INTERNATIONAL JOURNAL OF PHARMACEUTICAL AND CHEMICAL SCIENCES
ISSN: 22775005
Research Article
Corrosion Inhibition by Sodium Dihydrogen Phosphate
- Zn2+ System
P. Rengan¹ and S. Rajendran²*
1
Department of Chemistry, Yadhava College, Thiruppalai, Madurai - 625019, Tamil Nadu, India.
2
Corrosion Research Center, Post Graduate and Research Department of Chemistry,
GTN Arts College, Dindigul-624005, Tamil Nadu, India.
_________________________________________________________________________________
ABSTRACT
The inhibition efficiency of Sodium dihydrogen phosphate (SDHP) in controlling corrosion of carbon
steel immersed in well water has been evaluated in the absence and presence of Zn² by weight loss
method. The SDHP-Zn2+ system shows a synergistic effect. Electrochemical studies such as
polarization and AC impedance spectra have been used to propose a suitable mechanism of corrosion
inhibition of the SDHP-Zn2+ system. The nature of the film formed on the surface of metal specimens
was analyzed by SEM and AFM analysis techniques.
Keywords: Carbon steel, corrosion inhibition, synergism, SEM, AFM.
INTRODUCTION
A wide variety of chemicals ranging from
simple ions such as sulphate to complex
molecules represented by tannins and
polymeric substances can inhibit the corrosion
of metals in aqueous solutions. The properties
of these inhibitors depend on the nature of the
metal and the composition of the environment.
These inhibitors may be used alone or in
formulation containing other chemicals which
process. Health and safety requirements
control the solution and use of inhibitors.
Chromates1,2,
Molybdates3,4,
nitrites5,6,
7,8
9
phosphates , silicates , metallic cations10,
carboxylatest11,12 and tannins13,14, have been
used as inhibitors in natural aqueous
environments. Mixture of inhibitors frequently
provides better inhibition than either of the
individual components’; i.e. the mixtures are
synergistic. This was recoganised by Speller15
in the mid-thirties, who reported finding
“compound films”, such as formed by
phosphate – chromate mixtures, to be more
effective than those of either alone. Review of
literature, reveals that much work has not
been done using sodium dihydrogen
phosphate (SDHP) as corrosion inhibitors.
Hence SDHP has been selected as a
corrosion inhibitor for the present study.
The present work is undertaken
i. To evaluate the inhibition efficiency of
Sodium dihydrogen phosphate (SDHP)
Vol. 2 (4) Oct-Dec 2013
ii.
iii.
iv.
v.
in controlling corrosion of carbon steel in
well water.
To study the influence of duration of
immersion and pH on the inhibition
efficiency of SDHP – Zn2 + system.
To make use of polarization study and
AC impedance spectra to know the
mechanistic aspects of corrosion
inhibition and
To study the surface of metal
specimens analyzed by SEM and AFM.
To propose a suitable mechanism of
corrosion inhibition based on the results
from the above studies.
EXPERIMENTAL
Preparation of the specimens
Carbon steel specimens (0.026% S, 0.06%P,
0.4%Mn, 0.1%C and rest iron) of the
dimensions of 1.0 x 4.0 x 0.2 cm were
polished to a mirror finish and degreased with
trichoroethylene and used for both weight loss
method an surface examination studies.
Weight-loss Method
The carbon steel specimens in triplicate were
immersed in 100ml of the solution containing
various concentrations of the inhibitors in the
+
presence and absence of Zn² for a known
period 3 days (three days). The weights of the
specimens before and after immersion were
determined using a balance, Shimadzu AY62
model. The corrosion products were cleaned
with Clarke’s16 solution. The inhibition
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Surface examination study
The carbon steel specimens were immersed in
various test solutions for a period of one day.
After one day the specimens were taken out
and dried. The nature of the film formed on the
surface of metal specimens was analyzed by
various surface analysis techniques.
efficiency (IE) was then calculated using
equation.
