Materials Sciences and Applicatio ns, 2011, 2, 592-595
doi:10.4236/msa.2011.26079 Published Online June 2011 (
Copyright © 2011 SciRes. MSA
Corrosion Inhibition of Mild Steel in Hydrochloric
Acid by Tannins from Rhizophora Racemosa
Makanjuola Oki1,2, Ebitei Charles3, Collins Alaka4, Tambari Kayode Oki1
1Department of Technical Services, Greenfield-Oaks Limited, London, UK; 2Department of Petroleum Engineering, Covenant Uni-
versity, Ota, Nigeria; 3Department of Mechanical Engineering, Niger Delta University, Yenagoa, Nigeria; 4Department of Pure &
Industrial Chemistry, University of Port Harcourt, Port Harcourt, Nigeria.
Received December 11th, 2010; revised February 15th, 2011; accepted April 11th, 2011.
Studies on the corrosion behaviour of mild steel electrodes in inhibited hydrochloric acid are described. Conventional
weight loss measurements show that a maximum concentration of 140 ppm of tannin from Rhizophora racemosa is re-
quired to achieve 72% corrosion inhibition. Similar concentration of tannin: H3PO4 in ratio 1:1 gave 61% inhibition
efficiency, whereas efficiency obtained for phosphoric acid as inhibitor in the same environment was 55%. Corrosion
rates obtained over six hours of exposure in 1 M HCl solution at inhibitor concentrations of 140 ppm are 2 mA/cm2,
2.4 mA/cm2, 2.6 mA/cm2 and 6 mA/cm2 for tannin, tannin/H3PO4 and H3PO4-inhibited and uninhibited specimens re-
spectively. Natural atmospheric exposure studies revealed that specimens treated in H3PO4 resisted corrosion for three
weeks, while tannin treated specimens suffered corrosion attack after one week of exposure tests.
Keywords: Inhibitor, Tannin s, Corrosion Rate, Rhizophora Racemosa, Phosphoric Acid
1. Introduction
Rhizophora racemosa is in abundance in the Mangrove
forests of southern Nigeria. The bark of its stem is rich in
tannins which can be described as any group of naturally
occurring phenolic compounds. Their basic structure
consists of garlic acid residues which are linked to glu-
cose via glycosidic bonds [1]. Thus tannins have an array
of hydroxyl and carboxyl groups through which the mo-
lecules can adsorb on corroding metallic surfaces. Fer-
rous materials, especially mild steel, on the other hand
are largely used in acidic media in most industries in-
cluding oil/gas exploration and ancillary activities. Dur-
ing such activities, inhibited hydrochloric acid is widely
used in pickling , descaling and stimulation of oil wells in
order to increase oil and gas flow. The inhibitors em-
ployed are varied and some have been found to be haz-
ardous to health and the environment at large [2]. Thus
efforts are now directed towards formulation of modern
environmentally safe inhibitors [3] in which plant ex-
tracts have become important as eco-friendly, economi-
cal, readily available and renewable sources of effective
corrosion inhibitors. Other researchers [4-8] have dem-
onstrated corrosion inhibition in the order of 80% and
above by extracts from Mentha Pulegium [4], Azadirachta
[5], and Zenthoxylum alatum [6], Kidney bean [7] and
Occimum viridis [8] amongst many others. The inhibi-
tion efficiency has been described as primarily due to
their adsorption at corroding metal surfaces [9]. A pro-
tective film forms due to adsorption of these inhibitor
molecules which restricts either the movement of ions
away from the corroding surface or the consumption of
electrons; however in most cases, they act as bo th anodic
and cathodic inhibitors [10]. It is not uncommon to use
either tripolyph osphates or phosphoric acid [11] and/or a
mixture of either with other inhibitors to inhibit the cor-
rosion of mild steel especially in the petrochemical in-
dustry. In such cases synergistic effects where improved
corrosion inhibitive efficiency is observed, lower costs
implications of using either of the two are the primary
Thus, the present investigation is directed at the evalu-
ation of tannin from Rhizophora racemosa, phosphoric
acid and a combination of the two as inhibitors for the
corrosion of mild steel in hydrochloric acid.
2. Experimental
2.1. Materials
Mild steel specimen which contained, Carbon, 0.16%;
Corrosion Inhibition of Mild Steel in Hydrochloric Acid by Tannins Rom Rhizophora Racemosa593
Magnesium 0.53%; Silicon 0.16% and Iron 99.25% was
made out into electrodes. All chemicals used were of
laboratory grade by BDH Chemicals, UK, while tannin
was obtained from the bark of Rhizophora racemosa.
