CHEMICAL ENGINEERING TRANSACTIONS

VOL. 56, 2017

A publication of

The Italian Association
of Chemical Engineering

Online at
www.aidic.it/cet

Guest Editors: Jiří Jaromír Klemeš, Peng Yen Liew, Wai Shin Ho, Jeng Shiun Lim

Inhibitory Effect of Red Onion Skin Extract on the Corrosion of
Mild Steel in Acidic Medium

Choon Chieh Ong, Khairiah Abd Karim*
School of Chemical Engineering, Universiti Sains Malaysia Engineering Campus, 14300 Nibong Tebal, Penang, Malaysia
chkhairiah@usm.my

The inhibition efficiency of methanol extract of red onion peel on the corrosion of mild steel in hydrochloric acid
(HCl) solution has been investigated through weight loss measurements. This study was conducted by
immersing mild steel coupon into 1.0 M of HCl solution without and with the presence of red onion peel
extracts (ROPE) (0.5 – 2.0 g/L) and at various temperatures (303 to 333 K) for 7 d. Fourier Transform Infrared
(FT-IR) spectroscopy analysis indicated that the inhibition of mild steel corrosion was mainly contributed by C-
O and aromatics compounds. These compounds were adsorbed on the mild steel surface and formed a thin
layer of protective barrier. From the weight loss measurement, the inhibition efficiency of 90 % was obtained in
the HCl solution containing 2.0 g/L of ROPE at temperature of 303 K. The inhibition efficiency was found to
increase with increasing inhibitor concentration but decrease with increasing temperature of acid solution. This
is because mild steel is oxidised at a higher rate at high temperature which causes the ROPE to desorb from
the surface of mild steel. The adsorption mechanism of ROPE on the mild steel obeys the Langmuir
adsorption isotherm within the range of temperature studied.

1. Introduction
Metals such as steel are commonly used as piping system to transport acid or alkali in industrial process.
When metal is in contact with acid or alkali, an unavoidable metal corrosion on the metal surface will occur.
Metal corrosion can be defined as a gradual deterioration of metal by anodic and cathodic reactions when the
metal is exposed to weather, moisture or other corrosive medium (Ching, 2011). Metals are mostly inherently
unstable in the environment because metals are produced from its minerals or ores. Metal corrosion is also
known as natural electrochemical reaction. It consists of the dissolution of metals into the corrosive medium
and oxidants reduction that occurred at anodic and cathodic site of the electrochemical reaction (Sato, 2012).
Since metals have been converted into other stable form of products, the original physical and chemical
properties of the metals have been degraded. Continuous damage to the metal structure due to corrosion by
the environment will cause the plant unit to collapse or pipe leakage. Cost of maintenance, repair and
replacement will trigger significant loss of revenue for chemical industry (Patni et al., 2013). Corrosion of metal
cannot be prevented but it is controllable. Addition of corrosion inhibitor into the corrosive medium is the most
common method used to prevent metal corrosion. Small amount of inhibitor added will improve the lifetime of
the metal by physisorption or chemisorption of inhibitor compound on the metal surface (Fuchs-Godec, 2006).
Corrosion inhibitor will not induce any significant effect to the process since it is added in a small concentration
to the process stream (Raja and Sethuraman, 2008). It possesses strong benefit due to easy application,
effective in reducing corrosion rate, and inexpensive. It will inhibit corrosion by formation of enhanced
protective oxide film or formation of barrier due to adsorbed inhibitor compound on the metal surface (El
Rehim et al., 2001). Due to the toxic effects of synthetic or inorganic inhibitor, the researchers started to study
the inhibition efficiency of various types of natural corrosion inhibitor on metal corrosion. Plants are sources of
naturally occurring compounds. Corrosion inhibitors by using plant extracts are environmentally friendly, lower
cost, and less harmful to human health and the environment (James and Akaranta, 2014). These advantages
of using green corrosion inhibitor are more than enough to support that plant extracts are an alternative
inhibitor for non-renewable synthetic or inorganic inhibitor.

