Volume 20 Preprint 68
Corrosion Inhibition Properties of Dioctyl Phthalate on Mild Steel in 0.5 M Sulphuric Acid Medium
Michael Emmanuel, Bethrand T. Nwufo and Mbanefo M. Ekwenchi
Keywords: Mild steel, Dioctyl Phthalate, Weight loss, Corrosion inhibition.
The corrosion inhibition properties of dioctyl phthalate on mild steel in 0.5 M H2SO4 medium was carried out using the weight loss method at temperature range of 303 K to 313 K for three hours. The concentration of the inhibitor was set at a range of 0.01 M to 0.07 M and a maximum reaction time of three (3) hours. The adsorption of the inhibitor on the surface of the mild steel was studied using Langmuir, Freundlich and the newly proposed Adejoh â€“ Ekwenchi Isotherm. The maximum inhibition efficiency obtained for the corrosion of mild steel in H2SO4 was 85.11% at a temperature of 303 K. All values of âˆ†Hads were found to be positive indicating the endothermic nature of the dissolution of the mild steel. The values of entropy of adsorption âˆ†Sads was found to increase with increase in concentration of the inhibitor indicating that the rate determining combination step is a more orderly arrangement relative to the initial state. The values of Eads and those of âˆ†Hads varied in a similar manner, and the average values of Eads - âˆ†Hads equals 2.41 kJmol-1, which is very close to the value of RT (2.51 kJmol-1) indicating that the adsorption process is unimolecular in nature. The values of âˆ†Gads and Kads for the corrosion inhibition studies of mild steel in 0.5 M H2SO4 and dioctyl phthalate as the inhibitor were found to be suggestive of a physisorption mechanism. The adsorption values were in agreement with the three isotherms employed, with Freundlich isotherm as best fit for the modelling of the adsorption of dioctyl phthalate on mild steel in acid medium.
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Corrosion Inhibition Properties of Dioctyl
Phthalate on Mild Steel in 0.5 M Sulphuric
Michael Emmanuel1*, Bethrand T. Nwufo2 and Mbanefo M. Ekwenchi2
Department of Chemistry, Gombe State University, P.M.B. 127, Gombe, Gombe State,
2(Department of Pure and Industrial Chemistry, University of Jos, Jos, Plateau State, Nigeria)
The corrosion inhibition properties of dioctyl phthalate on mild steel in 0.5 M H2SO4
medium was carried out using the weight loss method at temperature range of 303 K to
313 K for three hours. The concentration of the inhibitor was set at a range of 0.01 M to
0.07 M and a maximum reaction time of three (3) hours. The adsorption of the inhibitor on
the surface of the mild steel was studied using Langmuir, Freundlich and the newly
proposed Adejoh – Ekwenchi Isotherm. The maximum inhibition efficiency obtained for the
corrosion of mild steel in H2SO4 was 85.11% at a temperature of 303 K. All values of ∆Hads
were found to be positive indicating the endothermic nature of the dissolution of the mild
steel. The values of entropy of adsorption ∆Sads was found to increase with increase in
concentration of the inhibitor indicating that the rate determining combination step is a
more orderly arrangement relative to the initial state. The values of E ads and those of ∆Hads
varied in a similar manner, and the average values of Eads - ∆Hads equals 2.41 kJmol-1, which
is very close to the value of RT (2.51 kJmol-1) indicating that the adsorption process is
unimolecular in nature. The values of ∆Gads and Kads for the corrosion inhibition studies of
mild steel in 0.5 M H2SO4 and dioctyl phthalate as the inhibitor were found to be suggestive
of a physisorption mechanism. The adsorption values were in agreement with the three
isotherms employed, with Freundlich isotherm as best fit for the modelling of the
adsorption of dioctyl phthalate on mild steel in acid medium.
Keyword: Mild steel, Dioctyl Phthalate, Weight loss, Corrosion inhibition.