IE = 100 (1-W2/W1)%
Where,
W 1 = Corrosion rate in absence of inhibitor
W 2 = Corrosion rate in presence of inhibitor
Potentio static polarization study
Polarization studies were carried out in an H
and CH electro chemical workstation
impedance analyzer model CHI 660. A threeelectrode cell assemble was used. The
working electrode was carbon steel with one
face of the electrode of constant 1cm² area
exposed and the rest being shielded with red
lacquer. A saturated calomel electrode (SCE)
was used as reference electrode. A
rectangular platinum, foil was used as the
counter electrode. The area of the counter
electrode was much larger compared to the
area of the working electrode. This can exert a
uniform potential field on the working electrode
and minimized the polarization effect on the
counter electrode.
RESULTS AND DISCUSSIONS
Analysis of potentiodynamic polarization
study
Polarization study has been used to confirm
the formation of protective film formed on the
metal surface during corrosion inhibition
process17-21. If a protective film is formed on
the metal surface, the linear polarization
resistance value (LPR) increases and the
corrosion current value (Icorr) decreases.
The potentiodynamic polarization curves of
carbon steel immersed in well water in the
absence and presence of inhibitors are shown
in Fig.1. The corrosion parameters are given
in Table 1. When carbon steel was immersed
in well water the corrosion potential was -680
mV vs SCE. When SDHP (250 ppm) and Zn2+
(50 ppm) were added to the above system, the
corrosion potential shifted to the noble side 640 mV vs SCE. This indicates that a film is
formed on the anodic sites of the metal
surface. This film controls the anodic reaction
of metal dissolution by forming Fe2+-SDHP
complex on the anodic sites of the metal
surface. The formation of protective film on
the metal surface is further supported by the
fact that the anodic Tafel slope (ba) increases
from 172 mV/decade to 200 mV/decade.
Further, the LPR value increases from 1.882 x
105 ohm cm2 to 4.005 x 105 ohm cm2; the
corrosion current decreases from 2.06 x 10-6
A/cm2 to 9.57 x 10-7 A/cm2. Thus, polarization
study confirms the formation of a protective
film on the metal surface.
Icorr ba, bc and Ecorr values were calculated.
AC Impendence Measurements
H and CH electrochemical work station
impedance analyzer model CHI 660 was used
to record AC impedance measurements. The
cell setup was the same as that used for
polarization measurements. The real part (Z’)
and imaginary part (Z”) of the cell impedance
were measured in ohms for various
frequencies. The Rt (change transfer
resistance) and Cdl (double layer capacitance)
values were calculated.
(a)
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(b)
Fig. 1: Polarization curves of carbon steel immersed in test solutions
(a) well water (blank), (b) Well water + SDHP (250 ppm) + Zn2+ (50 ppm)
Table 1: Corrosion parameters of carbon steel immersed in well water in the absence
and presence of inhibitor system obtained from potentiodynamic polarization study
System
Well water
Well water + SDHP
2+
(250 ppm) + Zn (50
ppm)
Ecorr
mV vs SCE
-680
bc
mV/decade
172
ba
mV/decade
177
-640
200
157
Analysis of AC impedance spectra
AC impedance spectra (electro chemical
impedance spectra) have been used to
confirm the formation of protective film on the
metal surface22,23. If a protective film is formed
on the metal surface, charge transfer
resistance (Rt) increases; double layer
capacitance value (Cdl) decreases and the
impedance log (z/ohm) value increases. The
AC impedance spectra of carbon steel
immersed in well water in the absence and
presence of inhibitors (SDHP-Zn2+) are shown
in Fig.2(a,b) (Nyquist plots) and Fig.3(a) and
Fig.3(b) (Bode plots). The AC impedance
parameters namely charge transfer resistance
(Rt) and double layer capacitance (Cdl) derived
Icorr
2
A/cm
-6
2.016 x 10
-7
9.574 x 10
LPR
ohm
2
cm
5
1.882 x 10
5
4.005 x 10
from Nyquist plots are given in Table 2. The
impedance log (z/ohm) values derived from
Bode plots are also given in Table 2.
It is observed that when the inhibitors
[SDHP(250 ppm) + Zn2+ (50 ppm)] are added,
the charge transfer resistance (Rt) increase
from 4107  cm2 to 17413  cm2. The Cdl
value decreases from 1.241 x 10-5 F/cm2 to
0.292 x 10-5 F/cm2. The impedance value [log
(z/ohm)] increases from 3.7 to 4.3 and the
phase angle of well water is to be 35° where
as well water + 250 ppm of SDHP + 50 ppm of
Zn2+ is to be 33°. These results lead to the
conclusion that a protective film is formed on
the metal surface.
