J Mater Sci (2009) 44:274–279

DOI 10.1007/s10853-008-3045-8

Raphia hookeri gum as a potential eco-friendly inhibitor for mild

steel in sulfuric acid
S. A. Umoren Æ I. B. Obot Æ N. O. Obi-Egbedi

Received: 17 August 2008 / Accepted: 9 October 2008 / Published online: 10 November 2008
Ó Springer Science+Business Media, LLC 2008

Abstract Exudate gum from Raphia hookeri (RH) was industry. As a result of corrosion, enormous loss is incurred
tested as corrosion inhibitor for mild steel in H2SO4 using due to loss of production, inefficient production, high
weight loss and hydrogen evolution techniques at 30– maintenance, and the cost of corrosion control chemicals.
60 °C. Results obtained revealed that RH act as corrosion Among the several methods devised to control metallic
inhibitor for mild steel in sulfuric acid medium. The cor- corrosion, the uses of inhibitors often remain the most
rosion rates in all concentrations studied increased with rise practical and cost-effective means. Inhibitors may be
in temperature. The inhibition efficiency was observed to regarded in general terms as a substance which when added
increase with increase in RH concentration but decreased in a small concentration to an environment, effectively
with rise in temperature, which is suggestive of physical reduces the corrosion rate of a metal exposed to that envi-
adsorption mechanism. The inhibitive action of RH is ronment. However, not just any chemical compound can be
discussed in view of the adsorption of its phytochemical used as a corrosion inhibitor. There are some requirements
components onto steel surface, which protects the metal that the compound must fulfill to do so. The chemical
surface and thus do not permit the corrosion process to take structure and behavior of the compound is an important
place. The adsorption of the exudate gum onto the steel consideration. In this regard, inorganic compound must be
surface was found to follow the Langmuir adsorption iso- able to oxidize the metal, forming a passive layer on its
therm. The free energies for the adsorption process and the surface. On the other hand, a molecule of an organic com-
apparent activation energies, enthalpies and entropies of pound should posses a large structure, p bond, an active
the dissolution process were determined. The fundamental center or group, etc. These features give the molecule the
thermodynamic functions were used to glean important ability to cover a large area of a metal surface with a firmly
information about the RH inhibitory behavior. The results attached film [1]. Other considerations when choosing an
were explained in terms of chemical thermodynamics. inhibitor include: (1) the cost of the inhibitor which can be
sometimes very high when the material involved is expen-
sive or when the amount needed is huge; (2) toxicity of the
Introduction inhibitor which can cause jeopardizing effects on human
beings, and other living species; (3) availability of the
Corrosion due to acids is important and expensive problem inhibitor, and (4) environmental friendliness.
in chemical industries including the petroleum refining Naturally occurring substances of both plants and ani-
mal origin otherwise tagged ‘green inhibitors’ are known to
meet these requirements. The use of plants extracts as
S. A. Umoren
I. B. Obot (&) corrosion inhibitors have generated a lot of interest in
Department of Chemistry, Faculty of Science, University recent times [2–7]. Among plant materials tested in our
of Uyo, P.M.B 1017, Uyo, Akwa Ibom State, Nigeria laboratory include Dacroydes edulis [8], Pachylobus edulis
e-mail: proffoime@yahoo.com
[9], Vigna unguiculata [10], and Gum arabic [11, 12]. The
N. O. Obi-Egbedi encouraging results obtained from previous investigations
Department of Chemistry, University of Ibadan, Ibadan, Nigeria permit us to test more plant materials.

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J Mater Sci (2009) 44:274–279 275

The aim of this present work is to study the effect of W0

Surface coverage ðhÞ ¼ 1  : ð3Þ
exuduate gum from Raphia hookeri (RH) on the corrosion W1
of mild steel in acidic environment at 30–60 °C using
weight loss and hydrogen evolution methods. Thermody- Gas-volumetric experiments
namics is used to effectively characterize the mild steel
dissolution and to properly elucidate the inhibition mech- The apparatus and procedure for gas-volumetric determi-
anism of the corrosion process. nation of corrosion rates have been described elsewhere
[11, 19]. The progress of the corrosion reaction was
monitored by careful volumetric measurement of the
evolved hydrogen gas at fixed time intervals. Experiments
Experimental details were conducted at 30–60 °C.

