An adequate and environmentally safe corrosion protection technique has been
linked to improved and sustainable pollution free environment. The challenges
associated with the use of harmful synthetic inhibitors in allied chemical industries
have continuously been heightened by concerns for the environment. Although
synthetic inhibitors like aniline and sodium nitrite (Okeniyi
et al., 2012) represent a rich source of effective inhibitors, its
damaging nature is a concern for the future. Thus, employing environmentally
friendly sources for corrosion protection has gained wide acceptance across
the globe. This is because these green sources have sustained potentials for
abundant and cheap supply.
The use of plants as corrosion inhibitors requires establishing if they contain
substances such as tannins, terpenes, alcohols, polyphenols, carboxylic acids
and alkaloid. These substances have been proven to have corrosion inhibiting
potentials (Loto et al., 2011; Saratha
et al., 2009; Singh et al., 2010;
Valek and Martinez, 2007; Oguzie,
2006, 2008; Okafor et al.,
2008, 2010; De Souza and Spinelli,
2009; El-Etre, 2003; Ajayi et
al., 2011a-c; Prabhu et
al., 2009). In the case of Bambusa Bambos (BB), its leaves have been
established to contain substances such as tannin and lignin which are capable
of inhibiting corrosion. In addition, it is environmentally friendly and not
harmful to humans.
Thus, several researchers have investigated the use of some plant extracts
on the corrosion of zinc in acidic media (Abiola and James,
2010; El-Etre et al., 2005; Mahmoud,
2008; El-Gaber et al., 2008; Shylesha
et al., 2011; Wang et al., 2003; Ghanbari
et al., 2009; El-Aila et al., 2011)
and all such studies suggest that, apart from the fact that green inhibitors
can be cheaply sourced and applied without contaminating the environment, they
are also organic compounds containing electron donor atoms particularly nitrogen,
sulfur and oxygen in their functional groups with aromatic and heterocyclic
rings with corrosion inhibiting capability. However, none of the investigations
were done on zinc in Hydrochloric acid (HCl) in the presence of BB. For instance,
Abiola and James (2010) reported the effects of Aloe
vera extract on the corrosion of zinc in HCl solution using weight loss methods,
while El-Etre et al. (2005) reported the outcome
of the effect of henna (lawsonia) leaf extract on the corrosion inhibition
of carbon steel, nickel and zinc in acidic, neutral and alkaline solutions employing
Thus, these reports are different from the focus of the present study whose
object is to investigate the effect of acid extract of BB on the deterioration
of zinc in 2 M HCl solution at a temperature of 298 K using gasometric method.
Furthermore, this investigation also analyzed the various indices that portrayed
the behavior of zinc metal in relation with extract quantity, metal-phytochemical
extract adsorption interaction mechanism and micrograph examination. In addition,
Inhibitor Efficiency (I.E) was determined by the method adopted by Ajayi
et al. (2011a) and Okafor et al. (2010).
MATERIALS AND METHODS
Corrosion tests using gasometric measurements were performed using zinc metal
coupons of dimensions 2.5x1.5 cm. The metal sheets were scraps purchased from
a metal stockist and it was cut into dimensions suitable for the experiment.
Surface preparation of the coupons involved degreasing in ethanol and drying
in acetone. The coupons were then stored in a moisture free desiccator to avoid
contamination before their use for the corrosion study. Weight percent compositions
of the metal samples were analyzed using Optical Emission Spectrometer (OES)
and the result obtained is shown in Table 1. Extracts of Bambusa
Bambos (BB) were made from its fresh leaves by drying and then pulverizing into
powder. The entire powdered mass was weighed, so that 20 g of it could be put
into a 200 cm3 flat bottom flask containing 100 mL of 2 M HCl solution.
