Volume 20 Preprint 78
Inhibitory Effects of Delonix regia(Gulmohor) extract on Corrosion of Aluminium in Hydrochloric Acid
N. I. Prajapati and R. T. Vashi
Keywords: Aluminium, Hydrochloric acid, Delonix regia , Corrosion, Inhibitor
The inhibition of the corrosion of aluminium in hydrochloric acid solution by the leaves extract of Delonix regia has been studied using weight loss, Potentiodynamic Polarization, Electrochemical Impedance Spectroscopic (EIS) techniques. Corrosion rate increases with the increase in acid concentration and temperature. As inhibitor concentration increases corrosion rate decreases while percentage of inhibition efficiency (I.E.) increases. The value of free energy of adsorption (ï„G0ads), heat of adsorption (Qads), energy of activation (Ea), enthalpy of adsorption (ï„H0ads) and entropy of adsorption (ï„S0ads) were calculated. The inhibition effect is discussed in view of Delonix regia molecules adsorbed on the metal surface and it obeys Langmuir adsorption isotherm. Polarization curve indicates that inhibitor act as mixed type and the I.E. was found up to 93.48 ï€¥. The results obtained showed that the leaves extract could serve as an effective green inhibitor for corrosion of aluminium in hydrochloric acid.
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Inhibitory Effects of Delonix regia(Gulmohor) extract on Corrosion of
Aluminium in Hydrochloric Acid
N. I. Prajapati and R. T. Vashi*
*Department of Chemistry, Navyug Science College, Rander Road, Surat-395005, Gujarat, India.
For correspondence: Email: firstname.lastname@example.org; Tel. No.: 09998572828.
The inhibition of the corrosion of aluminium in hydrochloric acid solution by the leaves extract of
Delonix regia has been studied using weight loss, Potentiodynamic Polarization, Electrochemical
Impedance Spectroscopic (EIS) techniques. Corrosion rate increases with the increase in acid
concentration and temperature. As inhibitor concentration increases corrosion rate decreases while
percentage of inhibition efficiency (I.E.) increases. The value of free energy of adsorption (DG0ads),
heat of adsorption (Qads), energy of activation (Ea), enthalpy of adsorption (DH0ads) and entropy of
adsorption (DS0ads) were calculated. The inhibition effect is discussed in view of Delonix regia
molecules adsorbed on the metal surface and it obeys Langmuir adsorption isotherm. Polarization
curve indicates that inhibitor act as mixed type and the I.E. was found up to 93.48 %. The results
obtained showed that the leaves extract could serve as an effective green inhibitor for corrosion of
aluminium in hydrochloric acid.
Key words: Aluminium, Hydrochloric acid, Delonix regia , Corrosion, Inhibitor.
Aluminium is one of the metals which is used in different human activities and many of important
application. Aluminum is widely used in various industrial operations due to light weight, high
thermal and electrical conductivity, relative high mechanical strength. Aluminium and its alloy are
widely used in construction, vessels, pipes, machinery and packing. Aluminium is used in electronics
due to it is super purity . Hydrochloric acid solution is one of the most currently used acids in the
pickling and electrochemical etching of aluminium capacitor foil .
In the recent years, there is an increasing awareness of environment and green chemistry. Therefore,
many works were conducted to use the environmentally safe, readily available and cheap substances,
as corrosion inhibitors, instead of the harmful synthetic chemicals [3-7]. Delonix regia contains large
amount of terpenoids, polyphenolic compounds, tannins, cardiac glycosides and anthroquinones .
The aim of the present study is to investigate the corrosion inhibition effect of Delonix regia as a
cheap and environment friendly corrosion inhibitor for aluminium in various concentration of HCl
medium by weight loss, effect of temperature, polarization and EIS techniques.
