Volume 20 Preprint 96
Synergistic effect and statistical model of terminalia avicennioides as anti-corrosion inhibitor of steel pipelines in acidic environment
I. Y. Suleiman,M. Z. Sirajo, T. A. Mohammed, Rufai Suleiman Ochu
Keywords: Terminalia avicenoides, potassium iodide, Corrosion inhibition, gravimetric method, phytochemical constituents.
Investigation of Terminalia avicennioides (Ta) extract as corrosion inhibitor for steel pipelines in acidic medium using gravimetric method with and without (0.5 M KI-) and statistical modelling was carried out. Qualitative analysis was used to determine the phytochemical constituents in the extract. The inhibitor concentration, temperature and time were varied in the range of 2.5-15.0 g/l at 2.5 g/l interval, 35-80 Â°C at 15 Â°C interval and 12- 72 hours at 12 hours interval in a static solution. A scanning electron microscopy (SEM) technique was used to characterize the coupon surfaces. A statistical design for the corrosion process was carried out to determine which parameters (inhibitor concentration, temperature and exposure time) that were statistically significant using ANOVA. The results revealed the presence of tannins, saponins, glycosides, flavonoids, alkaloids with reasonable amounts. Inhibition efficiency results without and with KI- at the optimum concentration of 10 g/l were 94.86% and 99.12%. Langmuir adsorption isotherm was followed. Ea in the presence of inhibitor is greater than in the absence of inhibitor at all the temperatures studied, less than 80KJ/mol and physiorption in nature. The surface of the steel pipeline at optimum point of 10 g/l had smooth surface compared to the surface in the absence of the inhibitor. The ANOVA results showed that the inhibitor was the most significant parameter having the highest statistical influence of 78.39% followed by time (11.09%) and temperature (10.43%). These results therefore confirmed the high inhibition efficiency values obtained in the presence of the inhibitor earlier.
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Synergistic effect and statistical model of terminalia avicennioides as anti-corrosion inhibitor of
steel pipelines in acidic environment
I. Y. Suleiman1, M. Z. Sirajo2, T. A. Mohammed3 and Rufai Suleiman Ochu4
Dept. of Metallurgical and Materials Engineering, University of igeria, sukka, igeria
Petroleum Technology Development Fund, Abuja, F. C. T; ,igeria
Dept. of Mechanical Engineering, Waziri Umaru Federal Polytechnic, Birnin Kebbi, igeria
Department of Chemistry, Ahmadu Bello University, Zaria. igeria
E- mail: firstname.lastname@example.org / +2348036057915
Investigation of Terminalia avicennioides (Ta) extract as corrosion inhibitor for steel
pipelines in acidic medium using gravimetric method with and without (0.5 M KI-) and
statistical modelling was carried out. Qualitative analysis was used to determine the phytochemical
constituents in the extract. The inhibitor concentration, temperature and time were varied in the
range of 2.5-15.0 g/l at 2.5 g/l interval, 35-80 °C at 15 °C interval and 12- 72 hours at 12 hours
interval in a static solution. A scanning electron microscopy (SEM) technique was used to
characterize the coupon surfaces. A statistical design for the corrosion process was carried out to
determine which parameters (inhibitor concentration, temperature and exposure time) that were
statistically significant using ANOVA. The results revealed the presence of tannins, saponins,
glycosides, flavonoids, alkaloids with reasonable amounts. Inhibition efficiency results without and
with KI- at the optimum concentration of 10 g/l were 94.86% and 99.12%. Langmuir adsorption
isotherm was followed. Ea in the presence of inhibitor is greater than in the absence of inhibitor
at all the temperatures studied, less than 80KJ/mol and physiorption in nature. The surface of the
steel pipeline at optimum point of 10 g/l had smooth surface compared to the surface in the absence
of the inhibitor. The ANOVA results showed that the inhibitor was the most significant parameter
having the highest statistical influence of 78.39% followed by time (11.09%) and temperature
(10.43%). These results therefore confirmed the high inhibition efficiency values obtained in the
presence of the inhibitor earlier.