The extracts were prepared by refluxing the resulting solution for 3 h at 343
K (70°C) and it was then left to cool overnight. Filtration was done on
the mixture and the filtrate obtained was taken as the stock inhibitor solution
from which different quantities of the inhibitor solution was made. The quantities
of the extract used for the study were 60,100 140 and 180 ml L-1
and they were prepared by serial dilution.
|| Composition of zinc sample employed for the investigation
||Relationship of volume of H2 (cm3) evolved
with time (min) of zinc coupons for different amounts of BB extract in 2
M HCl at a temperature of 298K
Experimental set up was analogous to the technique adopted elsewhere (Ajayi
et al., 2011a-c; Omotosho
et al., 2012). Each zinc sample was put into the mylius cell containing
50 cm3 of test solution and the experiments were conducted at ambient
temperature of 298 K. The volume of Hydrogen (H2) gas evolved and
trapped in the inverted burette per 2 min interval was recorded for 20 min in
a calibrated tube by downward displacement of water and the graph of volume
of hydrogen evolved against time interval was plotted and is shown in Fig.
The inhibition efficiency was computed by using Eq. 1 (Okafor
et al., 2010; Ajayi et al., 2011a-c;
Omotosho et al., 2012):
where, V and V1, are the volumes of H2 evolved from solutions without inhibitor (i.e., control experiment) and with inhibitor, respectively.
Since, it is a well known fact that hydrogen is given of when metals react
with acids, it is therefore correct to associate reaction rate of corrosion
deterioration of the zinc in HCl in the presence of BB extract with H2
evolution. The method employed in literature (Ajayi et
al., 2011a-c; Omotosho et
al., 2012) was used in this study to also model the corrosion rate which
explains the hydrogen evolution rate. From the foregoing, the hydrogen evolution
rate has a close link with the rate at which metal weight is lost. Thus, in
line with Ajayi et al. (2011c) corrosion rate
analysis from the direction of H2 gas evolution rate is completely
a way of modeling weight loss rate when the link between the weight loss and
H2 gas discharged is proven.
Hence, Eq. 2 was generated (Ajayi
et al., 2011a-c; Omotosho
et al., 2012):
|| Volume of H2 gas evolved
|| Metal weight loss due to corrosion reaction
|| Rate of corrosion
Equation 2 was generated by the analysis of relating volume
evolved with the time of evolution. Consequently, a polynomial regression analysis
of the volume of H2 gas evolved against time was done and it led
to Eq. 3 (Ajayi et al., 2011a-c;
Omotosho et al., 2012):
For measurements linked to 100 mL extract quantity, the corrosion rate model
is represented as Eq 6. Via the alignment of Eq.
3 and 4 according to the technique used elsewhere (Ajayi
et al., 2011a-c; Omotosho
et al., 2012), Eq. 5 and 6 sufficed.
RESULTS AND DISCUSSION
Figure 1 showed that the rate of corrosion of the zinc coupon in the control as indicated by the amount of H2 evolved was the highest. Corrosion rate of zinc coupons immersed in the inhibited solutions declined significantly in contrast to the control. It was also shown to some extent that the amount of H2 evolved also reduced as the quantity of the extract increased, considering that the 100, 140 and 180 mL extract quantities showed closely related values. This suggests that the BB extract in the solution slowed down the corrosion of zinc in HCl. As a result, the level of inhibition can be said to be controlled by the quantity of BB extract in solution. A slightly identical trend was also depicted in Fig. 2 in which the percentage inhibition efficiency (% I.E) was represented. The % I.E. readings for samples in the 60 and 140 mL extract quantities were closely related from the beginning of experiment to the 6th min of the experiment. The samples immersed in the 100 and 180 mL extract quantities also showed a similar behavior from the beginning to the 6th min of the experiment. However, in both cases % I.E. readings became slightly different as the experiment progressed to the end. From Fig. 2, it was also shown that peak % I.E. values of 0.714 and 0.736 was attained by samples in the 60 and 140 mL extract quantities at the 6th of the experiment. At the 10th min of the experiment % I.E. values attained by samples in all extract quantities were lower than at the 6th min. The % I.E. readings on the basis of average values in increasing order can be indicated as; 60 mL extract<180 mL extract<100 mL extract<140 mL extract.
