Investigation of inhibitive action of urea-Zn2+ system in the corrosion control of carbon steel in sea water

Abstract

The inhibition efficiency (IE) of Urea in controlling corrosion of carbon steel in sea water in the absence and presence of Zn 2+ has been evaluated by weight loss method. The formulation consisting of 250 ppm urea and 50 ppm Zn 2+ has 94% IE. It is found that the inhibition efficiency (IE) of urea increases by the addition of Zn 2+ ion. A synergistic effect exists between urea and Zn 2+ . Polarization study reveals that Urea – Zn 2+ system controls the cathodic reaction predominantly and suggests the formation of protective film on the metal surface. The nature of the protective film formed on the metal surface has been analyzed by FTIR spectra and AFM analysis. The protective film is found to consist of Fe 2+ – Urea complex and Zn(OH) 2 . Based on the above studies a suitable mechanism has been proposed for the corrosion inhibition.

Full text

INVESTIGATION OF INHIBITIVE
ACTION OF UREA- ZN2+ SYSTEM IN
THE CORROSION CONTROL OF
CARBON STEEL IN SEA WATER
M. MANIVANNAN *1
1 Department of Chemistry, Chettinad College of Engineering and Technology,
Karur – 639 114, Tamil Nadu, India
Email: manichem76@gmail.com, Mobile: +91 9789739587
S. RAJENDRAN 2, 3
2 Corrosion Research Centre, GTN Arts College,
Dindigul – 624 005, Tamil Nadu, India
3 Department of Chemistry, RVS School of Engineering and Technology,
Dindigul – 624 005, Tamil Nadu, India
Abstract :
The inhibition efficiency (IE) of Urea in controlling corrosion of carbon steel in sea water in the absence and
presence of Zn2+ has been evaluated by weight loss method. The formulation consisting of 250 ppm urea and
50 ppm Zn2+ has 94% IE. It is found that the inhibition efficiency (IE) of urea increases by the addition of Zn2+
ion. A synergistic effect exists between urea and Zn2+. Polarization study reveals that Urea – Zn2+ system
controls the cathodic reaction predominantly and suggests the formation of protective film on the metal surface.
The nature of the protective film formed on the metal surface has been analyzed by FTIR spectra and AFM
analysis. The protective film is found to consist of Fe2+ – Urea complex and Zn(OH)2. Based on the above
studies a suitable mechanism has been proposed for the corrosion inhibition.
Keywords: Corrosion inhibition, Urea, carbon steel, synergistic effect, FTIR, AFM, sea water
1. Introduction
Corrosion is the deterioration of metals and alloys by electrochemical reaction with its environment. It is a
natural phenomenon which cannot be avoided, but it can be controlled and prevented using the suitable
preventive measures like metallic coating, anodic protection, cathodic protection and using inhibitors, etc.
Inhibitors are playing very good role in the process of corrosion control. The organic inhibitors containing
hetero atoms like oxygen, nitrogen, sulphur and phosphorus, etc shows better corrosion inhibition by forming
protective layer over the metal surface. Also the corrosion inhibition efficiency follows the order O < N < S < P
1-4. Among various organic compounds, urea and its derivatives shows significant corrosion inhibition of metals
and alloys in corrosive media. As urea molecule (Fig. 1) contains one oxygen and two nitrogen atoms, hence
urea and its derivatives can act as very good corrosion inhibitors 5-7.
Fig. 1. Structure of urea
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The aim of the present study was to investigate synergistic corrosion inhibition for the urea and Zn2+
combination to carbon steel in sea water collected from Bay of Bengal at Marina beach which is located at
Chennai, Tamil Nadu, India (Table 1). The corrosion inhibition efficiency was calculated using weight loss
method and polarization study. The synergistic effect of the urea – Zn2+ system has been studied using
synergism parameters and F – test. The protective film formed on the metal surface characterized using surface
morphological studies such as Fourier Transform Infrared Spectra (FTIR) and atomic force microscopy (AFM).
2. Experimental
2.1. Preparation of the specimens
Carbon steel specimens (0.026% S, 0.06% P, 0.4% Mn and 0.1% C and rest iron) of the dimensions 1.0 X 4.0 X
0.2 cm were polished to a mirror finish and degreased with trichloroethylene and used for the weight-loss
method and surface examination studies.
