivand_Salooki1, M. Esfandyari2*, M.Koulivand3
1-Gas Research Division, Research Institute of Petroleum Industry, Tehran, Iran.
2- Department of
chemical engineering, University of Bojnord, Bojnord, Iran,
3- Department of Engineering, Borujerd Branch, Islamic Azad University, Borujerd, Iran
*Corresponding author: M.esfandyari (firstname.lastname@example.org)
The use of inhibitors for the control of corrosion of metals and alloys which are in contact
with aggressive environment is an accepted practice. Large numbers of organic compounds
were studied and are being studied to investigate their corrosion inhibition potential. All these
studies reveal that organic compounds especially those with N, S and O showed significant
inhibition efficiency. But, unfortunately most of these compounds are not only expensive but
also toxic to living beings. It is needless to point out the importance of cheap, safe inhibitors
of corrosion. Plant extracts have become important as an environmentally acceptable, readily
available and renewable source for wide range of inhibitors. They are the rich sources of
ingredients which have very high inhibition efficiency The title compound Liquorices
(Glycyrrhiza glabra) was synthesized and its inhibiting action on the corrosion of mild steel
in 4% hydrochloric acid solutions was investigated by means of weight loss and
electrochemical experiment. Results obtained revealed that Liquorices treated excellently as a
corrosion inhibitor for mild steel in 4% hydrochloric acid media and its efficiency attains
more than 62.4% at 0.5% of inhibitor in weight loss experiment.
Key words: Corrosion, Liquorices, Electrochemical, Inhibitor
A corrosion inhibitor is a chemical compound that, when added to a fluid or gas, decreases
the corrosion rate of a metal or an alloy. The effectiveness, or corrosion inhibition efficiency,
of a corrosion inhibitor is a function of many factors like: fluid composition, quantity of
water, flow regime, etc. Some of the mechanisms of its effect are formation of a passivation
layer (a thin film on the surface of the material that stops access of the corrosive substance to
the metal), inhibiting either the oxidation or reduction part of the redox corrosion system
(anodic and cathodic inhibitors), or scavenging the dissolved oxygen.
Excessive corrosion attack is known to occur on mild steel in acidic aqueous media. Acid
solutions are widely used in different industrial processes, for example, in acid pickling of
iron and steel, scale removal in metallurgy, acid cleaning of boilers and oil-well acidizing,
therefore the study of corrosion of steel in acid solutions and its inhibition have practical
importance. Different corrosion inhibitors are used to control and reduce corrosion of metals
in corrosive media. Due to toxicity of most inorganic corrosion inhibitors such as chromates
and nitrites and also because of restrictive environmental regulations, these inhibitors are
being replaced by new environment-friendly organic compounds. Steel corrosion in different
Solutions have also been effectively controlled using organic inhibitors containing nitrogen,
oxygen or Sulphur [1-9]. The problems that exist with current inhibitors are that they are
toxic and expensive; therefore a new less toxic and inexpensive material or method to reduce
corrosion is needed, necessary and proposed. The goal of this work is investigate effective
and environmentally safe inhibitors such as liquorices. Acid solutions are commonly used for
removal of undesirable scale and rust in the metal working, cleaning of boilers and heat
exchangers [10, 11]. In these situations hydrochloric acid is one of the most widely used
agents. To prevent unexpected metal dissolution and excess acid consumption in pickling
processes of mild steel, inhibitors are added to the acid [12, 13]. The effective inhibitors are
organic compounds that have π bonds, heteroatom's (P, S, N and O)[14, 15]. Now the
development of novel corrosion inhibitors of natural source and non-toxic type has been
considered more important and desirable . In present study, Liquorices (Glycyrrhiza
glabra) was investigated as an inhibitor for the corrosion of mild steel in 4% hydrochloric
acid (HCl) using weight loss, electrochemical experiments, The liquorices plant is a legume
(related to beans and peas) and native to southern Europe and parts of Asia. It is an
herbaceous perennial, growing to 1 m in height, with pinnate leaves about 7-15 centimeters
(3-6 inches) long, with 9-17 leaflets. Liquorices' inhibition is from the inhibition of the
enzyme ([11β-hydroxysteroid dehydrogenize). In this research we use the extraction of roots
of liquorices. In figures 1, 2 the pictures of liquorices plant and its root's extraction are
Fig1. Liquorices plant liquorices Fig 2. Extraction of roots of liquorices
Liquorices was dissolved in 4% HCl solution at various percents (from 0.5 to 10 %) in 100
mL solution. And the solution in the absence of Liquorices was taken as blank for
The composition (wt. %) of used mild steel given in Table 1.