Figure 3 revealed that in comparison to the control the corrosion
rate at a temperature of 298 K, decreases in the presence of BB extract. The
extract quantity of 60 mL showed the least effect of reducing the corrosion
rate of zinc coupon.
||Percentage inhibition efficiency of varying quantities of
BB extracts with time (min) on zinc coupons in 2 M HCl solution at a temperature
of 298 K
||Corrosion rate of varying quantities of BB extract with time
(min) on zinc coupon in 2 M HCl solution at a temperature of 298 K
The corrosion rate values for the sample immersed in the 60 mL extract began
at 0.018 cm3 sec-1 and gradually increased until the end
of the experiment at 0.21 sec-1. Corrosion rate of sample immersed
in the 140 and 180 mL extract solution which followed the 60 cm3
extract in terms of reducing corrosion rate began at 0.017 cm3 sec-1
and ended with values of 0.147 and 0.143 cm3 sec-1, respectively.
Sample immersed in the 100 mL extract produced the best result; its corrosion rate values began and ended at 0.006 and 0.0090 cm3 sec-1, respectively. Corrosion rate values on the average for the 60, 100, 140 and 180 mL extract quantities were 0.114, 0.048, 0.082 and 0.00722 cm3 sec-1, respectively. When these average values are weighed against that of the control (0.229 cm3 sec-1) it is obvious that the inhibitor was effective. The results also indicate that corrosion rate reduced as extract quantity increased to some extent since from the plot in Fig. 3 the 100 mL extract quantity is shown to be the best while the values for the 140 and 180 were closely related but better than the 60 mL extract. Corrosion rate reduction in terms of the average values from Fig. 3 was therefore observed to follow the order; control<60 cm3 extract<140 cm3 extract<180 extract<100 cm3 extract.
Further corrosion study involved a regression analysis of the values of corrosion
rate R against that of extract quantities. This was done to estimate the reaction
constant and the specific reaction constant of the reaction by examining the
relationship between corrosion rate and varying inhibitor quantity. This same
procedure was adopted according to literature (Ajayi
et al., 2011a-c).
|| Plot of Log of corrosion rate against log of the acid extract
It was revealed that corrosion rates can be linked with acid concentration
with Eq. 7 (Mathur and Vasudevan, 1982;
Noor and Al-Moubaraki, 2008; Ajayi
et al., 2011a-c; Omotosho
et al., 2012):
|| Reaction constant,
|| Specific reaction constant and
Thus, in order to determine reaction constant it became essential to use same
unit (mol min-1). This was done by assuming that hydrogen evolution
took place at 1.01325x10-5 Pa. The connection between Log R and Log
C shown in Fig. 4 is for the zinc coupon sample and a correlation
coefficient of 0.314 was obtained for the linear plot. From the linear expression
obtained from the graph, the k and B values were calculated to be 1.309x10-7
mol min-1 and -0.245, respectively. The negative value of B obtained
in this investigation suggests a decreasing slope which is at variance to other
studies (Mathur and Vasudevan, 1982; Noor
and Al-Moubaraki, 2008) where no inhibitors were used. This is worthy of
note because it describes the inhibitive action of the BB extract on zinc coupon
corrosion. Consequently, Eq. 7 was streamlined to obtain Eq.
And for the particular reaction Eq. 8 is written as:
Equation 9 completely explains the observations in Fig. 1 and 3 which shows an apparent variance between the uninhibited and inhibited solution.
Adsorption studies: The mechanism of interaction at the interface between
phytoconstituents in the BB extract and the metal can be validated using various
adsorption isotherms namely Langmuir (0.827), Freundlich (0.518), Temkin (0.495),
Frumkin (0.866) and Boris-Swinkels (0.657). The degree of surface coverage,
θ, for the different extract quantity was evaluated based on volume of
H2 gas evolved measurements. Also, the degree of corrosion inhibition
depends on the surface conditions and mode of adsorption of inhibitors.
||Variation of Ln (θ/C(1-θ)) with surface coverage
(θ) of acid extract showing conformity with Frumkin isotherm
The Frumkin isotherm was found to be best fitted to the θ values as experimental
data were made to fit with the different adsorption isotherms. The Frumkin isotherm
expression (Ajayi et al., 2011c) is represented
|| Quantity of inhibitor based on serial dilution
||Adsorption reaction binding constant
|| Lateral interaction term describing the molecular interactions in the
adsorption layer and the heterogeneity of the surface (it is a measure for
the steepness of the adsorption isotherm)
The precise relationship between Ln (θ/C(1-θ)) and θ as shown
in Fig. 5 was obtained by carrying out a linear regression
analysis of Ln (θ/C(1-θ)) against θ which lead to Eq.