DOI: 10.3303/CET1756153

Please cite this article as: Ong C.C., Karim K.A., 2017, Inhibitory effect of red onion skin extract on the corrosion of mild steel in acidic
medium, Chemical Engineering Transactions, 56, 913-918 DOI:10.3303/CET1756153

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Onion peels are commonly disposed as solid waste from almost every kitchen in Malaysia since it is one of the
major vegetables consumed every day. From the survey by Hertog et al. (1992), onion (Allium cepa L.) mainly
composed of quercetin content as compared with other 28 vegetables and nine fruits. Quercetin is the major
flavonoid compound that can be found in red onion peel (James and Akaranta, 2011). The research studied by
Patil et al. (1995) showed that the colour of onion is not the main criterion that affects the quercetin content in
onion. Red onion is still the most commonly used vegetables in every household compared with white or
yellow onion. The objective of this study was to investigate the inhibitory effect of red onion peel extract
(ROPE) for mild steel in HCl solution. The inhibition behaviour of ROPE on mild steel was determined using
weight lost measurements.

2. Materials and Methods
2.1 Materials preparation

Mild steel coupons were obtained locally and were mechanically press-cut into dimensions of 5 cm x 3 cm with
thickness of 0.1 cm. The coupons were first degreased with absolute ethanol (James and Akaranta, 2014),
immersed in acetone and air dried overnight. The initial mass of metal coupons was recorded and stored in a
desiccator before use. The red onion peel (6.25 g) was extracted using ultrasound-assisted extraction unit with
80 vol% methanol (250 mL) as a solvent. The extraction process took 5 min and the mixture was taken out
from the ultrasonicator and stirred at 200 rpm for 10 min at room temperature. These two steps were repeated
for six times to optimise the extraction process. The mixture was filtered and the methanol was evaporated
using a rotary evaporator at 373 K. The residues were collected and weighted. The ROPE was characterised
by Fourier Transform Infrared (FT-IR) analysis. The wavelength in the range from 4,000 –
400 cm-1 was used. The spectrum was expressed in terms of % transmission against the wavelength.

2.2 Experimentation

To study the effect of inhibitor concentration, five beakers which contained concentrations of 0 (control), 0.5,
1.0, 1.5 and 2.0 g/L of ROPE in 1 M HCl solution at temperature of 303 K were used to determine the
corrosion rate of mild steel coupons. The cleaned and weighed mild steel coupons were each fully immersed
into the HCl solution. The duplicate sets of experiments were conducted for 7 d. The experiment was repeated
for temperature of 313, 323 and 333 K. The temperature of solution was maintained at specified temperature
by using a water bath. Each beaker was covered with parafilm to minimise the amount of solution vaporised at
elevating temperature. The coupons were retrieved from the HCl solution after 7 d of immersion time. The
surface of metal coupon was cleaned using tissue paper and then immersed in absolute ethanol. The surface
was cleaned again using tissue paper and then dried in acetone. The weight of the metal coupon was
measured using analytical balance. The weight loss for each metal coupon was recorded and tabulated.

2.3 Weight loss determination

Weight loss of mild steel in HCl solution at specified time (Wt) was calculated as in Eq(1) (Iroha et al., 2015):

∆Wt = Winitial − Wt (1)

where, Winitial is initial weight of mild steel before immersed in HCl solution and Wt is the weight of mild steel
retrieved after t days of immersion in HCl solution.

2.4 Corrosion rate and inhibition efficiency

Corrosion rate, CR of the mild steel in hydrochloric acid solution was calculated using Eq(2);

Corrosion Rate, CR (mm y⁄ ) = 87.6 (
∆Wt

ρ. As. t
) (2)

where, Wt is the weight loss,  is metal density, As is area of metal surface, t is time of exposure. The
corrosion rate of mild steel is expressed on the basis of the apparent surface area. The approximated density
of mild steel is 7.85 g/cm3.
Inhibition efficiency (IE) and surface coverage,  were determined by using Eq(3) and Eq(4):

% IE =
CRwithout − CRwith I

CRwithout
× 100 % (3)

θ =
CRwithout − CRwith I

CRwithout
(4)

where CRwithout and CRwith I are the corrosion rates in the absence and presence of the extract.

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The high corrosion rate of mild steel indicates that the inhibition efficiency of corrosion inhibitor is low, and vice
versa.

2.5 Langmuir adsorption isotherm

The linearised Langmuir isotherm equation as in Eq(5) that has been modified is given by (El-Aila et al., 2011):

θ
=

Kads
+ nC (5)

where, Kads is equilibrium constant of adsorption, C concentration of inhibitor and n is corrective factor for the
linearised Langmuir equation with slope not equal to unity. By plotting a graph of C/ against C, the value of
Kads can be determined from the y-intercept value of the plot.