Table 2: Corrosion parameters of carbon steel immersed in well water in the
absence and presence of inhibitor system obtained from AC impedance spectra.
Nyquist plot
System
Well water
2+
Well water + SDHP (250 ppm) + Zn (50
ppm)
Vol. 2 (4) Oct-Dec 2013
Rt
ohm
2
cm
4107
17413
Cdl
2
F/cm
-5
1.241 x 10
-5
0.292 x 10
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Bode plot
Impedance value log
(z/ohm)
3.7
4.3
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ISSN: 22775005
(a)
(b)
Fig. 2: AC impedance spectra of (a) Well water,
(b) Well water + 250 ppm of SDHP + 50 ppm of Zn2+
SEM Analysis of Metal Surface
SEM provides a pictorial representation of the
surface. To understand the nature of the
surface film in the absence and presence of
inhibitors and the extent of corrosion of carbon
steel, the SEM micrographs of the surface are
examined24,25.
The SEM images of different magnification (X
1000) of carbon steel specimen immersed in
well water for 7 days in the absence and
presence of inhibitor system are shown in
Fig.4 (a,b,c).
The SEM micrographs of polished carbon
steel surface (control) in Fig.4(a) shows the
smooth surface of the metal . This shows the
absence of any corrosion products (or)
inhibitor complex formed on the metal surface.
Vol. 2 (4) Oct-Dec 2013
The SEM micrographs of carbon steel surface
immersed in well water (Fig.4(b)) show the
roughness of the metal surface which
indicates the highly corroded area of carbon
steel in well water.
However in Fig.4(c)
indicate that in the presence of inhibitor (250
ppm SDHP and 50 ppm Zn2+) the rate of
corrosion is suppressed, as can be seen from
the decrease of corroded areas. The metal
surface almost free from corrosion due to the
formation of insoluble complex on the surface
of the metal. In the presence of SDHP and
Zn2+, the surface is covered by a thin layer of
inhibitors which effectively controls the
dissolution of carbon steel.
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(a)
(b)
Fig. 3: AC impedance spectra of (a) Well water,
(b) Well water + 250 ppm of SDHP + 50 ppm of Zn2+
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Atomic Force Microscopy Characterization
Atomic force microscopy is a powerful
technique for gathering roughness statistics
26
from a variety of surfaces . AFM is becoming
an
accepted
method
of
roughness
investigation27.
All atomic force microscopy images were
obtained in a VEECO Lab incorporation AFM
instrument operating in contact mode in air.
The scan size of all the AFM images are 5 µm
x 5 µm areas at a scan rate of 6.835
µm/second.
The two dimensional (2D), three dimensional
(3D) AFM morphologies and the AFM crosssectional profile for polished carbon steel
surface (reference sample), carbon steel
surface immersed in well water (blank sample)
and carbon steel surface immersed in well
water containing the formulation of SDHP 250
ppm and 50 ppm of Zn2+ are shown as Fig.5
(a,d,g), (b,e,h), (c,f,i) respectively.
Vol. 2 (4) Oct-Dec 2013
ISSN: 22775005
Root– mean-square roughness, average
roughness and peak-to-valley value
AFM image analysis was performed to obtain
the average roughness, Ra (the average
deviation of all points roughness profile from a
mean line over the evaluation length), rootmean-square roughness, Rq (the average of
the measured height deviations taken within
the evaluation length and measured from the
mean line) and the maximum peak-to-valley
(P-V) height values (largest single peak-tovalley height in five adjoining sampling
28
heights) . Rq is much more sensitive than Ra
to large and small height deviations from the
mean.
Table 3 is the summary of the average
roughness (Ra), rms roughness (Rq) maximum
peak-to-valley height (P-V) value for carbon
steel
surface
immersed
in
different
environments.