Corrosion tests were performed on a mild steel of the

following percentage composition: 0.21% C; 0.38% Si; Results and discussion
0.09% P; 0.01% Al; 0.05% Mn; 0.05% S, and the
remainder iron. The metal was mechanically cut into Weight loss, corrosion rate, and inhibition efficiency
coupons of dimensions 5 cm 9 4 cm (total surface
area = 20 cm2). The coupons were used as cut without Figure 1 shows a representative plot of weight loss against
further polishing. However, they were degreased using time (days) for mild steel in 0.1 M H2SO4 solution con-
absolute ethanol (BDH) dried in acetone (BDH) and stored taining no inhibitor (blank) and in the presence of different
in a dessicator devoid of moisture prior to use in corrosion concentrations of RH exudate gum at 30 °C. Similar plots
studies [13, 14]. RH exudate gum was sourced locally. The were obtained for other temperatures (40–60 °C). The
exudate gum was purified following the method of Ekpe figure clearly shows a reduction in weight loss of the metal
et al. [15] and reported elsewhere [8, 9]. The concentra- coupons in the presence of different concentrations of the
tions of inhibitor (RH) prepared and used in the study were exudate gum compared to the free acid solution (blank).
0.1–0.5 g/L. The concentration of H2SO4 (BDH) used was The corrosion rates of the mild steel coupons in 0.1 M
0.1–1 M. H2SO4 with and without different concentrations of exu-
The apparatus and procedure followed for weight loss date gum were determined using weight loss at 30–60 °C.
measurements were similar to that earlier reported [12–14]. The results obtained are listed in Table 1. The corrosion
Experiments were carried out under total immersion con- rate decreases with increasing concentration of the gum.
ditions in 100 mL of test solutions maintained at 30–60 °C. This indicates that the exudate gum in the solution inhibits
All tests were made in aerated solutions. The coupons were the corrosion of mild steel in H2SO4 and that the extent of
retrieved at 24 h intervals progressively for 168 h, corrosion inhibition depends on the amount of the extract
immersed in 20% NaOH solution containing 200 g/L of present.
zinc dust, scrubbed with bristle brush, washed, dried, and Table 1 also shows the calculated values of %I for mild
weighed [16]. The results reported are averages of triplicate steel in 0.1 M H2SO4 in the presence of different
determinations. The corrosion rates of mild steel in H2SO4
without and with different concentrations of exudate gum 1.6
Blank 0.1 g/l
were calculated using the expression [17]: 1.4
0.2 g/l 0.3 g/l
534W
Corrosion rate ðmpyÞ ¼ ; ð1Þ 1.2
qAt 0.4 g/l 0.5 g/l
Weight loss (g)

1
-3
where W is the weight loss (gdm ), q the density of
0.8
specimen (gcm-3) (1.15 g/cm3), A the area of specimen
(cm2), and t the exposure time (h). 0.6

From the values of corrosion rate in the presence (W1) 0.4

and absence of inhibitor (W0), the inhibition efficiency (%I)
0.2
of RH was calculated from [18]:
  0
W0 1 2 3 4 5 6 7
Inhibition efficiencyð%I Þ ¼ 1   100: ð2Þ Time (days)
W1
The degree of surface coverage (h) was obtained from Fig. 1 A plot of weight loss against time for mild steel corrosion in
0.1 M H2SO4 in the absence and in the presence of different
Eq. 3 concentrations of exudate gum from RH at 30 °C

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276 J Mater Sci (2009) 44:274–279

Table 1 Calculated values of corrosion rate (mpy) and %I for mild steel in 0.1 M H2SO4 in the absence and presence of various concentrations
of RH at different temperatures using weight loss
Concentration (g/L) Corrosion rate (mpy) 9 10-3 %I
30 °C 40 °C 50 °C 60 °C 30 °C 40 °C 50 °C 60 °C