2.6 Arrhenius equation

Arrhenius equation was used to explain the corrosion phenomenon at higher temperature, as given in Eq(6):

CR = A exp (
−Ea
RT

) (6)

where, Ea is activation energy, A is pre-exponential factor, R is molar gas constant and T is operating
temperature (in K). By taking natural log on both sides of the equation, the plot of linearised Arrhenius
equation can be used to determine Ea and A using the slope and y-intercept of the graph.

3. Results and Discussions
3.1 FT-IR analysis

Red onion peel extract (ROPE) was characterised using FT-IR spectroscopy. The FT-IR spectrum of ROPE is
shown in Figure 1. A peak with broad and intense band at the region of 3,500 - 3,200 cm-1 is the O-H stretch
of alcohols since antioxidant compound of ROPE is extracted by using methanol. The peaks at wavenumber
of 1,622.20 cm-1 and 1,438.96 cm-1 are due to the C-C stretch of aromatic rings. This showed the presence of
polyphenolic component. The peak at wavenumber of 1,622.20 cm-1 is also corresponded to the C=O bond
(Nnaji et al., 2013). The peaks at 1,276.93, 1,188.20 and 1,065.72 cm-1 signalled the presence of C-O-C and
C-OH stretch bonds. Bonding of C-H in aromatic ring is corresponded to the peak at wavenumber of 812.07
cm-1. At wavenumber of 614.35 cm-1, the band showed the C-H bend bonding. The analysis above showed
the same agreement with that from other studies. The bond that present in the ROPE sample analysed using
FT-IR is consistent with quercetin compound (Figure 2) that is mainly found in red onion peel (Lee et al.,
2015).

Figure 1: FT-IR spectrum of ROPE Figure 2: Chemical structure of quercetin that

present in red onion peel

3.2 Effect of ROPE inhibitor concentration on its inhibition efficiency

The inhibition efficiency was increased with increasing inhibitor concentration from 0.5 to 2.0 g/L at specified
temperature with constant HCl concentration (Figure 3). This is due to the decrease of corrosion rate of mild

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steel. Since more active molecules are able to be adsorbed on the metal surface at higher concentration of
ROPE, thus, prevent the contact between corrosive medium and metal surface (Ibrahim et al., 2011). ROPE
with concentration of 2.0 g/L in HCl solution reached its highest inhibition efficiency of 89.97, 89.49, 80.49 and
69.43 % at temperature of 303, 313, 323 and 333 K. The inhibition efficiency of ROPE at each specified
temperature showed no significant changes from concentration of 1.5 to 2.0 g/L. This behaviour indicated that
ROPE achieved its optimum inhibition ability at concentration of 1.5 g/L. Further increase in the inhibitor
concentration did not show any significant increase in inhibition efficiency on mild steel under this operating
condition.

Figure 3: Effect of various concentrations of ROPE on its inhibition efficiency on mild steel in 1.0 M HCl

solution at different temperatures

3.3 Effect of temperature on inhibition efficiency of ROPE

From Figure 4, the inhibition efficiency decreased with increasing temperature of corrosive medium. This is
due to the increase in corrosion rate of mild steel and decrease in the ability of adsorption process at high
temperature. When temperature increased from 303 to 333 K, disintegration of mild steel in HCl solution
outweighs the adsorption of ROPE active molecules on the mild steel surface. This can also prove that the
adsorption of inhibitor molecule on the metal surface is a physisorption interaction (Kairi and Kassim, 2013).

Figure 4: Effect of various temperatures on inhibition efficiency of different concentration of ROPE on mild

steel in 1.0 M HCl solution

3.4 Langmuir adsorption isotherm

Figure 5 shows the plots of Langmuir isotherm. The experimental data fit the isotherm with a linear best fit line.
The R2 of Langmuir isotherm is closed to unity for temperature range between 303 and 333 K. This
demonstrated that ROPE inhibitor obeyed Langmuir adsorption isotherm on mild steel surface. According to
Langmuir isotherm model, the adsorption of inhibitor molecules on metal surface is monolayer of adsorbed
molecules and even distribution of the molecules at all sites (Masel, 1996). One inhibitor molecule will only be
adsorbed on an active site of metal surface. There is no interaction between the adsorbed molecules.
The Langmuir’s equilibrium constant of adsorption, Kads can be calculated based on the slope and y-intercept
of the linearised Langmuir isotherm equation (5). The Kads values decreased with increasing temperature; i.e
Kads, 303 K = 35.83 L g