Keywords: Terminalia avicenoides, Potassium iodide, Corrosion inhibition, Gravimetric method,
Sulphuric acid is being produced than any other chemicals in the world. It is being used
directly or indirectly in nearly all industries and is a vital commodity in national economy. The
consumption rate of sulphuric acid, like steel production or electric power could be used as a
yardstick to judge economic conditions of nation [1-2]. P ickling of steel and other metals,
manufacture of fertilizers, dyes, drugs, pigments, explosives, synthetic detergents, rayon, and other
textiles, petroleum refining, metal refining and production of rubbers give rise to the corrosion of
equipment . However, these constructional materials such as steel pipelines had been subjected to
dissolution when exposed to high concentration of sulphuric acid solution. Deterioration and
weakness of the constructional material can cause serious problems that had lead to the losses of lives
and equipment, so it becomes imperative to protect materials against corrosion in such aggressive
environments . Protection of this constructional material from being corroded is not only important
for increasing the lifespan of materials, but also important in protecting our environments from
hazards resulting from the disasters . However, an important approach adopted for the protection of
this material against corrosion is by the use of inhibitors which is one of the numerous ways of
mitigating corrosion in any environments .
The use of chemical inhibitors is often the most practical and cost effective means of corrosion
mitigation. However, most of these synthetic inhibitors such as chromates, nitrates, borates,
molybdates had proved good anticorrosive action but highly toxic to both human beings and
environment . An alternative has to be searched to replace the expensive and toxic synthesized
corrosion inhibitors because of environmental regulations in order to promote the sustainable
greenness to the atmosphere. The issue of toxicity has led to the use of naturally occurring substances
in order to find low-cost and non-hazardous inhibitors.
Plant extracts have become important as an environmentally acceptable, readily available and
renewable source of materials for a wide range of corrosion prevention formulations. Therefore,
finding naturally occurring substances as corrosion inhibitors is a subject of great practical
significance . Extracts of plants contain organic compounds such as N, O, P and S atoms which are
considered to be effective corrosion inhibitors. The effectiveness of organic inhibitors however,
depends on the nature and the condition of the metallic surface, the chemical composition and
structure of the inhibitor . All plant products are organic in nature and their constituents are tannins,
organic and amino acids, saponins, alkaloids, flavonoids, glycosides and pigments are known to
exhibit inhibiting action [10-17].
The use of organic inhibitors in acid solutions cannot be complete without mentioning the
phenomena of synergism. This effect has been observed since the earliest days of inhibitor technology
and continues to be a potent tool in the development of acid inhibitors for specialized uses [18,19].
The synergistic inhibition between halide ions and some organic compounds have been investigated
Terminalia avicennioides is a much-branched shrub or a small tree; it can grow from 2 - 12
metres tall. The tree is harvested from the wild for local use as a medicine and source of materials. It
has potential use as a pioneer to restore woodland and also for stabilizing the soil. A decoction of the
root is taken by draught and by enema to remedy diarrhoea and dysenteries .
In the present work, inhibitive action of Terminalia avicennioides as ant-corrosion behaviour
of steel pipelines in 0.5 M acidic medium in the presence of potassium iodide as synergy at (35-80 °C)
was investigated using gravimetrical weight loss method. The results obtained were compared with
that obtained by synergy and statistical model results.
2.1 Solution Preparation
Weight loss studies of the sized coupons were conducted in 0.5 M H2SO4 and 0.5 M KIprepared in double distilled deionized water, with or without different concentration of the inhibitor.
2.2 Preparation of inhibitor
Six hundred grams of Terminalia avicennioides roots after cleanings in water and dried was
extracted in 1.5 L of 70 % ethanol and 30 % distilled water as solvent and followed by maceration
method. The extract and the final stage of collecting the liquid at 100 °C before evaporation were
presented in Figure 1. The concentration of the stock solution was expressed in terms of (g/l) and the
concentration of 2.5g/l – 15g/l) of the extract was prepared .
Fig. 1. Terminalia avicennioides roots and the extraction process
2.3 Weight loss method
The coupons used for this research was cut from steel pipeline with the following
compositions presented in Table 1 of size 24 x 2 mm. The specimens were abraded with various
grades of wax coated emery papers from 600 - 1800 grits and degreased in absolute ethanol, dried in
acetone, weighed and stored in moisture-free desiccators prior to use to avoid reaction with
atmospheric air. Shimadzu Corporation, Model AUW120D was used to weigh the samples. The metal
coupons were then suspended with the help of glass hooks into beakers containing 200 mL of
corrosive electrolyte for complete immersion test. The potassium iodide (0.5 M KI-) was then added to
the solution of 0.5 M H2SO4 already prepared. The corrosion rate of each specimen in mmpy,
inhibition efficiency and surface coverage were determined using equations (1-2) according to the
. . . .1
Where CR is corrosion rate (mm/yr), W is weight loss (g), D is the density (g/cm ), A is the area (cm )
and T is time and K is a constant equal to 87500.
Where W is the weight loss (g), Wi is the initial weight (g) and Wf is the final weight (g).