2.2. Weight loss method
Carbon steel specimens in triplicate were immersed in 100 mL of the sea water containing various
concentrations of the inhibitor in the presence and absence of Zn2+ for three days. The corrosion product cleaned
with Clark’s solution 8. The parameter of the sea water is given in Table 1. The weights of the specimens before
and after immersion were determined using a balance, Shimadzu AY62 model. Then the inhibition efficiency
was calculated using the equation (1).
Table 1. Physico – Chemical Parameters of Sea Water
Parameters Value
pH 7.66
Conductivity 44200 µmhos/cm
Chloride 16050 ppm
Sulphate 2616 ppm
TDS 30940 ppm
Total hardness 2800 ppm
Calcium 120 ppm
Sodium 6300 ppm
Magnesium 600 ppm
Potassium 400 ppm
Where W1 and W2 are corrosion rate in the absence and presence of the inhibitor respectively.
The corrosion rate (CR) was calculated using the equation (2).
Where W = weight loss in mg, D = 7.87 g/cm3, A = surface area of the specimen (10 cm2) and T = 72 hrs.
2.3. Synergism parameter
Synergism parameters are indications of synergistic effect existing between the inhibitors. SI value is found to
be greater than one suggesting that the existence of synergistic effect between the inhibitors 9-12.
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Where θ1+2 = (θ1 + θ2) – (θ1 θ2)
θ1 = Surface coverage of inhibitor Urea
θ2 = Surface coverage of inhibitor Zn2+
θ’1+2 = Combined inhibition efficiency of inhibitor Urea and Zn2+.
2.4. Analysis of Variance (F – test)
F - test was carried out to investigate whether synergistic effect existing between inhibitor systems is
statistically significant 13-14. If F - value is above 5.32 for 1,8 degrees of freedom, it was proved to be at
statistically significant. If it is below the value of 5.32 for 1,8 degrees of freedom, it was statistically
insignificant at 0.05 level of significance confirmed.
2.5. Potentiodynamic Polarization Study
Polarization study was carried out in Electrochemical Impedance Analyzer model CHI 660A using a three
electrode cell assembly. The working electrode was used as a rectangular specimen of carbon steel with one face
of the electrode of constant 1 cm2 area exposed. A saturated calomel electrode (SCE) was used as reference
electrode. A rectangular platinum foil was used as the counter electrodes. Polarization curves were recorded
after doing iR compensation. The corrosion parameters such as Tafel slopes (anodic slope ba and cathodic slope
bc), corrosion current (ICorr) and corrosion potential (ECorr) values were calculated. During the polarization study,
the scan rate (V/s) was 0.005; Hold time at Ef (s) was zero and quiet time (s) was 2.
2.5. 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 surface analytical technique, FTIR spectra and AFM.
2.5.1. FTIR Spectra
The carbon steel specimens immersed in various test solutions for one day were taken out and dried. The thin
film formed on the metal surface was carefully removed and thoroughly mixed with KBr, so as to make it
uniform throughout. The FTIR spectra were recorded in a Perkin – Elmer – 1600 spectrophometer.
2.5.2. Atomic Force Microscopy (AFM)
The carbon steel specimens immersed in various test solutions for one day were taken out, rinsed with double
distilled water, dried and subjected to the surface examination. The surface morphology measurements of the
carbon steel surface were carried out by atomic force microscopy (AFM) using SPM Veeco diInnova connected
with the software version V7.00 and the scan rate of 0.7 Hz.
3. Results and Discussion
3.1. Analysis of results of weight loss study
The calculated inhibition efficiencies (IE) and corrosion rates of urea in controlling corrosion of carbon steel
immersed in sea water both in the absence and presence of Zn2+ ion are given in Table 2. Urea alone shows
some IE. But the combination of 250 ppm urea and 50 ppm Zn2+ shows 94% IE. The calculated value indicates
the ability of urea to be a good corrosion inhibitor. The IE is found to be enhanced in the presence of Zn2+ ion.
This suggests a synergistic effect exists between urea and Zn2+ ion 9, 15-16.
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Table 2. Inhibition efficiencies (IE %) and corrosion rates (mmpy) obtained from urea - Zn2+ systems, when carbon steel immersed in sea
water.