Corrosion is a major industrial problem all over the world and a nightmare to most
manufacturing and construction companies. According to the American Galvanizers
Association, steel corrosion is an expensive problem costing about $2.2 trillion annually
Although corrosion could be said to be nature’s method of recycling, or of returning of a
metal to its lowest energy form, it is an insidious enemy that destroys our cars, plumbing
systems, buildings, bridges, engines, factories etc
In Nigeria, especially in the oil
producing states where there is a large amount of emitted acid gases, zinc roofs and
metallic parts have been corroded and appear as a brownish colour on the surfaces. These
tends to be washed down by rain leading to a considerable reduction in metal thickness and
loss of strength, which may as a result leads to perforation of roofs and leaking
Corrosion inhibitors are classified into inorganic and organic corrosion inhibitors, the
mechanism of corrosion can be described as; chemical adsorption or chemisorption of the
inhibiting material to the surface of the steel or by combination between inhibitor ions and
Phthalates are esters of 1,2-benzenedicarboxylic acid. The most widely used phthalates
are the dibutyl phthalates and the dioctyl phthalate
Dioctyl phthalate (DOP) with IUPAC
nomenclature dioctyl benzene-1,2-dicarboxylate, is a clear, colourless, and viscous liquid
with a slight, characteristic odor, soluble in ethanol, ether, mineral oil, and the majority of
organic solvents, immiscible with water, resistant to hydrolysis and air oxygen activity. Its
high plasticizing efficiency, fusion rate, viscosity, low volatility, UV-resisting property,
water-extracting proof, cold-resisting property, and also good softness, found a lot of
applications in many offshoots of the industry
Many researchers reported that the inhibition effect mainly depends on some physicochemical and electronic properties of the organic inhibitor which relate to its functional
groups, steric effects, electronic density of donor atoms, and orbital character of donating
electrons, and so on
Most of the research conducted on corrosion inhibition are
Zenthoxylum alatum plant
extract , extract of Ricinus communis var minor , Newbouldia leavis leaf extract ,
leaf extract of Euphorbia hirta , e.t.c. Most of these extracts studied do not give a clear
through the use of plant extracts; from the juice of grapes
view of the compound in the extract responsible for the inhibition process, and as such
makes the application ambiguous. This particular study is centred on one compound –
dioctyl phthalate, and described it corrosion inhibition properties, hence its mechanism of
adsorption on the surface of the mild steel.
Materials and Method
All the materials used for this research are of analytical grade. The method used for the
study of the corrosion inhibition properties of Dioctyl phthalate on mild steel is the weight
with coupons of size 5 cm by 4 cm, and thickness of 0.5 mm obtained
from the mechanical workshop of the University of Jos, Nigeria.
The evaluation of the inhibition efficiency (%I) was determined from the weight loss
measurement at temperatures ranging from 303K to 318K
The kinetic and
thermodynamic parameters were determined as described below. The adsorption
parameters and surface coverage were also determined as outlined below.
Determination of Parameters
The data obtained from the experiment above was used to determine the following
Inhibition Efficiency (%IE) and Surface Coverage (θ)
The inhibition efficiency (%IE) and the surface coverage (θ) were determined from the
%I = [1 – (
Where W0 = initial weight of the mild steel before suspending in the solutions,
W= weight of the mild steel without the inhibitor, that is, weight in blank solution,
Wt = weight of the mild steel with the inhibitor
The inhibitor efficiency gives important information about the performance of the inhibitor
in various medium
This is a method of monitoring corrosion. The rate of corrosion C r, is the weight loss per
unit area per unit time of a metallic material
This is given by the equation
Cr (mg cm-2 hr-1) =
Where A = surface area of the coupon (cm2)
T = time of exposure in hrs
W0 – Wt = weight loss
These parameters tell more of the adsorption properties of the inhibitor. This includes the
Activation energy of adsorption Eads, the heat of adsorption Qads, enthalpy of activation
∆Hads, entropy ∆Sads and free energy change ∆Gads.
