O-Nitro Phenol as a Corrosion Inhibitor for Carbon Steel

 

H. BenitaSherine¹*, A. Jency Angela¹ and S. Rajendran²

¹Department of Chemistry, Holy Cross College, Tiruchirappalli – 620 002, Tamilnadu, India.

²Department of Chemistry, GTN Arts College, Dindigul – 624 005, Tamilnadu, India.

*Corresponding Author E-mail: beni2@rediffmail.com

ABSTRACT:

The inhibition efficiencies of inhibitor systems constituting various combinations of o-nitrophenol (ONP) and zinc ions in controlling corrosion of carbon steel immersed in ground water for seven days had been evaluated by weight loss method. The nature of the protective film formed on the metal surface has been analysed by FTIR spectra. The protective film was found to consist of Fe²⁺ - ONP complex. Synergism parameters and Analysis of variance (ANOVA) had been used to evaluate the synergistic effect existing between various inhibitors.

 

 

 

INTRODUCTION:

Corrosion problems have received a considerable amount of attention because of their attack on materials.  The use of inhibitors is one of the most practical methods for protection against corrosion. Several works have studied the influence of organic compounds containing nitrogen on the corrosion of steel in acidic media^1-9, most organic inhibitors act by adsorption on the metal surface⁴. Organic compounds containing lone pairs and/or multiple bonds usually are effective inhibitors ^10-12.

 

The influence of aminophenols on the corrosion and hydrogen permeation of mild steel in 1M HCl and 0.5 M H₂SO₄ has been been studied by Rengamani¹³et al  using weight loss and gasometric measurement. Muller et al., have studied on the use of phenol and the substituent phenols such as amino phenols and nitro phenols for aluminium pigment in acid and alkaline medium as corrosion inhibitors¹⁴. The corrosion inhibiting effect of 2-amino and 2-nitrophenol is excellent for aluminium pigment whereas 4-nitro and 4-aminophenol do not inhibit this corrosion reaction. Ortho and para substituted phenol as corrosion inhibitors for N80 steel in hydrochloric acid by weight loss method and potentiostatic polarization method has been studied Viswanatham et al. The extent of decrease in the corrosion rates was found to depend on the nature of the corrosion inhibitors and their concentration¹⁵.

 

Kulkarni et al¹⁶ have studied the inhibition by phenols on the corrosion of mild steel in nitric acid, sulphuric acid  and hydrochloric acid media. A trend observed among the phenols in corrosion inhibition efficiency is p-cresol>m-cresol>phenol>2-naphthol>1-naphthol>2-nitrophenol. 

Among naphthols, 2-napthol was stronger inhibitor than 1-naphthol.  In general 2-naphthol and p-cresol are found to be stronger over the range of concentrations of acids used.

 

The present study was undertaken:

1. To evaluate the inhibition efficiencies of o-nitro phenol (ONP) in controlling the corrosion of carbon steel immersed in ground water in the presence and absence of zinc ions;

2. To evaluate the synergistic effect of ONP and zinc ions by determining synergism parameters;

3. To investigate whether the synergistic effect existing of this inhibitor system is statistically significant or not by doing F-Test using the Analysis of variance (ANOVA);

4. To propose a suitable mechanism of corrosion inhibition based on the results obtained from weight loss method, FTIR, AC impedance, polarization study and cyclic voltammetry.

 

EXPERIMENTAL:

Preparation of the Specimen:

Carbon steel specimens (0.03% S, 0.05% P, 0.5% Mn, 0.15% C and the rest iron) of the dimensions 1.0×4.0×0.2cm were polished to mirror finish and degreased with trichloroethylene and used for the weight loss method and surface examination studies.  For electrochemical studies, the surface area of metal surface was 1 cm².

 

Weight Loss Method:

The water quality parameters for ground water are given in Table 1. Three carbon steel specimens were immersed in 100ml of the solutions containing ground water and various concentrations of the inhibitor in the absence and presence of Zn²⁺ for a period of seven days. The weights of the specimens before and after immersion were determined using a Shimadzu balance AY62. Inhibition efficiency (IE) was calculated from the relationship

IE = (1-W₂/W₁) × 100

Where, W₁ = corrosion rate in the absence of inhibitor and

W₂ = corrosion rate in the presence of inhibitor.

 

Table 1.   Water Quality Parameters.