Inhibition efficiency (IE %) =
where CRo and CRi are corrosion rates in the absence and presence of inhibitors, respectively.
Table 1. Chemical Composition of Steel pipeline coupon
Reminder 0.25 0.51 0.72 0.015 0.014 0.041 0.013 0.182 0.001
The synergy was evaluated by the following parameters in equation 3 
Synergy parameter, !" = ( +
= inhibition efficiency of the extract,
– ( . " ),
" = inhibition efficiency of the coupon in the presence of the
!" = inhibition efficiencies of the coupon in the presence of
the extract and iodide.
Characterization of the coupons:
A Philips model XL30SFEG Scanning Electron Microscope with high resolution was used to study
the morphology and chemical analysis of the corroded steel pipelines surfaces. The surface
morphology of the coupons before and after corrosion was characterized .
Development of mathematical model
To investigate which of the corrosion parameters that significantly controls the corrosion rate
studied the standard L8 orthogonal array was adopted in the design of the experiment. The
independently process parameters considered for the investigation are temperature, concentration and
exposure time. Two levels of each of the three factors were used for the statistical analysis. The levels
for the three factors are shown in Tables 2 and 3 respectively. The model equation was obtained by
representing the corrosion rate value by CR, which is a function of the three variables as expressed in
equation (4) below:
CR = ƒ (A, B, C)
. . . .4
where A is the temperature, B is the inhibitor and C is the exposure time. The model selected includes
the effects of first order main variables and second-order interactions of all variables in equation 5.
Hence the general model is written as;
CR = βo + β1A+ β2B+ β3C+ β4AB+ β5AC+ β6BC + β7ABC
Where βo is average response of CR andβ1, β2, β3, β4, β5, β6, β7 are coefficients associated with each
variable A, B, C and interactions .
Table 2. Factorial design of the corrosion rate
Table 3. Factorial design of the corrosion process with the treatment combinations
Coded= -1(low level), +1(upper level or high)
3. Results and Discussion
Table 4 shows the qualitative and quantitative analyses of phytochemical constituents present
in the root extract of Terminalia avicenoides.
Table 4. The qualitative and quantitative analyses of phytochemical constituents
3.1 Effect of Inhibitor concentration
Corrosion rates of the steel pipeline coupons in 0.5 M H2SO4 were evaluated in the absence
and presence of solutions and the results obtained were presented in Table 5. Corrosion rate increases
with increase in temperature and decrease with increase in the plant extract. Compared to the inhibited
solution, corrosion rate was higher in the free acid solution, which implies that the Terminalia
avicennioides extract actually retarded the pipeline steel corrosion. The effectiveness of the extract is
concentration dependent and corrosion rate on the other hand decreases as concentration of the extract
increases. Similar trends have been reported in literatures [19,20]. The variation of corrosion rates
with temperature was in the sequence of 80 oC >65 oC > 50 oC >35 oC. It can be attributed to the rate of
chemical reaction occurring during corrosion process and a well known fact that increase in
temperature brings more chemical reaction hence, decrease in the surface coverage .
Inhibition efficiency (% IE) increases with increase in inhibitor concentration and decreases
with increase in temperature (Table 5) and illustrated in Figure 2. Maximum inhibition efficiency of
94.86% was obtained from weight loss at 303 K. This efficiency maybe attributed to electrostatic
interactions between its charged sites of the organic phytochemicals such as tannins, alkaloids,
saponins, volatile oil [19,21].
Table 5. Optimum values of Corrosion
pipeline at 35, 50, 65 & 80 oC.
Corrosion Rate (mmpy)
Temp. (K) 308K 323K
Blank (0) 30.78 34.65
rate and Inhibition efficiency from weight loss on steel
Inhibition Efficiency (IE %)
Fig. 2. Variation of Inhibition efficiencies (IE %) with different concentration of TA at different
temperatures (35, 50, 65 and 80 oC) in 0.5 M H2SO4
3.2 Effect of immersion time
To assess the stability of inhibitive properties of the extract on a time scale, weight loss
measurements were performed in 0.5 M H2SO4 in the absence and presence of the extract at 2.5- 15.0
g/l concentrations. The effect of immersion period on steel pipeline corrosion inhibition by Terminalia
avicennioides (TA) was studied for 12, 24, 36, 48, 60 and 72 hours immersion time in 0.5M H2SO4 at
35 oC were summarized in Table 6 and illustrated in Figure 3. It shows that inhibition efficiency of the
extract increased with increasing immersion time from 12 to 72 hours and decrease with increase in
temperature. The increase in inhibition efficiency up to 36 hours reflects the strong adsorption of
constituents present in the extract (tannins, alkaloids, Volatile oil), resulting in a more protective layer
formed on steel pipeline-sulphuric acid solution interface . Thus, the extract effectively inhibits
the samples in 0.5 M sulphuric acid solution.