1. Activation Energy Eads: this is the energy that adsorbing species must acquire in order
to be adsorbed on the surface of the mild steel to form a protective monolayer. This
is obtained from a modified form of the Arrhenius equation
Cr = A 𝑒 −𝐸𝑎𝑑𝑠/𝑅𝑇
This equation can be linearized to give
ln Cr = ln A – (Eads/RT)
Where Cr = rate of corrosion
A = a frequency factor
R = Gas constant (Atm. L. mol-1 K-1)
T = temperature (K)
A graphical plot of ln Cr against 1/T, gives a straight line graph with slope equals to E ads/R
and an intercept of ln A
The values of Ea greater than 20kJ indicate that the adsorption
process is controlled by the surface reactions.
Heat of Adsorption Qads
This parameter is used to tell whether an adsorption process is endothermic or exothermic.
A positive value of Qads indicates an endothermic process while a negative value indicates
exothermic process of adsorption. This can be evaluated from equation 3.4 reported by
Where 𝜃 1 = is the initial surface coverage at temperature T1
= is the final surface coverage at temperature T2
R = the gas constant
Equation 3.4 can be written as;
ln( 1− 𝜃)
A graphical plot of ln(
against 1/T gives the value of the Qads obtained from the slope
of the graph which is Qads/R. Negative Qads values indicate an exothermic adsorption
process. This exothermic reaction can either be physisorption or chemisorption. This can
be distinguished by considering the values of the heat of adsorption. Value of energy less
than 40kJ is indicative of physical adsorption, otherwise, chemical adsorption.
Enthalpy of Adsorption ∆Hads and Entropy of Adsorption ∆Sads
The enthalpy of adsorption and the entropy of adsorption (Hads and Sads) can be determined
from the Eyring transition state equation;
exp ( 𝑅 ) exp(− 𝑅𝑇 )
Rearranging the equation gives;
A plot of ln Cr/T against 1/T is a straight line with a slope equal to
the graph is equal to
𝑙𝑛 𝑁ℎ + ( 𝑅 ).
the intercept of
Where Cr is the corrosion rate, T is the temperature in
Kelvin, N is the Avogadro’s number 6.03 x 1023 mol-1, h is the Planck’s constant 6.62 x 1034
Js, and R is the gas constant. The values of ∆Hads indicates whether the adsorption is
endothermic (+∆H) and exothermic (-∆H).
Free Energy of Adsorption (∆Goads)
This value also indicates the type of adsorption mechanism, whether by physical or
chemical adsorption mechanism. Values of ∆Goads from -20kJ or more indicates physical
adsorption mechanism, while values of -40kJ or less is an indicative of chemical adsorption.
This value was calculated from the equation below;
Kads = ln(55.5) − ( 𝑅𝑇 )
Kads = (
∆Gads = - RT(ln 55.5Kads)
Where Kads is the equilibrium constant for the adsorption of the inhibitor on mild steel
surface, which is obtained from the adsorption isotherm given by equation 3.9 – 3.11, T is
the Absolute temperature of the system, R is the gas constant, and C is the concentration of
Corrosion reactions are usually first order and are expected to obey the equation of a first
order reaction as given below
ln wf = ln wi – kt
ln wf/wi = -kt
ln wi/wf = kt
Where wi and wf are respectively the weights of the mild steel in blank acid solution and
after immersion in the inhibitor solution. This value tells how fast the adsorption process is
taking place. A plot of ln(wi/wf) against t, gives a straight line with a slope which is equal to
Also the half life of this process was determined from the value of the rate constant that
was obtained from above using
The variation at constant temperature of the surface coverage of adsorbing species with the
concentration or pressure of the adsorbing solutes is known as the adsorption isotherm.
Adsorption parameters obtained from adsorption studies give important information that
will help in understanding the type of adsorption taking place during the inhibition.