2. Experimental section
2.1 Preparation of sample and solution
The aluminium specimens with a chemical composition of 99.54 % Al, 0.090 % Si, 0.320 % Fe,
0.0012 % Cu, 0.0034 % Mn, 0.0014 % Mg, 0.0042 % Cr, 0.0046 % Ni, 0.0020 % Zn, 0.0079 % Ti,
0.0005 % Pb, and 0.0026 % Sn were used in the present study. The metal sheet, test specimens of size
5.0 x 2.50 x 0.198cm having an effective area of 0.279 dm2were used. The specimens were cleaned by
washing with distilled water, degreased by acetone, washed once more with doubled distilled water
and finally dried and weighted by using electronic balance. Hydrochloric acid was used as corrosive
solution having concentration of 0.75, 1.0 and 1.25 M prepared by diluting analytical grade of HCl
purchased from Merck using double distilled water.
2.2 Preparation of extract
The extract of leaves of Delonix regia was prepared as follows: fresh leaves of Delonix regia were
dried and ground into powder, then 0.2g of powder was put into a 200ml flat bottom flask containing
100ml of 2 M HCl. The resulting solution was boiled for 30 min and left overnight before filtering.
The filtrate was diluted with the appropriate quantity of 2 M HCl to obtain 0.6, 0.8, 1.0 and 1.2g/L
concentrations. The above process was repeated for the preparation of the acid leaves extract .
2.3 Weight loss measurements
For weight-loss measurement, the aluminium specimen having an area of 0.2797 dm2 were each
completely suspended in 230 ml of 0.75, 1.0 and 1.25 M HCl solution with and without different
Delonix regia leaves extract concentrations using glass hooks at 301± 1 K for 24h. The coupons were
retrieved after 24h, washed with distilled water, dried well and reweighed. From the weight loss data,
corrosion rate (CR) was calculated.
2.4 Temperature effect
To study the effect of temperature on corrosion rate, aluminium coupons were completely immersed
in 230 ml of 0.75M HCl solution without and with different concentrations of Delonix regia leaves
extract at 313, 323 and 333K for 2h. From the data corrosion rate, inhibition efficiency (I.E.),
activation energy (Ea) and heat of adsorption (Qads) and free energy of adsorption (∆G0ads) were
2.5 Potentiodynamic polarization measurements
Both the potentiodynamic and EIS measurement were carried out using CHI608C –series, U.S. Model
with CH- instrument. For polarization study, metal specimens were immersed with and without
Delonix regia leaves extract in 0.75 M HCl solution. In the electrochemical cell aluminium
specimens having an area of 1 cm2was used as a working electrode, Ag/AgCl electrode as a reference
electrode and platinum electrode as an auxiliary electrode and allowed to establish a steady-state open
circuit potential (OCP) for approximately 65 min. The polarization curves were plotted with current
Vs potential. An anodic and cathodic polarization curve gives corresponding anodic and cathodic
Tafel lines.The intersect point of Tafel lines gives the corrosion potential(Ecorr) and corrosion current
2.6 Electrochemical Impedance Spectroscopy (EIS) measurements
EIS measurements were made at corrosion potentials over a frequency range of 1 to 105 Hz by a sine
wave with potential perturbation amplitude of 5 mV. A graph was drawn by plotting real impedance
(Z’) versus imaginary impedance (-Z’’). From the Nyquest plots of Z’ Vs-Z’’ the charge transfer
resistance (Rct) and double layer capacitance (Cdl) were calculated. An experiment was carried out in
absence and presence of inhibitor.
3. Results and discussion
3.1 Weight loss experiments
The weight loss experiments was carried out in 0.75, 1.0 and 1.25 M HCl solution containing 0.6, 0.8,
1.0 and 1.2 g/L concentration of Delonix regia leaves extract at 301±1 K for a exposure period of 24h
was investigated. Corrosion rate (C.R.) was calculated using following equation:
C.R. (mg/dm 2d) = Weight loss (gm.) x 1000 / Area in dm2 x day
Inhibition efficiency (I.E.) was calculated by using following equation,
I. E. ሺ%ሻ = ቄ
൫౫ ି ൯
ቅ × 100
Where: Wu = Weight loss in absence of inhibitor, Wi = Weight loss in presence of inhibitor.