Table 1 Chemical composition of the used mild steel
Specimens were mechanically cut into 2cm×2.94cm×5.92cm (used in the weight loss and
electrochemical experiment ) dimensions, polished with SiC abrasive papers up to 1200
grade, washed in distilled water, degreased ultrasonically in ethanol and acetone, dried in
2.1. Weight loss experiment
Mild steel specimens in triplicate for each inhibitor concentration were immersed in the test
acid solutions for 6 h at 298 K. After that, the specimens were removed, scraped, rinsed in
water and acetone, and finally dried in desiccators. Then the loss in weight was determined
by analytic balance. (Fig3)
Figure 3 . Experiment 1 - weight loss Experiment
In second part of first experiment we have three samples with below description, after six
hours we again measure the mass removal of three steel pieces, noting that we put liquorices
in the three samples with theses portion, 1%, 2 %. 5% of the volume of the solution means
that 2 gr, 4 gr, and 10 gr liquorices in each solution of 4 % HCl (Fig 4).
Figure 4 . Experiment 1-2 - weight loss Experiment
These experiments show that our inhibitor (liquorices) only on a definite range of
concentration has the positive action on the corrosion, and on out of this rang has the inverse
action. For example in the solution of (4gr and 10 gr) has the inverse action.
2.2 Electrochemical experiment
Electrochemical experiments were performed in a conventional two electrodes cell, mild
steels sealed by epoxy resin with exposure surface (2.94cm×5.92cm) as working electrodes
and a saturated calomel electrode (SCE) provided with a Luggin capillary as reference
electrode. Without any inhibitor measured the voltage and current, and then begin our
experiment by adding the inhibitor in mL in the solution and measured the current and
voltage in this case. (Figure 5)
Figure 5. Experiment 3 - Measuring current and voltage with Calomel Ref. electrode by adding inhibitor
3. Results and discussion
3.1. Weight loss measurements
Values of inhibition efficiency IE
(w) and corrosion rate (gr cm-2 h-1) obtained from weight
loss method for various concentrations of Liquorices at 298 K. The inhibition efficiency IE
was obtained by:
Where R0 and Ri are the corrosion rates of mild steel in the absence and presence of the
2-cm1-R: rate of corrosion (gr hr
T: time (hour)
It can be found that the increscent in the inhibitor concentration lead to an increase in the
inhibition efficiency. At the highest concentration of 1% M, the compound Liquorices has a
highest inhibition efficiency of about 62.4% in weight loss measurement. For weight loss,
plot the change in weight of each cleaned coupon (metal sample) both before and after
exposure using multiple samples. Each one of these can be removed at predetermined times,
see the representative curve Figure 6. The slope of the curve at any point is the corrosion rate
at that time. The bottom curve shows the corrosion rate for the uninhibited system. The
weight loss of the inhibited coupon divided by the loss of the uninhibited coupon provides a
measure of inhibition efficiency.
1 2 3 4 5 6
Solution with 1%
Solution with 2%
Solution with 5%
Figure 6-Weight loss of mild steel by weight loss
Figure 6 present the results of weight loss measurements for the corrosion of mild steel, in 1 ,
2 , 5% HCl solutions devoid of and containing different concentrations of the liquorices
extract. Inspection of the data in the figures reveals that the addition of liquorices extract
decreases markedly the corrosion rate of mild steel. This result indicates the inhibitive effect
of the added extract on mild steel corrosion in the acidic solution. The inhibition efficiency
increases as the concentration of added extract is increased till 0.5 % of inhibitor after that by
increasing inhibitor content efficiency will decrease.