Blank 48 57 72 76 – – – –
0.1 37 39 71 75 63.1 60.2 57.4 40.8
0.2 34 35 68 73 65.2 62.4 60.5 45.2
0.3 31 33 66 67 69.1 65.2 64.2 50.0
0.4 29 30 52 51 70.8 68.5 67.0 54.1
0.5 27 29 46 49 71.9 71.1 70.3 56.5

concentrations of exudate gum at 30–60 °C from weight loss H2 gas, which bubbles from the surface [20]. The relative
method. The %I was found to increase with increase in the rapidity and effectiveness of the gas-volumetric technique
concentration of the exudate gum. This observation is in as well as its suitability for monitoring in situ, any per-
agreement with those previously reported in H2SO4 [20–22]. turbation by an inhibitor with respect to gas evolution in
The %I reached a maximum of 71.9% at 0.5 g/L exudate metal/corrodent systems have been established in earlier
concentration at 30 °C. From the values of %I, it is clear that reports [11, 14, 23]. Figure 2 shows the plot of the volume
the corrosion inhibition may be due to the increase in the of H2 evolved as a function of time for mild steel corrosion
adsorption of phytochemical constituents of the exudate gum in 1 M H2SO4 in the absence and presence of different
on the metal surface. The adsorption may also be due to the concentrations of the inhibitor (exudate gum from RH) at
negatively charged metal surface and the protonated species 30 °C. Similar plots were obtained at 40–60 °C. The
of the constituents in the acidic solution on the metal surface results illustrate the decrease deflection of H2 gas evolution
[19]. Owing to the complex chemical composition of the rate on introduction of the exudate gum into the corrodent,
exudate gum, it is quite difficult to assign the inhibitive effect indicating that the exudate gum from RH actually affords
to a particular constituent. Initial physiochemical analysis of corrosion inhibition of mild steel in the acidic
the exudate gum identified the presence of hexuronic acid environments.
and neutral sugar residues, volatile monoterpenes, canaric The %I was calculated using Eq. 7:
and related triterpene acids, reducing and non-reducing  1

VHt
sugars [15]. Mutual adsorptive effects of these compounds %I ¼ 1  0  100; ð7Þ
VHt
and other components present in the exudate gum cannot be
ruled out in the adsorption process. The adsorption of these where VH1 is the volume of hydrogen evolved at time ‘t’ for
components on the mild steel surface reduces the surface inhibited solution and VH0 that for uninhibited solution.
area available for corrosion [9]. Further investigation using Results shown in the table follow the same trend
surface analytical techniques will enable the characterization observed for weight loss measurements. %I increases with
of the active materials in the adsorbed layer and identifica-
tion of the most active species. 45
Blank 0.1g/l
Volume of Hydrogen evolved (cm3)

40
Hydrogen evolution measurements 35
0.2g/l 0.3g/l
30
The spontaneous corrosion of iron can be represented by
25 0.4g/l 0.5g/l
the anodic dissolution reaction:
20
Fe ! Fe2þ þ 2e: ð4Þ
15
There must normally be a cathodic reaction to consume 10
the electrons produced. At acidic pH, the cathodic reaction
5
results in hydrogen evolution as follows:
0
2Hþ þ 2e ! 2Hads ; ð5Þ 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30
Time (min)
2Hads ! H2 : ð6Þ
Fig. 2 A plot of volume of hydrogen evolved against time for mild
Hads, atomic hydrogen adsorbed on the metal surface, steel corrosion in 0.1 M H2SO4 devoid and in the presence of
reacts by combining with other adsorbed H atom to form different concentrations of exudate gum at 30 °C

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J Mater Sci (2009) 44:274–279 277