Inhibitor system: Urea + Zn2+ Immersion period: 3 days
Urea
ppm
Zn2+ = 0 ppm
Zn2+ = 25 ppm
Zn2+ = 50 ppm
IE% CR
mmpy IE% CR
mmpy IE%
CR
mmpy
0
- 0.1124 12 0.0989 17 0.0933
50
8 0.1034 25 0.0843 38 0.0696
100
14 0.0966 38 0.0696 60 0.0449
150
30 0.0786 60 0.0449 75 0.0281
200
48 0.0584 70 0.0337 80 0.0224
250
56 0.0494 74 0.0292 94 0.0067
3.2. Synergism Parameters (SI)
The values of synergism parameters are shown in Table 3. Here the values of SI are greater than one, suggesting
a synergistic effect. SI approaches 1 when no interaction exists between the inhibitor compounds. When SI > 1,
this points to synergistic effects. In the case of SI < 1, the negative interaction of inhibitors prevails (i.e.
corrosion rate increases). From Table 3, it can be seen that the values of SI are greater than unity, suggesting that
the phenomenon of synergism existing between urea and Zn2+. Also the synergism parameter (SI) for the
formulation consisting of 250 ppm of urea and 50 ppm of Zn2+ is 6.08, which is greater than one. Thus, the
enhancement of the inhibition efficiency caused by the addition of Zn2+ to urea is only due to the synergistic
effect.
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Table 3. Synergism parameters (SI) for carbon steel immersed in sea water in the absence and presence of inhibitor.
Inhibitor system: Urea + Zn2+ Immersion period: 3 days
Urea
ppm
θ1
θ2
(Zn2+ =
25 ppm)
θ1+2
θ’1+2
SI
θ1
θ2
(Zn2+ =
50 ppm)
θ1+2
θ’1+2
SI
50
0.08 0.12
0.19 0.25 1.07 0.08 0.17 0.23 0.38 1.23
100
0.14 0.12
0.24 0.38 1.22 0.14 0.17 0.28 0.60 1.78
150
0.30 0.12
0.38 0.60 1.54 0.30 0.17 0.41 0.75 2.32
200
0.48 0.12
0.54 0.70 1.52 0.48 0.17 0.56 0.80 2.15
250
0.56 0.12
0.61 0.74 1.48 0.56 0.17 0.63 0.94 6.08
3.3. Analysis of Variance (ANOVA)
To investigate whether, the influence of Zn2+ on the inhibition efficiencies of urea is statistically significant,
F – test was carried out. The results are given in Table 4. The results of Analysis of Variance (ANOVA) shows
the influence of 25 ppm and 50 ppm of Zn2+ on the inhibition efficiencies of 50 ppm, 100 ppm, 150 ppm, 200
ppm and 250 ppm of urea. The obtained F – value 2.75 for 25 ppm Zn2+, is not statistically significant, since it is
less than the critical F – value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore, it is
concluded that the influence of 25 ppm of Zn2+ on the inhibition efficiencies of various concentrations of urea is
not statistically significant. The obtained F – value 8.11 for 50 ppm Zn2+, is statistically significant, since it is
greater than the critical F – value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance. Therefore, it is
concluded that the influence of 50 ppm Zn2+ on the inhibition efficiencies of various concentrations of urea is
statistically significant.
Table 4. Distribution of F – value between the inhibition efficiencies of various concentrations of Urea – Zn2+ system.
Zn2+
(ppm) Sources of
Variance Sum of
Squares
Degrees
of
Freedom
Mean
Square F
Level of
Significance of F
25
Between
242 1 242
2.75 p<0.05
Within
704 8 88
50
Between
722 1 722
8.11 p>0.05
Within
711 8 89
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3.4. Analysis of Polarization curves
The potentiodynamic polarization curves of carbon steel immersed in sea water in the absence and presence of
inhibitors are shown in Fig 2. The calculated corrosion parameters such as corrosion potential (ECorr), Tafel
slopes (anodic slope ba and cathodic slope bc), linear polarization resistance and corrosion current (ICorr) values
are given in Table 5. When carbon steel is immersed in sea water the corrosion potential is – 731 mV vs
saturated calomel electrode (SCE). The corrosion current is 3.80 x 10-4 A/cm2. When urea (250 ppm) and Zn2+
(50 ppm) are added to the above system the corrosion potential is shifted to the cathodic side (from -731 mV to -
783 mV). This suggests that the cathodic reaction is controlled predominantly. More over in presence of the
inhibitor system, the corrosion current decreases from 3.80 x 10-4 A/cm2 to 3.23 x 10-4 A/cm2 and LPR value
increases from 1.0756 x 104 ohm cm2 to 1.4451 x 104 ohm cm2. These observations indicate the formation of
protective film on the metal surface 17-20.