-1; Kads, 313 K = 19.46 L g
-1; Kads, 323 K = 7.65 L g

-1. This indicates that the interaction

100

0.0 0.5 1.0 1.5 2.0 2.5

In
hi

bi
tio

n
E

ff
ic

ie
nc

y
(%

)

Inhibitor Concentration, C (g/L)

T = 303 K
T = 313 K
T = 323 K
T = 333 K

100

300 310 320 330 340

In
hi

bi
tio

n
E

ff
ic

ie
nc

y
(%

)

Temperature, T (K)

0.5 g/L

1.0 g/L

1.5 g/L

2.0 g/L

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between inhibitor molecule and metal surface is weak due to the physical adsorption of the molecules on
metal surface (El Rehim et al., 2001). At higher temperature, the inhibitor molecules were desorbed easily
from the metal surface due to the weak attraction force with metal surface. This eventually lowered the
inhibition efficiency and induced higher rate of corrosion of mild steel at higher temperature.

Figure 5: Langmuir isotherm for the adsorption of ROPE on mild steel in 1.0 M HCl solution at different

temperatures

3.5 Arrhenius equation

The values of R2, activation energy and pre-exponential factor for different concentration of ROPE inhibitor are
calculated and tabulated in Table 1. The activation energy for acid solution without addition of inhibitor
(74.92 kJ/mol) is lower than that with corrosion inhibitor. With the presence of ROPE, the activation energy
increased which induced the reduction of corrosion rate. Active molecules of inhibitor are able to be adsorbed
on the metal surface easily at high inhibitor concentration (Essa, 2012). More energy is needed to overcome
this high activation energy for corrosion of metal surface to proceed. Reducing of available reaction area and
changing the activation energy of the anodic and/or cathodic reactions can affect the adsorption of an inhibitor
molecule on metal surface. The inhibition of mild steel corrosion is due to the physical adsorption of organic
inhibitor on the metal surface with the formation of protective layer.

Table 1: Activation energy and pre-exponential factor of corrosion of mild steel in HCl solution with different

concentration of inhibitor.

Concentration of Inhibitor (g/L) R2 Activation Energy, Ea (kJ/mol) Pre-exponential Factor, A
Blank 0.9652 74.92 1.79 x 1013
0.5 0.9675 108.01 1.13 x 1018
1.0 0.9566 108.17 1.01 x 1018
1.5 0.9662 108.68 1.07 x 1018
2.0 0.9635 109.05 1.18 x 1018

4. Conclusions
The present study shows that red onion peel extract (ROPE) has successfully retarded the mild steel
corrosion in hydrochloric acid solution. Protective film is formed on the metal surface through the interaction
between the active site of metal surface and the lone pair of electrons of oxygen atom and/or aromatic ring.
These chemical bonds are found in the extract of red onion peel analysed using FT-IR and also consistent
with the chemical structure of quercetin. Inhibition efficiency of 90 % by the ROPE inhibitor on the corrosion of
mild steel in 1 M HCl solution containing 2.0 g/L of inhibitor at 303 K was achieved through weight loss
measurement. Increasing concentration of inhibitor has increased the inhibition efficiency but it decreased with
rising temperature. The adsorption of ROPE on the metal surface obeys Langmuir adsorption isotherm. The
Kads values decreased with increasing temperature which indicated the physisorption of inhibitor molecules on
metal surface. The activation energy observed experimentally has also proposed that the corrosion inhibition
was due to the adsorption of inhibitor molecules on the mild steel surface. It is clear from the study that ROPE
can be used as an effective solution to the mild steel corrosion in acidic medium.

R² = 1
R² = 1

R² = 0.9988

R² = 0.9999

0.00
0.50
1.00
1.50
2.00
2.50
3.00
3.50

0.0 0.5 1.0 1.5 2.0 2.5

C
/

(m
ol

/L
)

Inhibitor Concentration, C (g/L)

303 K
313 K
323 K
333 K

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Acknowledgments

The authors would like to express their gratitude to the Ministry of Education of Malaysia for the financial
supports through the Fundamental Research Grant Scheme (FRGS) (6071270).

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