Adsorption isotherms are important in the optimization of the adsorption mechanism and
aiding the understanding of the surface properties of the inhibitor.
Most frequently used adsorption isotherms are Langmuir, Freundlich, Frumkin, Temkin,
Forry – Huggins, BET, e.t.c. The newly proposed Adejo – Ekwenchi isotherm was also used
to confirm the claims about its effectiveness in deciphering the mechanism of adsorption.
The isotherms used to test the linearity of the adsorption results are;
(𝜃) = 𝑘 + 𝐶
log 𝜃 = log 𝑘 + 𝑛𝑙𝑜𝑔 𝐶
Adejoh – Ekwenchi
log( 1− 𝜃) = log 𝐾AE + blog C
k is the equilibrium constant for adsorption in all the equations. The values of k, gives
information about how favourable the inhibition process is, and higher values of k indicates
favourable adsorption process. C is the concentration of the solution of the inhibitor.
Results and Discussion
The corrosion inhibition properties of Dioctyl phthalate was determined at temperature
ranges of 303 – 313 K, in 0.5 M H2SO4 solution. The average initial weight of the mild steel
coupon before immersion into the inhibitor solution is 2.512 g. Table 1 represents the
weight loss of mild steel at different concentration of the inhibitor in 0.5 M H 2SO4 solution
as the inhibitor.
Table 1: weight loss of mild steel in inhibitor and 0.5 M H 2SO4 solution at different
Weight Loss (g) (303 K)
(w0 – wt)
Weight Loss (g) (308 K)
(w0 – wt)
Weight Loss (g) (313 K) (w0
The percentage inhibition efficiency (%IE) was calculated according to equation (1a) and the
values obtained at different concentration of the inhibitor and different temperatures in 0.5
M H2SO4 solution are recorded in Table 2.
Table 2: Percentage Inhibition Efficiency (%IE) of the inhibitor on mild steel in 0.5M H 2SO4
% IE at 303K
% IE at 308K
% IE at 313K
Also, the surface coverage of the inhibitor solution on mild steel was calculated according
to equation (1b). Table 3, represents the surface coverage (θ) of the inhibitor on mild steel
in 0.5 M H2SO4 solution
Table 3: Surface coverage (θ) of the inhibitor on mild steel in 0.5M H2SO4
θ at 303K
θ at 308K
θ at 313K
The rate of corrosion of mild steel in solution of the inhibitor was also calculated according
to equation 2. These values are represented in Table 4 for the inhibitor in 0.5 M H2SO4.
Corrosion rate (Cr) increases with increasing temperature from 303 K to 313 K in H 2SO4
medium, and the corrosion rate decreases with increase in the concentration of the
inhibitor. This is because as the concentration of the inhibitor increases, more molecules of
the inhibitor become available to cover the surface of the mild steel, preventing it from the
grip of the corrodant.
Table 4: Corrosion rate (Cr) of the mild steel in inhibitor/0.5M H2SO4
Cr at 303K
Cr at 308K
Cr at 313K
As presented in table 2, there is a significant increase in inhibition efficiency as the
concentration of the inhibitor increases and it could be observed that the efficiency
decreases with increase in temperature. This is suggestive of a physisorption mechanism of
adsorption of the inhibitor onto the surface of the metal
Activation energy of adsorption Eads, the quantity of energy Qads involved in the adsorption
process, the enthalpy of adsorption ∆Hads, and the entropy of the system ∆Sads, where
determined according to equations, 3b, 4b and 5b respectively. The results obtained for
these parameters are shown in Table 5.
Table 5: Thermodynamic parameters of the dioctyl phthalate inhibitor in 0.5 M H 2SO4
solution at different concentrations of the inhibitor
Enthalpy of adsorption and
Entropy of Adsorption
Free Energy of
A change in the value of Eads in the presence of the inhibitor may be due to the modification
of the mechanism of the corrosion process in presence of adsorbed inhibitor molecules.