It is common practice to assume that the inhibitive effect of the added compound is a result of
adsorption of its molecules on the metal surface. Therefore, there is a direct relationship
between the inhibition efficiency and the fraction of surface covered by the adsorbed
3.2. Electrochemical experiments
Measurements are based on calculation of mass of dissolution W (
gr hr-1 cm-) by the relations
given in Table 2 .
Table 2 relations of evaluating electrochemical experiment
r: rate of weight loss (mm/year)
W: weight lost (milligrams)
3D; density (gr/cm
2A: surface area (cm
T: time (hr)
F: faraday number (=96485)
i: current density (Acm-2)
W: mass of dissolution (grhr-1cm-2)
T: time (hr)
M: (atomic mass/valance number)
Figures 7, 8
represent the curves of current and voltage of the cell versus added volume of
predicted inhibitor in mild steel 4% HCl solutions devoid of and containing different
concentrations of liquorices extract. Inspection of the figures reveals that the presence of
liquorices extract shifts the current and voltage magnitude toward the low amount and the
cathodic curves toward active direction. This behavior suggests the inhibitive effect of the
Figure 7- mL of Liquorices vs Current of cell
Figure 8 mpy diagram of mild steel vs inhibitor percent
Figure 9 - mL of Liquorices vs efficiency of inhibitor
Figure 10 -A - Counter plot of mpy
Figure 10 -B - Counter plot of mpy
Figure 11 -A - Counter plot of Efficiency
Figure 11 -B - Counter plot of Efficiency
The result indicated that the increase of inhibitor efficiency with concentration may be
attributed to the formation of a barrier film, which prevent acid medium to attack the metal
surface, because of the adsorption of Liquorices on the mild metal surface
The chemicals which can act as corrosion inhibitors may be inorganic or organic. The
inorganic compounds such as chromates inhibit the corrosion process via formation of
passive oxide film on the metal surface and thus prevent the corrosive medium to attack the
bar metal. On the other hand, the organic compounds adsorb on the metal surface forming a
barrier between the metal and the corrosive environment. Some structural features of the
organic compounds help them to do so. These include the presence of oxygen, nitrogen or
sulfur atoms as well as presence of double bonds. The lone pair electrons of the mentioned
atoms facilitate the adsorption process. Some criteria should be considered when making a
choice of chemical compounds for inhibition of corrosion. Inhibition of metallic corrosion is
mainly an economical process. Therefore, the first criterion must be fulfilled by the used
inhibitors is their prices. The other very important criterion should be considered when
dealing with corrosion inhibitor is its effect on the human and environment. Unfortunately,
most of the effective corrosion inhibitors are synthetic chemicals with high cost. At the same
time, the use of such synthetic compounds can cause harm to human and environment. Upon
looking around we'll find very rich resources for substances with wide varieties of chemical
structures. So, why we rush to synthesize harmful chemical while the nature around us full of
the safest ones? The plant is the great chemical factory which can supply us with the
chemicals required to inhibit the corrosion process. Most of the naturally occurrence
substances are safe and can be extracted by simple and cheap procedures. Recent literature
full of researches which test different extracts for corrosion inhibition applications.
This plant (Liquorices) showed an excellent inhibition performance. The
from weight loss test and electrochemical measurements
were in good agreement.
Weight loss and electrochemical method are used to study corrosion of mild steel in 4% HCl
solution and the inhibiting effect of liquorices derivatives. The weight loss measured within 6
hr shows that all studied compounds act as inhibitors in this acid medium. The highest
inhibition efficiency is obtained by 0.5% solution of inhibitor in weight loss experiment.
Analysis of the electrochemical data show that the inhibiting properties increase with
The authors would like to thank Dr Peykari for sample preparation and chemical analysis and
Dr .Kiani_Rashid for useful comments and grammatical revision and Ahwaz-Abadan
Petroleum University of Technology.
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