Table 6. Effect of immersion time on percentage inhibition efficiency of mild steel in 0.5 M H2SO4 at
35 oC in the presence of optimum concentrations of the extract
Name of the plant extract
10 g/l of (TA))
Inhibition efficiency (IE %)
Immersion time (Hours)
Fig. 3. Variation of inhibition efficiency of Terminalia avicennioides extract in 0.5 M H2SO4 with
immersion time of the solution
3.3 Synergism considerations
The synergistic parameters were calculated using the relation initially given by  in
equation 5 to evaluate its synergistic effects on the protection of steel pipelines in sulphuric acid at
different temperatures. The results obtained from the equation 5 were presented in Table 7 and
illustrated in Figure 4. There was decrease in corrosion rates from 35-80 oC and inhibition efficiencies
of the system were enhanced.
Table 7. Synergistic parameters (Sp) for different concentrations of TA extract at 35, 50 and 65 and
80 oC in 0.5 M H2SO4
Conc. of Inhibitor (g/l)
The halide ions introduced into the solution change the situation and the mechanism of action of
anion-active substances was connected with the fact that adsorbed anion-active substances created
connecting bridges between metal atoms and organic cations. It was assumed that many organic
inhibitors in acidic electrolytes become protonated, changing into cations according to the scheme
. According to the previous works, if S1 approaches 1, it implies no interaction between the
inhibitor compound existences, if S1 > 1, it implies synergistic effects between the inhibitor and the
halide and if S1 < 1, the antagonistic interaction prevails, which may be attributed to competitive
adsorption . From the studies presented in Table 7, there were synergistic effects between the
extract and halide ion investigated and greater than 1. It was observed that the inhibitive effects of
halide ions increase in the order of Cl– < Br– < I– .
Fig. 4. Variation of Inhibition efficiencies (IE %) with different concentration of TA at different
temperatures (35, 50, 65 and 80 oC) in the presence of 0.5 M KI
3.4 Effect of temperature
Temperature investigation was carried out in order have an idea about the stability of the
inhibitor films deposited during protection at various temperatures studied. From the weight loss
results presented in Table 5, the effect of temperature on inhibition efficiency and activation energy of
corrosion process was carried out. It was observed that corrosion rates decreases with increase in
temperature. The increase in corrosion rate on temperature increase can be attributed to the etching,
rupture and desorption of inhibitor molecules at higher temperature and physorption in nature .
However, the dependency of corrosion rate on temperature can be best represented by the Arrhenius
equation given in equation 6 :
Log (CR) = −Ea/2.303RT + A
Table 8. Activation energy for steel pipelines dissolution in 0.5 M H2SO4 in the absence and presence
of an optimum concentration (10 g/l) of inhibitor
Where Ea is the apparent activation energy, R is the molar gas constant, T is the absolute temperature
(K) and A is the Arrhenius pre-exponential factor, known as frequency factor.
Figure 5 shows the plot of log (CR) against 1/T for corrosion of steel pipelines in 0.5 M H2SO4
solution giving straight line with regression coefficient (R2) value close to one. It is also clear from
Table 10 that the value of Ea is higher in the presence of inhibitor than in its absence. This increase of
Ea in presence of inhibitor indicated the adsorbed inhibitor film on steel pipelines surface increases
the energy barrier for corrosion reaction .
Fig. 5. Arrhenius plots for steel pipelines in 0.5 M H2SO4 solution in the absence and presence of
optimum concentrations at 35-80 °C of TA
3.5 The Adsorption Isotherms
Adsorption isotherm gives the relationship between the coverage of an interface with the
adsorbed species and the concentration of species in solution. The use of adsorption isotherms
provides useful insight into the corrosion inhibition mechanism. The values of the degree of surface
coverage (θ) were evaluated at different concentrations of the inhibitor in 0.5 M H2SO4 solution.