Bouyanzer et’al in Ejikeme et’al
indicated that generally, higher values of Eads in presence
of additives support physical adsorption mechanism whereas an unchanged or lower value
of Eads for inhibited systems compared to the blank is indicative of chemisorption
mechanism. From table 5, it can be seen that the values of the activation energies for the
inhibited systems are higher than that of the uninhibited or blank system. This is an
indication of a physisorption mechanism of adsorption.
The values of Enthalpy of adsorption and Entropy of Adsorption; ∆H ads and ∆Sads were
obtained from the graphical plot of the Eyring transition state equation (equation 6). The
values of ∆Hads and Eads are nearly the same in 0.5 M H2SO4, and are higher in the presence
of the inhibitor. This indicates that the energy barrier of the corrosion reaction increased in
the presence of the inhibitor without changing the mechanism of dissolution
positive values of ∆Hads for both corrosion processes with and without the inhibitor reveal
the endothermic nature of the mild steel dissolution process and indicate that the
dissolution of steel is difficult
The values of entropy of adsorption ∆Sads listed in Table
5 for 0.5M H2SO4, clearly showed that entropy of activation increases in the presence of the
studied inhibitor compared to free acid solution. The negative Qads values indicate that
adsorption and inhibition efficiency (Table 2) decreases with rise in temperature which is in
line with the findings of Onen et al,
and indicates that the adsorption of the inhibitor to
the surface of the mild steel is spontaneous, and also, this is indicative of a strong
interaction between the inhibitor and the mild steel
Three isotherms were selected for the adsorption study; these are Langmuir, the Freundlich
and the newly proposed Adejoh – Ekwenchi isotherms.
Table 6: Values for adsorption studies of the inhibitor on mild steel in 0.5 M H 2SO4
solutions at different temperatures.
“b” (from the Adejoh – Ekwenchi Isotherm) is a parameter called “variation factor” and it
relates the surface coverage by the adsorbate to the change of adsorbate bulk
concentration. A decrease in the value of b with rise in temperature is an indication of
physical adsorption mechanism, while increase or fairly constant in its value with rise in
temperature shows the mechanism of adsorption to be chemical adsorption
KAE is the
isotherm adsorption equilibrium constant known as Adejo-Ekwenchi constant. Just like any
isotherm it is a temperature dependent parameter, which gives the adsorption strength.
High value of it is indicative of favourable adsorption process. It can be seen that K AE,
followed the required trend similar to those of Langmuir and Freundlich, indicating a
favourable adsorption process. It can also be seen that the values of “b” decreases with
increase in temperature. This supports the claims by Adejoh et’ al,
that the mechanism
for this type of trend is physisorption. This is in line with all the experimental data analysed
previously in this research.
The positive adsorption equilibrium constant KF (Freundlich Isotherm) values are indication
of favourable adsorption. The parameter nF in the Freundlich isotherm relates to intensity of
adsorption and it varies with heterogeneity of the material
positive, but not an integer, with typical value is 0.6
and the value is always
The average value of nF is 0.798 for
the inhibitor in 0.5 M H2SO4 indicating that this adsorption process can reasonably be
modelled by Freundlich isotherm and is heterogeneous
The adsorption equilibrium
constant Kads (for all the Isotherms tested) decreases with increase in experimental
temperature in the corrosion media, indicating that the interactions between the adsorbed
molecules and the metal surface are weakened and consequently, the adsorbed molecules
could become easily removable. Such data explains the decrease in the inhibition efficiency
with increasing temperature
Free Energy of Adsorption ∆Goads
The standard free energy of adsorption, ΔG°ads, which can characterize the interaction of
adsorbed molecules and metal surface, was calculated using Equation 10. The values of K ads
were determined from the different isotherm plots, and are represented in table 7 for 0.5 M
H2SO4 solution. The negative values of ΔG°ads ensure the spontaneity of adsorption process
and stability of the adsorbed layer on the metal surface. Generally, the values of ΔG°ads
below 40kJ/mol are consistent with physisorption, while those above 40 kJ/mol, involve
chemisorption . From table 7 below, the values of the standard free energy of adsorption
are negative. This indicates that the adsorption process is spontaneous. The values also,
were found to be below 40 kJ/mol; which concur with the aforementioned physisorption
mechanism for the adsorption process in 0.5 M H2SO4 solution.