Table 5. Corrosion Parameters of carbon steel immersed in sea water in the absence and presence of inhibitors obtained by polarization
method
Urea
Ppm
Zn2+
ppm Ecorr
mV vs SCE bc
mV/decade ba
mV/decade LPR
ohm cm2 Icorr
A/cm2
0
0 - 731 135.5 162.3 1.0756 x 104 3.80 x 10-4
250
50 - 783 146.0 200.7 1.4451 x 104 3.23 x 10-4
Fig. 2. Polarization curves of carbon steel immersed in various test solutions
(a) Sea water (b) Sea water + Urea (250 ppm) + Zn2+( 50 ppm)
3.5. Analysis of FTIR spectra
The FTIR spectrum of pure urea is shown in Fig. 3 (a). The C=O stretching frequency is appears at 1677 cm-1.
The N – H stretching and deformation frequencies appear at 3455 cm-1 and 1625 cm-1 respectively. The C – N
stretching frequency appears at 1453 cm-1. The FTIR spectrum of the film formed on the metal surface after
immersion in marine media consisting urea (250 ppm) and Zn2+ (50 ppm) is shown in Fig. 3 (b). The C=O
stretching frequency has shifted from 1677 cm-1 to 1660 cm-1. The N – H stretching frequency has shifted from
3455 cm-1 to 3434 cm-1. The N – H deformation frequency has shifted from 1625 cm-1 to 1639 cm-1. The C – N
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stretching frequency has shifted from 1453 cm-1 to 1413 cm-1. This indicates that the urea has coordinated with
Fe2+ on the metal surface through oxygen atom of C=O group and nitrogen atom of N – H group resulting in the
formation of Fe2+ - Urea complex. The peak at 1386 cm-1 is due to Zn – O stretching. These observations
indicate the presence of Zn(OH)2 formed on the metal surface. Thus the FTIR study leads to the conclusion that
the protective film consist of Fe2+ - Urea complex and Zn(OH)2 formed on the metal surface 21- 24.
Fig. 3. (a) FTIR Spectrum of pure Urea
Fig. 3. (b) FTIR Spectrum of the film formed on the metal surface.
3.6. Atomic force microscopy (AFM)
Atomic force microscopy is a powerful technique for the gathering of roughness statistics from a variety of
surfaces 25-26. AFM is becoming an accepted method of roughness investigation 27-31. All atomic force
microscopy (AFM) images were obtained using SPM Veeco diInnova AFM instrument operating in contact
mode in air. The scan size of all the AFM images is 4.91µm x 4.91 µm areas at a scan rate of 0.7 Hz. The two
dimensional (2D), three dimensional (3D) AFM morphologies and the AFM cross-sectional profile for polished
carbon steel surface (reference sample), carbon steel surface immersed in sea water (blank) and carbon steel
surface immersed in sea water containing the formulation of 250 ppm of Urea and 50 ppm of Zn2+ are shown as
Fig.4. (a, d, g), (b, e, h), (c, f, i) respectively.
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3.6.1. Root-mean-square roughness, average roughness and peak-to-valley value
AFM images 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), root-mean-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-to-valley height in five adjoining sampling
heights) 32. Rq is much more sensitive than Ra to large and small height deviations from the mean 33. The
summary of the average roughness (Ra), rms roughness (RRMS) and maximum peak-to-valley height (P-V) value
for carbon steel surface immersed in various test solutions are given in Table 6.
The value of Ra, Rq and P-V for the polished carbon steel surface (reference sample) are 5.6241nm, 8.1069 nm
and 44.40 nm respectively, which shows a more homogeneous surface, with some places in where the height is
lower than the average depth 34. Fig.4. (a, d, g) displays the uncorroded metal surface. The slight roughness
observed on the polished carbon steel surface is due to atmospheric corrosion. The average roughness, rms
roughness and P-V height values for the carbon steel surface immersed in sea water are 32.9000 nm, 40.2000
nm and 140.60 nm respectively. These data suggests that carbon steel surface immersed in sea water has a
greater surface roughness than the polished metal surface, which shows that unprotected carbon steel surface is
rougher and was due to the corrosion of the carbon steel in sea water. Fig.4. (b, e, h) displays corroded metal
surface with few pits.