The degree of surface coverage (θ) of the aluminium specimen for different concentration of HCl
solution have been evaluated by weight loss experiments using the following equation,
ሺ౫ ି ሻ
Results showed in Table-1 indicate that as the acid concentration increases corrosion rate increase
while I.E. decreases. Corrosion rate was 2195.20, 6163.74 and 8283.87 mg/dm 2d corresponding to
0.75, 1.0 and 1.25 M HCl concentrations respectively for an immersion period of 24 h at 301± 1 K
(Figure-1). At constant acid concentration, as the inhibitor concentration increases corrosion rate
decreases while I.E. increases. For example, in 0.75 M HCl solution, the I.E. was found to be 56.02,
66.61, 83.55 and 93.48 % corresponding to 0.6, 0.8, 1.0 and 1.2 g/L inhibitor concentration
respectively (Table-1 and Figure-2).
Table-1. Effect of HCl concentration on corrosion rate (C.R.) and inhibition efficiency (IE)
of aluminium having different concentration of Delonix regia leaves extract.
Table-2: Inhibition efficiency (I.E.), Corrosion rate (ρ) and Surface coverage (θ) of
Delonix regia leaves extract on aluminium in 0.75 M HCl for an immersion
period of 24h at 301 ± 1 K.
Corrosion rate (mg/dm2d)
Inhibitor concentration (g/L)
Fig.1: Corrosion rate of aluminium in 0.75 M HCl solution in absence and presence
of different concentration Delonix regia leaves extract for 24 h.
Inhibition efficiency (I.E.)
Inhibitor concentration (g/L)
Fig.2: Inhibition efficiency of aluminium corrosion in 0.75 M HCl solution in presence
of different concentration Delonix regia extract for an immersion period of 24h.
3.2 Temperature effect
To investigate the influence of temperature on corrosion of aluminium, the weight loss experiments
were also carried out at 313, 323 and 333K in 0.75 M HNO3in absence and presence of Delonix regia
for an immersion period of 2h. The results in Table-3 shows that corrosion rate increases with rise in
temperature. Corrosion rate was 21451.44, 26642.76 and 43417.92 mg/dm 2d corresponding to 313,
323 and 333K respectively. The value of energy of activation (Ea) has been calculated with the help of
Arrhenius equation .
ቀ − ቁ
Where ρ1 and ρ2 are the corrosion rate at temperature T1 and T2 respectively.
Table-3. Temperature effect on corrosion rate (CR), activation energy (Ea) and heat of
adsorption (Qads) for aluminium in 0.75 M HCl in absence and presence of Delonix
regia extract for an immersion period of 2 h.
( kJ/ mol)
log ρ (Corrosion rate)
Results given in Table-3, indicates that the values of Ea were higher in inhibited acid ranging from
71.54to 92.83 kJmol-1 than Ea value for uninhibited acid (30.92 kJ/mol) which indicates physical
adsorption of the inhibitor on metal surface and the adsorption of inhibitor causes an increase in the Ea
value of the process . Results of Table-3 indicates that as temperature increases, rate of corrosion
increase while percentage of I.E. decreases. The value of Ea were also calculated from the slope of the
Arrhenius plot of log ρ versus 1/T x 1000 (Figure-3) shows good agreement with the calculated
1/T × 1000 (K)
Fig.3 : Arrhenius plots for aluminium in 0.75 M HCl in absence and presence of the different
concentration of Delonix regia extract.
The values of heat of adsorption (Qads) were calculated by using the following equation .
Qୟୢୱ = 2.303R ቂlog ቀ
ቁ − log ቀଵିభ ቁቃ ቂ భିమ ቃ
Where, θ1 and θ2 are the fraction of the metal surface covered by the inhibitor at temperature T1 and
T2respectively. It is evident that in all cases, the value of Qୟୢୱ were negative and ranging from -18.84
to -119.10 kJ/mol. The negative values shows that the adsorption and hence the I.E. decreases with
rise in temperature.