Parameters	Value
pH	7.30
TDS	1236 ppm
Chloride	232 ppm
Sulphate	75 ppm
Total Hardness	416 ppm
Conductivity	1792 μmhos/cm

 

Synergism Parameter (S_I):

Synergism parameters are indications of synergistic effect existing between the inhibitors ^17-19.  If S_I  value is found to be greater than one it suggests that a synergistic effect exists between the inhibitors.

                      1- ₁₊₂

S₁            =

                     1- ’₁₊₂

Where,

₁₊₂       = (₁+₂) – (₁₂)

₁       = Surface coverage () of inhibitor o-nitrophenol

₂       = Surface coverage () of inhibitor Zn²⁺

’₁₊₂   = Combined surface coverage () of inhibitor o-nitrophenol and  Zn²⁺.

 

Analysis of Variance (F-Test):

F-Test was carried out to investigate whether synergistic effect existing between inhibitor systems is statistically significant ^20-21.  If the F-value is above 5.32 for 1,8 degrees of freedom, it was proved to be  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.

 

Surface Examination Study:

The carbon steel specimens were immersed in various test solutions for a period of seven days. After seven days, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimens was analysed by surface analysis technique.

 

FTIR spectra:

These spectra were recorded with the Perkin Elmer 1600 spectrophotometer. The FTIR spectrum of the protective film was recorded by carefully removing the film, mixing it with KBr and making the pellet.

 

Potentiodynamic polarization study:

Polarization study was carried out in H and CH electrochemical workstation impedance Analyzer Model CHI 660A provided with IR compensation facility, using a three electrode cell assembly. Carbon steel was used as working electrode, platinum as counter electrode and saturated calomel electrode (SCE) as reference electrode. After having done IR compensation, polarization study was carried out at a sweep rate of 0.01 V/Sec. The corrosion parameters such as linear polarization resistance (LPR), corrosion potential E_corr, corrosion current I_corr and Tafel slopes (b_c and b_a) were measured.

 

Alternating Current Impedance Spectra:

AC impedance spectra were recorded in the same instrument used for polarization study, using the same type of three electrode cell assembly. The real part and the imaginary part of the cell impedance were measured in ohms for various frequencies. The charge transfer  resistance (R_t) and double layer capacitance (C_dl) values were calculated.

R_t = (R_s + R_t) – R_s

where R_s = solution resistance

C_dl = ½p R_t f _max

Where f _max = maximum frequency

 

Cyclic Voltammetry:

Cyclic Voltammetry was carried out H and CH electrochemical workstation impedance Analyzer Model CHI 660A provided with IR compensation facility, using a three electrode cell assembly. Carbon steel was used as working electrode, platinum as counter electrode and saturated calomel electrode (SCE) as reference electrode. The scan rate was 0.1 V/s. The graph between potential (V) vs current (A) was plotted.

 

RESULT AND DISCUSSION:

Weight loss method:

Corrosion rates of carbon steel in ground water, in the absence and presence of o-nitro phenol and Zn^2+, obtained by weight loss method are given in Table 3. The inhibition efficiencies are also given in Table 2.

 

Table 2.  Inhibition efficiencies (IE) of carbon steel in groundwater obtained by weight loss method.

Inhibitor: ONP + zn²⁺

ONP (ppm)	Inhibition efficiency (IE)% , Zn2+ (ppm)
	0	10	25	50	75	100
0	0	-30	-12	25	32	45
50	93	97	95	95	99	97
100	94	97	97	95	98	96
150	94	97	98	94	98	96
200	96	98	98	94	97	95
250	96	98	99	93	97	94

 

When carbon steel was immersed in ground water, the corrosion rate was 195.7 mdd. Upon addition of various concentrations of ONP, the corrosion rate decreased. There was protection of the metal from corrosion. 250 ppm of ONP had 96% inhibition efficiency.

Table-3   Corrosion rates (cr) of Carbon Steel in ground water obtained by weight loss method.