Table 7: values for the standard free energy of adsorption ∆Goads of the solution of the
inhibitor in 0.5 M H2SO4 at different temperatures
The corrosion of mild steel follows a first order rate law with respect to the concentration of
A decrease in the rate constant k, with increase in temperature indicates
that the rate of the mild steel corrosion is greatly reduced in the presence of the inhibitor
(dioctyl phthalate) in H2SO4 Solution. The half – life presented in table 8, indicates that the
life span of the mild steel is increased as the temperature increased.
Table 8: rate constant k, and half – life t1/2 of mild steel in the inhibitor/0.5 M H2SO4
solution at different temperatures
Rate Constant k (hr-1)
Dioctyl phthalate can be said to be a good corrosion inhibitor, and gave a maximum
percentage inhibition efficiency of 85.11% at a concentration of 0.07 M of the inhibitor in
0.5 M H2SO4 solution at a temperature of 303 K.
The Freundlich adsorption isotherm was found to be the best fit for the adsorption studies
of the corrosion inhibition of dioctyl phthalate in H2SO4 medium with an average R2 value of
0.9. It was also discovered that the Adejoh – Ekwenchi Isotherm can be used to model
adsorption process for corrosion inhibition studies.
All values of ∆Hads were found to be positive indicating an endothermic nature of the
dissolution of the mild steel. The values of entropy of adsorption ∆S ads was found to
increase with increase in concentration of the inhibitor indicating that the rate determining
combination step is a more orderly arrangement relative to the initial state.
It was also discovered that the values of Eads increases with increase in the concentration of
the inhibitor. The values of Eads and those of ∆Hads varies in a similar manner, and the
average values of Eads - ∆Hads equal to 2.41 kJmol-1, which is very close to the value of RT
(2.51 kJmol-1) indicating that the adsorption process is unimolecular in nature.
All values of Qads were found to be negative, indicating that the process of adsorption of the
inhibitor on mild steel is exothermic and spontaneous. It also indicates strong interaction
between the inhibitor and the mild steel surface.
The corrosion kinetics showed an adherence to first order kinetics and the half- life values
showed that the presence of the inhibitor extends the lifespan of the mild steel.
The values of ∆Gads and Kads for the corrosion inhibition studies of mild steel in 0.5 M H 2SO4
and dioctyl phthalate as the inhibitor were found to be suggestive of a physisorption
The corrosion inhibition properties of dioctyl phthalate were determined in 0.5 M H 2SO4
solution and at a temperature range of 303 to 313 K using the weight loss method. The
results obtained, showed that dioctyl phthalate is a good corrosion inhibitor of mild steel.
This efficiency was found to decrease with increasing temperature and increases with
increase in the concentration of the inhibitor. The most suitable isotherm to be used for
modelling the corrosion inhibition studies of the dioctyl phthalate is the Freundlich
adsorption isotherm. The values obtained for the newly proposed Adejoh – Ekwenchi
isotherm agrees with the predictions of Adejoh and Ekwenchi, such that their isotherm can
also be used in modelling adsorption processes. Physisorption mechanism was proposed
for the adsorption of dioctyl phthalate in 0.5 M H2SO4 and 1.0 M HCl solution on mild steel.
We wish to thank the University of Jos, for furnishing us with the laboratory space used in
conducting the research and the contributions of Gombe State University for providing the
funds for the research.
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