The value of RRMS, Ra and P-V height for the
polished carbon steel surface (reference
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sample) are 6 nm, 4 nm and 10 nm
respectively,
which
shows
a
more
homogeneous surface, with some places in
which the height is lower than the average
depth. Fig.5(a,d,g) displays the uncorroded
metal surface. The slight roughness observed
on the polished carbon steel surface is due to
atmospheric corrosion. The rms roughness,
average roughness and P-V height values for
the carbon steel surface immersed in well
water are 26 nm, 21 nm and 120 nm
respectively. These data suggest that carbon
steel surface immersed in well water has a
greater surface roughness than the polished
metal surface.
This shows that the
unprotected carbon steel surface is rougher
and is due to the corrosion of the carbon steel
in well water.
Fig.5(b,e,h) displays the
corroded metal surface with few pits.
.
ISSN: 22775005
The presence of 250 ppm of SDHP and 50
ppm of Zn2+ in well water reduces the Rq by a
factor of 9 nm from 26 nm and the average
roughness is significantly reduced to 6 nm
when compared with 21 nm of carbon steel
surface immersed in well water. The maximum
peak-to-valley height also was reduced to 52
nm from 120 nm. These parameters confirm
that the surface appears smoother. The
smoothness of the surface is due to the
2+
formation of a compact protective film of Fe SDHP complex and Zn(OH)2 on the metal
surface thereby inhibiting the corrosion of
carbon steel.
Also the above parameters observed are
somewhat greater than the AFM data of
polished metal surface which confirms the
formation of the film on the metal surface,
which is protective in nature.
Table 3: AFM data for carbon steel surface immersed in
inhibited and uninhibited environments
Samples
Polished carbon steel (control)
Carbon steel immersed in well
water
Carbon steel immersed in well
2+
water SDHP (250 ppm) + Zn
(50 ppm)
RMS (Rq)
Roughness
(nm)
6
Mechanism of Corrosion inhibition
Analysis of the results of weight loss method
reveals that the combination consisting of 250
ppm of SDHP and 50ppm of n2+ offers an I
of 92%. Results of polarization study suggest
that the formulation functions as anodic
inhibitor. The AC impedance spectral studies
indicate that a protective film is formed on the
metal surface.
In order to explain all these observations the
following mechanism of corrosion inhibition is
proposed.
 When mild steel specimen is
immersed in an aqueous solution
the anodic reaction is
2+
Fe  Fe + 2e
and the cathodic reaction is
2H2O + O2 + 4e -  4OH  When the system consisting of 250
2+
SDHP and 50 ppm of
n
is
2+
prepared, there is a formation of n
- SDHP complex in solution.
n2+ + SDHP  n2+ - SDHP
 When the mild steel is immersed in
2+
the solution, the n - SDHP complex
Vol. 2 (4) Oct-Dec 2013
4
Maximum peakto-valley height
(nm)
10
26
21
120
9
6
52
Average (Ra)
Roughness (nm)
diffuses from the bulk of the solution to
the metal surface.
On the surface of the metal, Zn2+ SDHP complex is converted into Fe2+ SDHP complex at the local anodic
regions.

2+
2+
n - SDHP + Fe
2+
2+
 Fe - SDHP + n
2+
The released n ions combine with
OH - ions to form Zn(OH)2 on the
cathodic sites.
n2+ + 2OH -  Zn(OH)2 
Thus the protective film consists of Fe2+ SDHP complex and Zn(OH)2

CONCLUSIONS
The present study leads to the following
conclusions
 A synergistic effect exists between Tris
Hydroxymethyl
(aminomethane)
2+
THMAM and Zn
in controlling
corrosion of carbon steel immersed in
aqueous solution containing 60 ppm Cl–.
 The formulation consisting of 50 ppm of
THMAM and 50 ppm of Zn2+ has 62%
IE.
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 As the immersion period increases IE
decreases.
 Polarization study reveals that THMAM
2+
– Zn system functions as cathodic
inhibitor.
 AC Impedance spectra reveal that a
protective film is formed on the metal
surface
Vol. 2 (4) Oct-Dec 2013
ISSN: 22775005
 SEM and AFM images confirm the
presence of the protective film on the
metal surface.
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ACKNOWLEDGEMENT
The authors are thankful to UGC, India and to
their respective Management for their help and
encouragement and to Mr. S. Elango for his
computer aided help.
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