Table 6. AFM data for carbon steel surface immersed in inhibited and uninhibited environments
Samples
Average (Ra)
Roughness
(nm)
RMS (Rq)
Roughness
(nm)
Maximum
peak-to-valley
(P-V) height
(nm)
Polished carbon
steel (control)
5.6241 8.1069 44.40
Carbon steel
immersed in sea
water
32.9000 40.2000 140.60
Carbon steel
immersed in sea
water
containing Urea
(250 ppm)and
Zn2+ (50ppm)
10.4000 14.5000 72.38
The presence of 250 ppm of Urea and 50 ppm of Zn2+ in sea water reduced the RRMS by a factor of 2.77
(14.5000 nm) from 40.2000 nm and the average roughness is significantly reduced to 7.1882 nm when
compared with 32.9000 nm of carbon steel surface immersed in sea water. The maximum peak-to-valley height
also was reduced to 72.38 nm. These parameters confirm that the surface appears smoother. The smoothness of
the surface is due to the formation of a compact protective film of Fe2+ - Urea complex and Zn(OH)2 on the
metal surface thereby inhibiting the corrosion of carbon steel. Also the above parameter 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.
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Fig. 4. 2D AFM images of carbon steel surface.
(a) Polished carbon steel (control)
(b) Carbon steel immersed in sea water (blank)
(c) Carbon steel immersed in sea water containing Urea (250 ppm) + Zn2+ (50 ppm)
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Fig. 4. 3D AFM images of carbon steel surface.
(a) Polished carbon steel (control)
(b) Carbon steel immersed in sea water (blank)
(c) Carbon steel immersed in sea water containing Urea (250 ppm) + Zn2+ (50 ppm)
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Fig. 4. The cross-sectional profiles, which are corresponding to as Shown broken lines (black colour) in AFM images of carbon steel
surface.
(a) Polished carbon steel (control)
(b) Carbon steel immersed in sea water (blank)
(c) Carbon steel immersed in sea water containing Urea (250 ppm) + Zn2+ (50 ppm)
4. Mechanism of corrosion inhibiton
With these discussions, a mechanism is proposed for the corrosion inhibition of carbon steel immersed in sea
water 250 ppm of Urea and 50 ppm of Zn2+.
 When the formulation consisting of 250 ppm of urea and 50 ppm of Zn2+ in sea water there is a
formation of urea – Zn2+ complex in solution.
 When carbon steel is immersed in this solution urea – Zn2+ complex diffuses from the bulk of the
solution towards the metal surface.
 Urea – Zn2+ complex is converted into urea – Fe2+ complex on the anodic sites of the metal surface
with the release of Zn2+ ion.
Zn2+ – Urea + Fe2+ ----------> Fe2+ – Urea + Zn2+
 The released Zn2+ combines with OH – to form Zn(OH)2 on the cathodic sites of the metal surface.
Zn2+ + 2 OH – ------------> Zn(OH)2 ↓
 Thus the protective film consists of Fe2+ – Urea complex and Zn(OH)2.
 In near neutral aqueous solution the anodic reaction is the formation of Fe2+. This anodic reaction is
controlled by the formation of Urea – Fe2+ complex on the anodic site of the metal surface. The
cathodic reaction is the generation of OH–. It is controlled by formation of Zn(OH)2 on the cathodic
sites of the metal surface.
Fe ----------> Fe2+ + 2 e– (Anodic reaction)
H2O + ½ O2 +2e– ----------->2 OH – (Cathodic reaction)
Fe2+ + Urea ----------> Fe2+ - Urea complex
Zn2+ + 2 OH – -----------> Zn(OH)2
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 This accounts for the synergistic effect of Urea – Zn2+ system.
5. Conclusions
The present study leads to the following conclusions:
The formulation consisting of 250 ppm Urea and 50 ppm Zn2+ shows 94% IE. The synergism parameter and F –
test confirms the existence of synergistic effect between urea and Zn2+ ion. The polarization study reveals that
Urea – Zn2+ system controls cathodic reaction predominantly. FTIR spectra reveal that the protective film
consists of Fe2+ – Urea complex and Zn(OH)2. The AFM images confirm the formation of protective layer on
the metal surface and hence the corrosion process is inhibited.
Acknowledgement
The authors are thankful to their respective managements and All India Council for Technical Education
(AICTE), New Delhi, India.
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M. Manivannan et al. / International Journal of Engineering Science and Technology (IJEST)
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