Distilled water was used throughout the ex periment.
2.2. Methods
Tannin was extracted from the bark of Rhizophora ra-
cemosa by the methods described by Ojinnaka [12]. Mild
steel specimens of dimensions 40 × 20 × 4 mm were
made into electrodes by fastening each on to Copper wire,
100mm in length through a notch at one end and con-
solidated with Araldite, an epoxy resin. The final ex-
posed area of each electrode was 20 × 20 × 4 mm respec-
tively, giving an exposed total surface area of 10.4 cm2.
Prior to exposure to various corrosive media, the speci-
mens were abraded with emery paper after which the
specimens were individually rinsed in ethanol, dried un-
der the fan and stored in desiccators prior to use in the
experiments. Weight loss measurements were carried out
by immersing each pre-weighed electrode in 100 ml of 1
M of HCl to act as control. Other electrodes were simi-
larly exposed in 1 M HCl media containing various con-
centrations of tannin. During exposure, the potentials of
the various specimens were measured with reference to
saturated calomel electrode connected through a high
impedance voltmeter. Triplicate experiments were per-
formed in each case and the mean values of the weight
loss recorded. The specimens were then examined under
Olympus optical metallographic microscope. Further,
electrodes immersed in concentrated inhibitor solutions
for 10 minutes each and dried under the fan for 30 min-
utes and untreated specimens were exposed vertically to
the outside environment behind the laboratory at Choba,
Port Harcourt, Nigeria. These specimens were observed
regularly and at the end of 21 days, the electrodes were
examined under an Olympus metallographic microscope.
Similar tests as described above were performed on
mild steel electrodes with phosphoric acid and tannin:
phosphoric acid in ratio 1:1 as inhibitors of interests.
3. Results and Discussions
3.1. Atmospheric Exposure
General Observations
To the naked eye, untreated mild steel specimens showed
some red-brown patches of rust on the first day of expo-
sure to the atmosphere. This is expected as the corrosion
product of iron exposed to moist air, ferrous hydroxide is
further oxidised to the hydrated oxide during exposure.
However, those specimens treated with tannin, tan-
nin/phosphoric acid and phosphoric acid, retained the
colourations imparted on th em by the respective inhibito r
For those specimens treated with phosphoric acid and
tannin/phosphoric acid, there was no rust exhibited over
three weeks of exposure as the phosphate formed a tena-
cious film which is integral with the substrate and pro-
tected the surface from the inclement atmosphere. How-
ever the specimens treated with tannin solution showed
some signs of rust after one week of exposure to the at-
mosphere. Researchers [5,6,13] agree that inhibitors of
organic origin perform by adsorbing on substrates
through weak bonds. The bonds formed by tannin with
the mild steel substrates were compromised thus expos-
ing the substrate to the atmosphere and corrosion reac-
tions with formation of rust occurred.
These observations were further confirmed with opti-
cal microscopy examination of the various specimens.
3.2. Corrosion Rates and Inhibition Efficiencies
During this investigation corrosion rates, CR an d perc ent
efficiency, E% were derived from Equations (1) and (2)
respectively as demonstrated by other researchers [6,8,
CRww At
where wo and w are, respectively, the weights of the spe-
cimens before and after exposure to 1 M hydrochloric
acid; A is the total surface area, 10.4 cm2 in this investi-
gation and t is the time of exposure.
% 100
where, CRo and CRi are the corrosion rates of mild steel
in 1 M HCl without and in the presence of various con-
centrations of different inhibitors respectively and E%
are the inhibition efficien cy.
The weight loss of mild steel in 1 M HCl solution ini-
tially increased rapidly with a rate that decreased with
time, Figure 1, as a result of formation of corrosion
products which may stifle corrosion reactions when de-
posited on the subst rate [1 4] .
After 6 hours of exposure, the we ight loss was 413 mg,
which translates to a corrosion rate, CR of 6.619 mg/cm2/h.
At that instance and beyond, the electrode potential, as
measured with respect to saturated calomel electrode
through a high impedance voltmeter, was –1.490V.
For iron corroding fr eely as Fe Fe2+ + 2e, 2.51 mdd
is equivalent to 1 × 10–6 A/cm2 [15]. Thus, 6.619 mg/cm2/h
is equivalent to about 6 mA/cm2.