Table 2 Calculated values of %I for mild steel in 1 M H2SO4 in the metal surface as a result of enhanced corrosion, may also
absence and presence of various concentrations of RH at different reduce the ability of the inhibitor to be adsorbed on the
temperatures using hydrogen evolution method
metal surface [20]. This result supports the idea that the
Concentration (g/L) %I adsorption of the exudate gum components onto the steel
30 °C 40 °C 50 °C 60 °C surface is physical in nature. Thus, as the temperature
increases, the number of adsorbed molecules decreases,
Blank – – – – leading to a decrease in the inhibition efficiency. It has
0.1 17.4 15.1 14.7 13.5 been suggested that adsorption of an organic inhibitor can
0.2 29.9 20.4 19.7 16.6 affect the corrosion rate by either decreasing the available
0.3 53.9 44.4 39.9 29.7 reaction area (geometric blocking effect) or by modifying
0.4 51.4 54.9 42.2 31.1 the activation energy of the anodic or cathodic reactions
0.5 62.0 56.6 44.1 41.2 occurring in the inhibitor-free surface in the course of the
inhibited corrosion process [26].
increase in concentration and decreases with temperature Figure 3 depicts an Arrhenius plot (corrosion rate
rise. against the reciprocal of temperature (1/T) for mild steel in
Comparison of inhibition efficiencies calculated from 0.1 M H2SO4 solution in the absence and presence of dif-
the weight loss and hydrogen evolution methods shows that ferent RH concentrations. Satisfactory straight lines of high
the values of %I obtained from hydrogen evolution method correlation coefficients were obtained and the activation
(Table 2) are lower than that obtained from weight loss energy can be obtained from the slopes. The activation
method (Table 1). This may be attributed to the difference energies obtained are listed in Table 3. The apparent acti-
in time required to form an adsorbed layer of the inhibitor vation energy obtained for the corrosion process was found
on the metal surface that can inhibit corrosion [24]. to be 13.2 kJ/mol. It is clear that Ea values in the presence
of RH are much higher than those in the absence of RH.
Effects of temperature The higher activation energies mean a slow reaction and
that the reaction rate is very sensitive to temperature. The
Rodovici [25] has classified inhibitors into three groups increase in apparent activation energy in the presence of
based on the effect of temperature viz: (i) Inhibitors whose exudate gum denotes physical adsorption [27]. This con-
%I decreases with temperature increase; the value of the clusion is denoted by the decrease in inhibition efficiency
apparent activation energy, Ea, found is greater than that in with increasing temperature (Table 1). Similar result has
the uninhibited solution. (ii) Inhibitors in whose %I does been reported by El-Etre [1] on the inhibition of acid
not change with temperature variation; the apparent acti- corrosion of carbon steel using aqueous extract of olive
vation energy Ea does not change with the presence or leaves. Moreover, the increase in activation energy is
absence of inhibitors. (iii) Inhibitors in whose presence the proportional to the inhibitor concentration, indicating that
%I increases with temperature increase, while the value of the energy barrier for the corrosion process is also
Ea for the process is smaller than that obtained in the
uninhibited solution. Thus in examining the effect of -1
temperature on the corrosion process in the presence of the Blank 0.1g/l 0.2g/l
exudates, the Arrhenius equation is helpful: -1.1
0.3g/l 0.4g/l 0.5g/l
Ea
log CR ¼ þ log A; ð8Þ
2:303RT -1.2
Log CR

where ‘CR’ is the corrosion rate, Ea the apparent activation

-1.3
energy, R the molar gas constant, T the absolute tempera-
ture, and A is the frequency factor.
The influence of temperature on the corrosion behavior -1.4

of mild steel in 0.1 M H2SO4 in the absence and presence

of exudate gum of varying concentrations was investigated -1.5

by weight loss method in the temperature range 30–60 °C

during 168 h of immersion. Inhibition efficiency decreases -1.6
3 3.09 3.19 3.3
with increase in temperature. This may be attributed to a
1/T (K-1) x10-3
possible shift of the adsorption–desorption equilibrium
toward desorption of adsorbed inhibitor due to increased Fig. 3 Arrhenius plot for mild steel corrosion in 0.1 M H2SO4 in the
solution agitation. This, as well as the roughening of the absence and presence of various concentrations of exudate gum

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Table 3 Activation parameters of mild steel in 0.1 M H2SO4 with decided by the pre-exponential factor. When the concen-
and without RH exudate gum tration is high, the decrease in steel corrosion rate is chiefly
Systems/ Ea (kJ/mol) DHo (kJ/mol) DSo (J/mol/K) decided by the kinetic parameters of activation. These
concentrations (g/L) results are in agreement to those obtained in literature [30].
The positive values of DHo mean that the dissolution
Blank 13.2 8.3 -26.6
reaction is an endothermic process and that the dissolution
0.1 22.6 19.9 -28.5
of steel is difficult [31]. Also, the entropy DSo increases
0.2 24.5 20.1 -29.3
negatively with the presence of the inhibitor than the non-
0.3 24.9 22.4 -29.6
inhibited one. This reflects the formation of an ordered
0.4 25.5 23.3 -33.7
stable layer of inhibitor on the steel surface [24].
0.5 26.7 24.0 -34.5