In a bid to deduce values for φ and μ, a comparison of Eq.
11 with the Frumkin isotherm equation is necessary.
The low value of φ shows that the investigated inhibitor is physically adsorbed on the zinc metal surface, while the positive value of μ infers that the interaction between the molecules boosts the adsorption energy with the increase of θ.
|| Relationship of surface coverage (θ) with extract quantity
(mL) at different time intervals
Additionally, the degree of surface coverage, θ, for the extract at different
quantities was plotted for time intervals of 6, 8 and 10 min as shown in Fig.
6 to examine if there is any effect of times of exposure to the relationship
between θ and C. The 6 min curve had the highest surface coverage value
when the extract quantity was 60 mL, while the 8 and 10 min curve though closely
related were second and third respectively. This implies that the 6 min time
frame is the best for the phytochemical in the extract quantity of 60 mL to
suitably adsorb to the metal surface. The 8 and 10 min curves showed closely
related values that were higher than the 6 min curve at extract quantity of
100 mL, implying better adsorption at this extract quantity. Again the 6 min
curve displayed better surface coverage when extract quantity was 140 mL, with
the 8 and 10 min curve having closely related lower θ values. Finally at
extract quantity of 180 mL the 8 and 10 min curves showed the best θ values
with that of the 6 min curve being the lowest. Overall in terms of average θ
values the best time frame across all extract quantities used is the 8 min curve.
This was closely followed by the 6 and 10 min curve. Also the highest θ
value of 0.736 was attained at extract quantity of 140 mL at the 6th minute
of the experiment.
The surface effects of the HCl action on the zinc metal in the presence of
BB extract were examined using optical microscope. The photomicrograph studies
were performed on these samples in order to evaluate the condition of the zinc
metal surface and grain structure. However, since increasing the extract quantities
used did not translate to increased reduction in corrosion rate, the surface
effects of all the samples used for the experiment was not done. Hence, the
investigation were carried out on three metal samples which include that of
the control experiment (having no inhibitor present), sample from the 60 and
100 mL extract quantity. These were chosen to study the phenomenon on the case
scenario of direct 2 M HCl attack, the least and next to the least inhibitive
effect scenarios, knowing that all others will fall within these limits. In
addition, the values of the various indicators that characterized the behavior
of samples in the 60 and 100 mL extract as well as 140 and 180 mL extract were
closely related after immersion. The surface analysis was carried out and the
micrograph of the metal before immersion as observed in Fig. 7a,
indicate the presence of all phases with even distribution. In Fig.
7b which is the control sample, the micrograph indicate general corrosion
with many localized pits at several sites. The black spots are evidences of
the localized pits and severe corrosion.. In Fig. 7c, the
chemical attack on the metal is such that the black spots are less in number
showing that the localized pits is not as much as what was observed in the previous
||Micrographs for zinc surface (a) before immersion in 2 M HCl
solutions (b) after immersion in 2 M HCl for 20 min (c) after immersion
in 60 mL of BB extract for 20 min (d) after immersion in 100 mL of BB extract
for 20 min
However, in Fig. 7d the uniform corrosion and localized pit
formation is far less than the control and the severity of attack is also drastically
reduced suggesting some level of corrosion inhibition. Therefore, exposure of
the metal to BB extract increased the adsorption efficiency at the metal extract
interface which slowed down the metallic deterioration.
The investigation examined and evaluated the damage of zinc by HCl acid in the presence of BB extract using the gasometric method. Several indices that described the behavior of the alloy in the medium at different inhibitor quantity were pinpointed and a relationship for corrosion rate measurement was also obtained. Results showed that maximum % I.E and lowest corrosion rate were obtained at extract quantities of 140 and 100 mL. The mechanism of interaction between the phytochemicals in the plant extract and zinc surface was best described by the Frumkin isotherm. The results also revealed that, the best time for phytochemicals to suitably adsorb to metal surface was 6 min at extract quantity of 140 mL. Statistical modeling of corrosion rate yielded a significant relationship suitable for estimating corrosion rate once quantity of BB extract is known. The superficial analysis revealed that rate of deterioration of the metal slowed down as extract quantity increased to some extent.