The surface coverage ‘θ’ value was calculated by using equation- 3. A plot of inhibitor concentration
Cinh versus Cinh/θ was presented in Figure-3 which gives straight line with slope values equal to unity
indicates that the system follows Langmuir adsorption isotherm . This isotherm can be
Where, Kads is the equilibrium constant and Cinh is the inhibitor concentration.
Fig.4: Langmuir adsorption isotherm plot for aluminium in 0.75 M HCl containing Delonix regia as
green inhibitor at 301 K.
Free energy of adsorption (ΔGºads) was determined by the Langmuir isotherm was given by a plot of
C/θ versus C (Figure-4) . From the intercepts of the straight line on the C/θ axis, Kads can be
calculated which was related to ΔGºads as given by the following equation [15,16].
ΔGºads = -RT ln (55.5 Kads )
Where , R is the gas constant, T is the concentration of water in solution in Molar [17 ], Kads is the
equilibrium constant of the adsorption/desorption process. The mean ΔGºads value was negative
(-13.82 kJ/mol) indicating that the adsorption mechanism of Delonix regia on aluminium in 0.75 M
HCl at the studied temperatures is physisorption with adsorptive layer having electrostatic character
. This is concluded on the fact that the values of ΔGºads -20 kJ/mol are consistant with
physisorption, while those around -40 kJ/mol or higher are associated with chemisorption .
The enthalpy of adsorption (ΔHºa) and entropy of adsorption (ΔSº a ) were calculated using the
equations (8) and (9),
ΔHºa = Ea –RT
ΔSº a = ΔHºa - ΔGºa /T
Results indicates that values of ΔHºa were positive and increase in presence of inhibitor indicating a
higher degree of surface coverage and higher protection efficiency attained due to raising the energy
barrier for the aluminium corrosion reaction. The enthalpy change ΔHºa was positive and ranging
between 56.18 to 126.41 kJ/mol indicating the endothermic nature of the reaction suggests that higher
temperature favours the corrosion process. Positive values of ΔSºa ranging from
0.225 to 0.450 kJ /mol indicates that corrosion process is entropically favourable.
3.3 Potentiodynamic polarization study
Figure-5 represents the potentiodynamic polarization curves of aluminium in 0.75 M HCl in absence
and presence of Delonix regia leaves extract. Electrochemical parameters such as corrosion potential
(Ecorr),corrosion current density (icorr), anodic Tafel slope(βa), cathodicTafel slope (βc) and percentage
inhibition efficiency (I.E.) were given in Table-4.
Table -4. Potentiodynamic polarization data and Inhibition efficiency (I.E.) of Delonix regia
leaves extract as green inhibitor for aluminium in 0.75 M HCl.
(mV / decade)
Fig.5(a):Polarization curve for corrosion of aluminium in 0.75 M HCl in absence of inhibitor.
Fig.5(b): Polarization curves for aluminium in 0.75 M HCl in presence of 1.2 g/L Delonix
From Table-4, it was observed that the addition of Delonix regia leaves extract in acid solution
indicates the significant decrease in corrosion current density (icorr) and decrease in corrosion rate
with respect to blank. There is significant change in the anodic and cathodic slopes after the addition
of the inhibitor. This Tafel curves indicate that Delonix regia function as a mixed type inhibitor with
the predominant cathode effectiveness (Figure-5 and Figure-6).
Inhibition efficiency (I.E.) from polarization study was calculated using following equation .
I. E. ሺ%ሻ =
3.4 Electrochemical impedance spectroscopy (EIS) measurements
Nyquist plots for the corrosion of aluminium in 0.75MHCl solution in absence and presence of
Delonix regia leavesextract wasexamined by EIS method at room temperature was shown in Figure-7
and EIS parameters in Table-5.