Adsorption isotherms
increased [28]. The increase in activation energy with
inhibitor concentration is often interpreted by physical It is a general assumption that the adsorption of the organic
adsorption with the formation of an adsorptive film of an inhibitors at the metal interface is the first step in the
electrostatic character [27] (Table 3). mechanism of the inhibitors action. Organic molecules may
An alternative formulation of Arrhenius equation is adsorbed on the metal surface in four types: (i) electrostatic
[29]: interaction between the charged molecules and the charged
   o   metal surface, (ii) interaction of unshared electron pairs in
RT DS DH o
CR ¼ exp exp ; ð9Þ the molecule and the metal, (iii) interaction of p-electrons
Nh R RT
with the metal, and (iv) a combination of the types (i)–(iii).
where h is the Planck’s constant, N the Avogadro’s num- The values of degree of surface coverage (h) defined as the
ber, T the absolute temperature, R the universal gas fraction of the mild steel surface that was covered by the
constant, DSo the entropy of activation, and DHo is the inhibitor and obtained directly from %I (h = %I/100) are
enthalpy of activation. Figure 4 shows a plot of log (CR/T) quite useful in determining inhibitor adsorption character-
as a function of 1/T. Straight lines were obtained with a istics. Surface coverage data are applied in construction of
slope of (–DHo/R) and an intercept of (ln R/Nh ? DSo/R) adsorption isotherms, which give detailed information on
from which the values of DHo and DSo were calculated, and adsorption mechanisms. Attempts were made to fit (h)
listed in Table 3. From the data presented in Table 3, it values to the Frumkin, Freundlich, Temkin, and Langmuir
seems that Ea and DHo vary in the same manner. Both the isotherms and correlation coefficient (R2) values were used
activation energy and the enthalpy of activation increase as to determine the best-fit isotherm. By far, best result was
the concentration of the exudate gum increases from 0.1 g/L obtained for Langmuir isotherm. The fitting to the Lang-
till it reaches 0.5 g/L of the exudate gum. This phenome- muir isotherm is shown by plotting C/h versus C (Fig. 5)
non is interpreted by the fact that at lower concentration of according to the equation [1, 31]:
RH, the reduction in mild steel corrosion rate is chiefly C 1
¼ þ C; ð10Þ
h Kads
-0.3

-0.32
Blank 0.1g/l 0.2g/l 1
-0.34 0.9
0.3g/l 0.4g/l 0.5g/l
-0.36 0.8
LogCR/T

-0.38 0.7

-0.4 0.6
C/θ

0.5
-0.42
0.4 30oC
-0.44
0.3 40oC
-0.46 50oC
0.2
60oC
-0.48
0.1
-0.5 0
3 3.09 3.19 3.3 0.1 0.2 0.3 0.4 0.5
1/T (K-1) x10-3 C (g/l)

Fig. 4 Transition state plot for mild steel corrosion in 0.1 M H2SO4 Fig. 5 The Langmuir adsorption isotherm for mild steel corrosion in
in the absence and presence of various concentrations of exudate gum 0.1 M H2SO4 at 30–60 °C

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J Mater Sci (2009) 44:274–279 279

Table 4 Adsorption parameters for the adsorption of exudate gum 4. The thermodynamics/kinetics parameters obtained
from RH in 0.1 M H2SO4 on mild steel at different temperatures indicate that the adsorption of RH onto the mild steel
Temperature (°C) Kads R2 DGoads (kJ/mol) surface is spontaneous.
5. Activation energies were higher in the presence of
30 32.25 0.999 -18.87 the exudates gum suggesting the physiosorption
40 21.74 0.997 -18.46 mechanism.
50 18.51 0.996 -18.62
60 9.17 0.994 -17.25

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