Inhibitor: ONP + Zn²⁺

ONP (ppm)	Corrosion rate (CR) mdd
	0	10	25	50	75	100
0	195.7	254.4	219.9	146.7	133.0	107.6
50	13.6	5.8	9.7	9.7	1.9	5.8
100	11.7	5.8	7.8	9.7	3.9	7.8
150	11.7	5.8	5.8	11.7	3.9	7.8
200	7.8	3.9	3.9	11.7	5.8	9.7
250	7.8	3.9	3.9	13.6	5.8	11.7

 

Influence of Zn²⁺ on the inhibition efficiency of ONP:

The influence of a divalent metal ion Zn²⁺, on the inhibition efficiency of ONP in controlling corrosion of carbon steel, is given in Table 2. The inhibition efficiencies of various concentrations of Zn²⁺, namely 10, 25, 50, 75 and 100 ppm were -30,-12, 25, 32 and 45% respectively. Negative sign indicated acceleration of corrosion.  It was observed from Table 2, that the synergistic effect exists between Zn²⁺ and ONP. For example 25 ppm of Zn²⁺ has -12% inhibition efficiency and 250 ppm of ONP has 96% inhibition efficiency. But their combination had 99% inhibition efficiency.  The role of Zn²⁺ is to transport the 1-naphthol inhibitor from the bulk of the solution onto the metal surface ^22-24. Generally it was observed that the inhibition efficiency remained almost constant upon addition of Zn^2+.

 

Synergism parameters:

The values of synergism parameters are shown in Table 4. S_I approaches 1 when no interaction exists between the inhibitor compounds. When S_I>1, this points to synergistic effect.  In the case of S_I < 1, the negative interaction of inhibitor prevails, (i.e. corrosion rate increases). From Table 4,  it can be seen that the values of S_I are greater than unity, for 10 ppm  25 ppm and 75ppm of Zn²⁺, suggesting that the phenomenon of synergism existing between ONP and Zn²⁺.  Thus, the enhancement of the inhibition efficiency caused by the addition of Zn²⁺ ions to ONP is only due to the synergistic effect. For other concentrations of Zn²⁺ namely 50 ppm and 100 ppm, the values of S_I are negative.  This suggested that there was no synergistic effect between ONP and Zn²⁺ at these concentrations.

 

Analysis of Variance (ANOVA):

To investigate whether, the influence of Zn²⁺ on the inhibition efficiencies of ONP is statistically significant, the analysis of variance (F-test) was carried out ²⁵. The results are given in Table 5.

 

In Table 5, the influence of 10 ppm, 25 ppm,50 ppm,75 ppm and 100 ppm of Zn²⁺ on the inhibition efficiencies of 50,100,150, 200, 250 ppm of ONP was investigated.

 

The obtained F-value 16 for 10 ppm Zn²⁺, 6.91 for 25 ppm Zn²⁺, 20.48 for 75 ppm Zn²⁺  was statistically significant, since it was greater than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance.  Therefore, it was concluded that the influence of 10 ppm Zn²⁺, 25 ppm Zn²⁺and 75 ppm Zn²⁺   on the inhibition efficiencies of various concentrations of ONP was statistically significant.

 

The obtained F-value 0.32 for 50 ppm Zn²⁺, 1.61 for 100 ppm Zn²⁺ was not statistically significant, since it was lesser than the critical F-value 5.32 for 1, 8 degrees of freedom at 0.05 level of significance.  Therefore, it was concluded that the influence of 50 ppm Zn²⁺ and 100 ppm Zn²⁺ on the inhibition efficiencies of various concentrations of ONP was not statistically significant.

 

Analysis of potentiodynamic polarization Curves:

The potentiodynamic polarization curves of carbon steel immersed in ground water, in the absence and presence of inhibitors are shown in Fig.1. The corrosion parameters such as corrosion potential (E_corr), corrosion current (I_corr), Tafel slopes (b_a, b_c) and linear polarization resistance (LPR) are given in Table 6.

 

Fig 1- Potentiodynamic polarization study

(a) Ground water (b) Ground water containing 50ppm of ONP and 75 ppm of Zn²⁺

 

Table 4. Synergism parameters derived from inhibition efficiencies of ONP – Zn²⁺ system.

ONP (ppm)		Zn2+ (ppm)
	0 (ppm)	10 (ppm)	SI	25 (ppm)	SI	50 (ppm)	SI	75 (ppm)	SI	100 (ppm)	SI
0	0	-30	-	-12	-	25	-	32	-	45	-
50	93	97	3.03	95	1.56	95	1.05	99	4.76	97	1.28
100	94	97	2.60	96	1.68	95	0.90	98	2.04	96	0.83
150	94	97	2.60	97	2.24	94	0.75	98	2.04	96	0.83
200	96	98	2.60	98	2.24	94	0.50	97	0.91	95	0.44
250	96	98	2.60	98	2.24	93	0.43	97	0.91	95	0.44

 

Table 5. Distribution of F – value between the inhibition efficiencies of various concentrations of ONP (0 ppm Zn²⁺) and the inhibition efficiencies of ONP in the presence of 10,25,50,75,100 ppm Zn²⁺.