Figure 2 describes the efficiency of tannin, tannin/
phosphoric acid and phosphoric acid respectively in 1 M
HCl, where it is observed that tannin showed a maximum
efficiency of about 72% at a concentration of 140 ppm
whereas, at the same concentration, efficiencies of about
61% and 55% were achieved by tannin/H3PO4 and H3PO4
Copyright © 2011 SciRes. MSA
Corrosion Inhibition of Mild Steel in Hydrochloric Acid by Tannins Rom Rhizophora Racemosa
Figure 1. Weight loss of mild steel specimen with time in
1 M HCl at 30˚C.
Figure 2. Efficiency of various inhibitors on the corrosion of
mild steel in 1 M HCl at 30˚C.
respectively. For tannin and tannin/H3PO4, efficiencies
increased with increase in concentration which is in
agreement with the findings of other researchers [8,13]
who employed organic based compounds as inhibitors
for mild steel in various concentrations [6,7] and types
[10,11] of acids. However, of interest is the characteristic
behaviour exhibited by the specimen in the presence of
H3PO4 as inhibitor in HCl solution, Figure 3. The curve
for the corrosion rate of mild steel in the presence of
H3PO4 , showed a maximum value on adding 60 ppm of
the inhibitor, which is also demonstrated as the lowest
point on the efficiency curve in Figure 2 Phosphates
are passivators which form mixed iron oxide/phosphate
films on mild steel in aqueous corrosion systems. How-
ever for passivation to occur, the corrosion current den-
sity for the specimen must be driven beyond a critical
Figure 3. The corrosion rates of mild steel in 1 M HCl in the
presence of various inhibitors at 30˚C.
point, icrit, when the corrosion rate will drop sharply as its
electrode potential for passivation, Epp, is attained. The
critical corrosion density was attained on adding 100
ppm of H3PO4 to the corroding system and the potential
recorded at this point and beyond was about –1.105 V
with respect to saturated calomel electrode, SCE. This
potential is more positive than –1.490 V, recorded for
mild steel in 1 M HCl without inhibitor, indicating the
formation of a film over the surface of the electrode. The
free corrosion potential recorded for the specimen in the
presence of tannin as inhibitor was about –1.042 V wrt
SCE, which is equally more positive than –1.490 V as
reported earlier in the absence of an inhibitor. In this re-
spect, although a film had formed over the surface of the
electrode, it may not be as compact as those formed in
the presence of phosphates in a similar environment.
However, the magnitudes of potential in all cases do not
signify the extent of corrosion which is determined by
kinetic factors; it gives an indication of the magnitude of
the pH at the electrode’s interphase with the electrolyte.
The corrosion rates as calculated from weight loss
measurements for the various specimens at inhibitor
concentration of 140 ppm for six hours of exposure are
equivalent to 2 mA/cm2, 2.4 mA/cm2, 2.6 mA/cm2 for
tannin, phosphoric acid and tannin/phosphoric acid re-
spectively, using the relationship quoted earlier.
For organic inh ibitors su ch as th e tann ins which inh ib it
corrosion by adsorption, the surface coverage, θ, often
gives insight into the mode(s) of interaction of their mo-
lecules with the corroding substrate. Some of the adsorp-
tion isotherms, [16,17] which relate θ to C, the concen-
tration of the inhibitor in the corroding system are
Langmuir isotherm, (C/θ Vs C), which assumes that
Copyright © 2011 SciRes. MSA
Corrosion Inhibition of Mild Steel in Hydrochloric Acid by Tannins Rom Rhizophora Racemosa
Copyright © 2011 SciRes. MSA
there is no interaction between adsorbed molecules on
the surface. Others are Temkin (θ Vs log C) and Frumkin
(θ Vs C); these assume the effect of multiple layer and
some interactions between molecules on the surface re-
spectively. For tannin and phosphoric acid as inhibitors,
the data derived from the gravimetric studies obeyed the
Langmuir adsorption isotherm. However for tannin/
H3PO4, as inhibitor, none of the isotherms clearly defined
its mode of interactions on the substrates.
4. Conclusions
At a concentration of 140ppm, tannin has efficiency of
72%, tannin: H3PO4 61% and H3PO4 showed 55% effi-
ciency in 1 M HCl.
Mild steel specimens treated in H3PO4 were protected
against atmospheric attack for three weeks while those
treated in tannin resisted corrosion for one week.
5. Acknowledgements
C. O Alaka acknowledges the Head of Department, De-
partment of Pure and Industrial Chemistry, University of
Port Harcourt for the use of laboratory facilities.
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