Attempts were made to fit θ values to various adsorption isotherms (El-Awady, Freundlich, Frumkin,
Temkin and Langmuir models) as described elsewhere [22-24]: The inhibitor was found to obey
Langmuir isotherm with a plot of C/θ vs. C which was linear. From the figure 6, the adsorption of
different concentrations of Terminalia avicennioides extract on the surface of steel pipelines was
found to obey Langmuir adsorption isotherm. The phytochemical constituents of terminalia
avicennioides showed that these compounds were hydrolysable and can easily be adsorbed on the
metal surface via the lone pair of electrons present on their oxygen atoms and make a barrier for
charge and mass transfer leading to decrease the interaction of the metal with the corrosive
environment and as a result, the corrosion rate of the metal decreased with increase in the extract .
The formation of film layer essentially blocks discharge of H+ and dissolution of metal ions was then
satisfied. Acid pickling inhibitors containing organic N, S and OH- groups behave similarly to inhibit
corrosion . It follows that inhibition efficiency (IE) is directly proportional to the fraction of the
surface covered by the adsorbed molecules (θ). Therefore, (θ) with the extract concentration specifies
the adsorption isotherm that describes the system.
Fig. 6. Langmuir adsorption isotherm plot for the adsorption of various concentrations of plant extract
on the surface of steel pipeline in 0.5 M H2SO4 solution
3.6 Surface Examination Studies
Philips model XL30SFEG scanning electron microscope (SEM) was used to examine the
action of the inhibitor on the steel pipeline specimens with the magnification of 1000X. The coupons
after immersion in 0.5 M H2SO4 solution for thirty six hours at 35 °C in the absence and presence of
optimum concentration of the plant extract were investigated using SEM. The SEM images at
different coupons were obtained at optimum concentrations. From figure 7, the images obtained
showed more pits were found in SEM image of steel pipelines immersed in 0.5 M H2SO4 solution in
the absence of the inhibitor. SEM image of steel pipelines immersed in 0.5 M H2SO4 solution in the
presence of the plant extract shows the presence of a protective film over their surfaces and the
protective film form was uniform . The SEM morphologies of the adsorbed protective films on
the steel pipelines surface have confirmed the high performance of inhibitive effect of the plant extract
Fig. 7. SEM microstructure of steel pipelines immersed in a) 0.5M H2SO4 b) 0.5M H2SO4 with 10 g/l
3.7 Analysis of variance and the effects of parameters on the corrosion rate
The results of ANOVA for the corrosion rate were presented in Tables 9 and 10 respectively.
The results showed that the inhibitor was the most significant parameter having the highest statistical
influence of 78.39% followed by time (11.09%) and temperature (10.43%). Since the F-value for the
models was less than 0.05, then the parameter or interaction can be considered to be statistically
significant . The coefficient of determination (R2) being the ratio of the explained variation to the
total variation and is a measure of the degree of fitness. For R2 to approach unity, a better response
model results is then indicated, which shows that it fits the actual data. The value of R2 was 0.9291
(92.91%), then high correlations with the experimental values established. The developed
mathematical model equation for the corrosion behaviour of the steel pipelines in the acidic
environment with the presence of the extract can be expressed in equation 7:
CR (TA) = 39.58 + 7.33A – 19.85B – 7.56C – 2.60BC -1.31ABC . . . .7
From Table 10 and equation 7, inhibitor (-19.85) is the most important variable followed by
time (-7.56) in the corrosion control studied. Similar results have been observed . Cconfirmation
test carried were presented in figure 8. Residual variation estimated for the corrosion was in the range
of -2.12 to 2.12. Hence, the regression models developed has demonstrated feasible and effective way
to predict the corrosion rate of the steel pipeline investigated and also reported .
Table 9. Analysis of Variance (ANOVA) for corrosion rate in the presence of Terminalia
Source of Sum
= F-value F (%)
Table 10. Effect of the variables at 95% confidence level for Terminalia avicennioides extract
Coefficient Degree of
95% CI Low 95%
C-Time, 1.00 -7.56
Fig. 8. Variation of experimental number with corrosion rate for mild steel with Terminalia
From the research work carried out so far, the followings can be drawn:
1. The root extract of Terminalia avicennioides acts as good and efficient inhibitor for corrosion
of pipeline steel in 0.5 M H2SO4 acid. The plant extract obeys Langmuir adsorption isotherm
and physorption in nature.
2. The mechanism involved in this study is the phytochemical constituents in the plant extract
was adsorbed on the steel pipelines surface forming a protective thin film layer preventing the
discharge of hydrogen ion (H+) ions.
3. The SEM morphology of the adsorbed protective film on the steel pipelines surface confirmed
high performance of inhibitive effect of the plant extract.
4. Results obtained from weight loss method were very much in good agreement with that of
statistical model showing that inhibitor was the most important variable.
5. The result of the synergy is greater than one and the corrosion rate was drastically reduced.
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