Table-5. EIS parameters for corrosion of Aluminium in 0.75 M HCl containing Delonix regia
(µF / cm2)
Weight loss method
Fig.6 : Nyquist plot for aluminium in (A) in 0.75 M HCl (Blank) (B) in 0.75 M HCl in presence of
1.2 g/L Delonix regia extract.
It is observed from Figure-6 that the impedance diagram is almost semi circular. The difference has
been attributed to frequency dispersion. The semi circular nature of the plots indicates that the
corrosion of aluminium is mainly controlled by charge transfer process.
The diameter of capacitive loop in the presence of inhibitor is bigger than that in the absence
of inhibitor. The high frequency capacitive loop is related to the charge transfer resistance (Rct). To
calculate the double layer capacitance (Cdl), the frequency at which the imaginary component of the
impedance is maximum was found as presented in the following equation .
Where’ f’ is the frequency at the maximum height of the semicircle on the imaginary axis and Rct is
the charge transfer resistance .
Inhibition efficiency (I.E.) from EIS method was calculated using following equation:
ܫ. ܧ. ሺ%ሻ =
The addition of inhibitor increase Rct value while decreases in Cdl values which is due to the
adsorption of inhibitor on the metal surface. The results suggest that the inhibitor acts by the
formation of a physical protective layer on the surface that retards the charge transfer process and
therefore inhibit the corrosion reaction, leading to increase in Rct values. Moreover, the adsorbed
inhibitor species decrease the electrical capacity of electrical double layer values at the
electrode/solution interface and therefore decrease the value of Cdl .
3.5 Mechanism of corrosion
Generally, aluminium dissolves in acid solutions due to hydrogen evolution type of attack, the
reactions taking place at the microelectrode of the corrosion cell being represented as [24,25]:
Al → Al+3 + 3 e− (anodic reaction) ….
H+ + e- → Hads (cathodic reaction)
Followed by the reactions:
H + H → H2(g)
The following secondary reactons can also take place in acid solutions :
2M + 2H+ → H2 + 2M+ (anodic) ….
O2 + 4H+ + 4e- → 2H2O (cathodic) ….
3.6 Mechanism of inhibition
Chou and Leu  noted that Delonix regia leaves extract contains some phytochemicals with hetero
atoms (N,S,O) having higher molecular weight such as 4-hydroxybenzoic acid, gallic acid, 3,4dihydroxycinnunmic acid, 3,5-dinitrobenzoic acid, alkaloids, L-azetidine-2-carboxylic acid, amine
oxide base and 3,4-dihdroxybenzaldehyde etc. Structure of some of these compounds are shown in
adsorbed on the surface of metal. Since, a physisorption mechanism is indicative of a weak
adsorption bond , it may be possible that at higher temperatures, the decrease in I.E. may be a result
of increased agitation of the solution resulting from higher rates of hydrogen evolution, thereby
reducing the ability of the molecules to be adsorbed on the surface of the metal.
On the basis of the study the following conclusions can be drawn:
1. As acid concentration increases corrosion rate increases while I.E. decreases.
2. At constant acid concentration, as inhibitor concentration increases corrosion rate decreases
while I.E. increases.
3. As temperature increase corrosion rate increases while I.E. decreases.
4. The leaves extract of Delonix regia showed maximum I.E. of 93.48% at an optimum
concentration of 1.2 g/L.
5. The values of Ea obtained in the presence of the extract were higher compared to the blank acid
solution which indicates that inhibitor was more effective at lower temperature.
6. The values of ΔGºads were negative, which reveals the spontaneous adsorption of inhibitor onto
7. Plot of C/θ versus C shows straight line with almost unit slope, which suggest that the
inhibitor cover both anodic and cathodic regions through general adsorption following
8. Polarization curves indicates that the Delonix regia extract act as mixed type of inhibitor.
The authors are thankful to the Department of Chemistry, Navyug Science College, Surat for
providing laboratory facilities.
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