Zn2+ (ppm)	Source of variance	Sum of squares	Degrees of freedom	Mean Square	F	Level of significance of F
10	Between	14.4	1	14.4	16	p>0.05
	Within	7.2	8	0.9
25	Between	12.1	1	12.1	6.9	p>.0.05
	Within	14	8	1.7
50	Between	0.4	1	0.4	0.3	P<0.05
	Within	10	8	1.2
75	Between	25.6	1	25.6	20.4	p>0.05
	Within	10	8	1.25
100	Between	2.5	1	2.5	1.6	P<0.05
	Within	12.4	8	1.55

 

Table-6     Corrosion parameters of carbon steel immersed in ground water  in the absence and presence of inhibitors obtained by  potentiodynamic polarization method. Inhibitor system: ONP+Zn²⁺

System	Ecorr mV vs SCE	bc mv/ decade	ba mv/ decade	LPR Ohm cm2	Icorr A/cm2
Ground water	-512	638	190	1.306x103	4.875x10-5
Ground water+50ppm  ONP +75ppm  Zn2+	-682	232	160	8.898x102	4.625x10-5

 

 

When carbon steel is immersed in ground water water, the corrosion potential (E_corr) is -512 mV vs SCE. The formulation consisting of 50 ppm of ONP and 75 ppm of Zn²⁺ shifts the corrosion potential to -682 mV vs SCE. i.e., the corrosion potential shifts to the cathodic direction. This suggests that the cathodic reaction is controlled predominantly indicating that the dissolution of the metal is reduced, since more inhibitor is transported to the cathodic sites in the presence of Zn²⁺.

 

The corrosion current for ground water is 4.875x10^-5 A/cm².  It is decreased to 4.625x10^-5 A/cm²   by the addition of 50 ppm of ONP and 75 ppm of Zn²⁺ . The current of the iron dissolution is decreased significantly, indicating that the metal surface was passivated by the formed inhibitor layer.  The passivity of iron is probably due to the formation of a ONP-Fe²⁺ surface layer. The significant reduction in corrosion current for inhibitor formulation may indicate more adsorption of the inhibitors and better inhibition performance ²⁶. This   indicates that a protective film is formed on the metal surface.

 

Analysis of AC-Impedance spectra:

The AC-impedance spectra of carbon steel immersed in ground water in the presence and absence of inhibitors are shown in Fig 2.

 

When carbon steel is immersed in ground water the R_t value is found to be 176.54 Ω cm² and the C_dl  value is 1.64x10^-8  m F/ cm².  When 75 ppm of Zn²⁺ and 50 ppm of ONP are added the R_t value has increased 227.43Ω cm² and C_dl value decreases to 1.28x10^-8  mF/cm².  The increased R_t value and decreased double layer capacitance value obtained from impedance studies confirm the formation of a protective film on the metal surface and justify the good performance of a compound as an inhibitor in well water ²⁷. This behavior shows that the film obtained acts as a barrier to the corrosion process that clearly proves the existence and formation of the film ^28,29.

 

Fig 2 – AC- Impedance spectra of carbon steel immersed in various test solutions (a) Ground water (b) Ground water containing 50ppm of ONP and 75 ppm of Zn²⁺

 

Analysis of bode plots:

The Bode plot of carbon steel immersed in ground water is shown in Fig 3.  The real impedance value [log (z/ohm)] was 2.383.  In the presence of inhibitors this value increased to 2.431. The plot obtained in the presence of inhibitor is characterized by a single time constant. This indicates the formation of a homogeneous film on the metal surface ^30,31. The impedance values increases in the presence of inhibitors.

Fig 3 – Analysis of Bode plot

(a)The Bode plot of carbon steel immersed in Ground water.

(b)The Bode plot of carbon steel immersed in Ground water containing 50 ppm of ONP and 75 ppm of Zn²⁺

 

Analysis of cyclic Voltammogram:

Cyclicvoltammograms did not show any redox couple or any characteristic peaks. [Fig 4]

 

Fig 4- Analysis of Cyclic voltammetry

(a) Cyclic voltammogram of ground water

(b) Cyclic voltammogram of ground water containing 50ppm of ONP and 75 ppm of Zn²⁺

 

Analysis of FTIR spectra:

The FTIR spectrum of pure ONP (KBr) is shown in Fig 5a.

The OH stretching frequency appeared at 3434 cm^-1.The C=C ring stretching frequency appeared at 1476 cm^-1.The symmetric N=O stretching frequency of NO₂ group appeared at 1532 cm^-1.The C-N stretching frequency of ArNO₂ appeared at 863 cm^{-1 32}.  The FTIR spectrum of the film formed on the metal surface after immersion in the solution containing ground water, 50ppm of ONP and 75ppm of Zn²⁺ was shown in Fig 5b. It was observed that the OH stretching frequency appeared has shifted from 3434-3444 cm^-1.The C=C stretching frequency has disappeared. The N=O stretching frequency has shifted from1532-1633 cm^-1.The C-N stretching frequency of ArNO₂ has disappeared. These observations indicated that ONP has coordinated with Fe²⁺ through oxygen atoms of phenolic group and also nitro group and through the п electrons of benzene ring resulted in the formation of Fe²⁺ -ONP complex formed on the anodic sites of the metal surface. The peak at 1340cm^-1 was due to Zn (OH) ₂ formed on the cathodic sites of the metal surface.

 

 

Fig 5 – FTIR spectra

(a) Pure ONP

(b)The FTIR spectra of film formed on the surface of carbon steel immersed in ground water containing 50ppm ONP and 75ppm Zn²⁺ Legends for figures

 

Table-7  AC-Impedance parameters of carbon steel immersed in the presence of inhibitors obtained from AC impedance spectra.

System	Rt Ohm cm2	Cdl mF/cm2	log(z/ohm)
Ground water	176.54	1.64x10-8	2.383
Ground water+50ppm  of ONP+75ppm of Zn2+	227.43	1.28x10-8	2.431

 

Mechanism of corrosion inhibition:

Weight loss study revealed that the formulation consisting of 50 ppm of ONP and 75ppm of Zn²⁺ had 99% inhibition efficiency in controlling corrosion of carbon steel immersed in ground water. Synergism parameters suggested that a synergistic effect existed between ONP and Zn^2+.  Polarization study revealed that this system functions as cathodic inhibitor. AC impedance spectra revealed that a protective film is formed on the metal surface. FTIR spectra revealed that the protective film consists of Fe²⁺-ONP complex and Zn (OH) ₂.

 

In order to explain the above fact in a holistic way the following mechanism of corrosion inhibition was proposed.

When the formulation consisting of 50 ppm of ONP and 75ppm of Zn²⁺ was prepared, there was formation of Zn²⁺ - ONP complex in solution.

 

When carbon steel was immersed in this solution, there was diffusion of Zn²⁺ -ONP complex towards the metal surface.

On the metal surface, it was converted in to Fe²⁺ -ONP complex. Zn²⁺ was released.

 

Zn (OH)₂ was formed.

Zn²⁺ - ONP + Fe²⁺  à Fe ²⁺ -ONP +    Zn²⁺

Zn²⁺ + 2OH^-          à Zn (OH) ₂ ↓

 

Thus the protective film consists of Fe²⁺ -ONP complex formed on anodic sites of the metal surface and Zn (OH) ₂ deposited on cathodic sites of the metal surface.

 

CONCLUSION:

The inhibition efficiency of ONP -Zn²⁺ system in controlling corrosion of carbon steel ground water has been evaluated by weight loss method.  The present study leads to the following conclusions.

 

Weight loss study revealed that the formulation consisting of 50 ppm of ONP and 75 ppm of Zn²⁺ had 99% inhibition efficiency in controlling corrosion of carbon steel immersed in ground water.

 

Synergism parameters suggested that a synergistic effect existed between ONP and Zn²⁺.

ANOVA test revealed that the synergistic effect of the above formulation was statistically significant.

Polarization study revealed that Zn²⁺ -ONP system functions as a cathodic inhibitor.

AC impedance spectra (Bode plots) revealed that a protective layer was formed on the metal surface.

 

Cyclic voltammograms revealed the absence of redox couple.

FTIR spectra revealed that the protective film consists of Fe²⁺ -ONP complex and Zn (OH) _2.

 

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Received on 12.12.2010        Modified on 05.01.2011

Accepted on 27.01.2011        © AJRC All right reserved