New Aromatic Azo-Schiff as Carbon Steel Corrosion Inhibitor in 1 M H₂SO₄

Introduction

Acidic solutions are usually used in industry for variety of purposes such cleaning and descaling of metallic parts; however, such processes are accompanied with the problem of corrosion which is considered as one of the most damaging problems that occur as a result of the dissolution of metals due to the exposure of the metallic parts to the acidic solutions.^1-2 It is well known that carbon steel alloy is the alloy of the choice to be used in the manufacturing of the machines due to its unique mechanical properties. Accordingly, reduction or inhibition of the carbon steel corrosion in acidic environments became an interesting research area. The use of organic materials as corrosion inhibitors is considered as one of the best ways to protect steel alloys form the aggressive acidic attack, they form a shield-like layer after the molecules of the applied inhibitor being adsorbed on the surface of the metal acting as an insulator between the metal and the environment and inhibit the corrosion process.^3-5 It is reported that the presence of π-electrons and heteroatoms such as N, O, P or S in the structure of the aromatic organic molecules makes them effective corrosion inhibitors since they have such adsorption centers that bind the molecules onto the metal surface forming a protective film.^6-11 Among the reported organic inhibitors ones that contain nitrile (-CN) [12, 13], imine (-C=N)^14,15 and/or azo (-N=N-)¹⁶ groups in their structures.

In the present work we report the design, synthesis, and use of new azo-Schiff compound as carbon steel corrosion inhibitor in 1.0 M H₂SO₄ solution. The new inhibitor is readily synthesized via two steps synthetic pathway from relatively inexpensive starting materials, it is designed to be rich with π-electrons, has azo (-N=N-), imine (-C=N) and methoxy (-OCH3) groups in its structure, a non-ionic surfactant tween 80 is used in this study to increase the solubility of the inhibitor and consequently improve the corrosion inhibition efficiency.

Experimental

Preparation of Steel Specimens

Cylindrical carbon steel specimens (24 mm diameter and 3 mm height) composed of: 0.22% C, 0.23% Si, 0.83% Mn, 0.006% P, 0.008% S, 0.79% Cr, 0.33% Mo, 0.08% Ni, 0.02% Al, 0.19% Cu and Fe were used. The specimens were mechanically grounded by grinding and polishing machine with emery papers grade (80 – 3000), a mirror-like surface was obtained after treatment with polishing paste (diamond suspension, 1μm). Finally, samples were rinsed in distilled-water then in absolute ethanol under ultrasonic conditions and kept in a desiccator until used.

Synthesis of the Organic Inhibitor (AS)

The target compound was synthesized via two steps as shown in Scheme 1.

Scheme 1: Synthesis of AS Click here to View scheme

 

Compound 3 was synthesized according to a literature procedure,¹⁷ then 3 (0.5 gm, 1.99 mmol) was dissolved in DCM (5 ml). 4-Methoxyaniline (0.245 gm, 1.99 mmol) was dissolved in absolute methanol (25 ml), the two solutions were mixed and catalytic amount of glacial acetic acid was added. The reaction mixture was subjected to reflux conditions for 5 h. The product was collected by filtration and washed with hot methanol. The structure of AS, Figure 1, was characterized by FT-IR, ¹H NMR and ¹³C NMR.

Figure 1: Structure of AS. Click here to View figure

 

Yield 80%. MP 238-240°C. 

FT-IR (KBr, cm-1): ν (3431, OH), (2227, C≡N), (1617, C=N), (1485, N=N), (1591, C=C).

¹H NMR (400 MHz, DMSO-d6) δ (ppm): 14.07 (s, 1H, H-7), 9.16 (d, J = 4.1 Hz, 1H, , H-4), 8.34 (d, J = 4.5 Hz, 1H, H-8), 8.09 – 8.00 (m, 3H, H-2,16,12), 8.00- 7.93 (m, 2 H, H-13,15), 7.57 – 7.46 (m, 4 H, H-20,21,23,24), 7.36 (apparently m, 1 H, H-1), 7.25 – 7.11 (m, 1 H, H-22)

¹³C NMR (101 MHz, DMSO-d6) δ 165.63 (C-6), 163.01 (C-8,11), 145.19 (C-19), 134.28 (C-3), 130.04 (C-13,15), 129.46 (C-21,23) , 128.01 (C-22), 127.91 (C-4) 123.37 (C-12,16), 121.87 (C-20,24), 119.76 (C-5), 118.95(C-17), 114.34 (C-1) , 113.10 (C-14).

Preparation of Solutions

Different concentrations (0.005, 0.01, 0.02, 0.04 and 0.08 mM) of AS were prepared, 0.029 gm of AS was dissolved it in DMSO (2 ml) and 0.01 gm of tween 80, then volume was brought up to 1000 ml with of 1.0 M H₂SO₄ to get 0.08 mM concentration which used as stock solution for the preparation of the other concentrations by dilution.

Potentiodynamic Polarization Measurements

Potentiodynamic polarization measurements were carried out by three electrodes cell. Saturated calomel electrode Hg / Hg₂Cl₂ was used as a reference electrode, Pt electrode was used as an auxiliary electrode. The carbon steel specimen was connected to the working electrode with exposure area of 1 cm². The working electrode was then dipped in the test solution for (30 min with a time step of 2 sec). The change in electrode potentials was ±250 mV and sweep rate 0.3 mVs^-1. By the extrapolation of Tafel plots segments, the corrosion current density (I_corr) was measured. The inhibition efficiency (Ƞ%) was calculated according to equation (1)¹⁸

Where I_corr and I_inh represent corrosion current density in the absence and presence of AS respectively.

Weight Loss Analysis

Carbon steel specimens were accurately weighed and immersed in the aggressive solution (200 ml, with and without different concentrations of the AS) for 3 h at 313 K. Specimens were then washed with distilled water and ethanol under ultrasonic conditions. After drying, specimens were accurately weighed. Duplicate experiments were done in each case. The inhibition efficiency (Ƞ_w%) and surface coverage were calculated according to equation (2)and (3) respectively^19,20

Where W and Wo represent the weight loss for the specimen after immersion with and without inhibitors.

Scanning Electron Microscope (SEM) Analysis

The test sample were immersed in the acidic solution (1 M H₂SO₄) for 3 h in the absence and presence of AS (0.08 mM) , sample was then washed thoroughly with methanol, dried and tested by SEM;

Results and Discussion

Potentiodynamic Polarization Measurements

Potentiodynamic polarization measurements were used to test the corrosion inhibition efficiency of carbon steel by the new compound AS in 1 M H₂SO₄. The experiments were carried out at different temperatures (303, 313, 323 and 333 K), in presence of different concentrations of AS (0.00, 0.005, 0.01, 0.02, 0.04 and 0.08 mM). Table 1 illustrates the measured parameters that include: corrosion potential (E_corr), corrosion current density (I_corr), cathodic and anodic Tafel slopes (βc and βa), inhibition efficiency (Ƞ%) and surface coverage (Ɵ). The results indicate that increasing the concentration increases the inhibition efficiency at each temperature, the maximum recorded inhibition efficiency was 93.9% in the presence of AS in 0.08 mM concentration at 313 K. It was also found that increasing the temperature above 313 K decreases the inhibition efficiency, such decrement in  the efficiency may be attributed to the desorption of AS molecules form the adsorbed protective film on the metal surface. The displacement in the E_corr values that results from the presence of AS in the acidic solution was found to be less than 85 mV (Table 1), this indicates that AS acts as a mixed type inhibitor which inhibits both anodic and cathodic reactions simultaneously [21]. Figure 2 shows the obtained polarization curves.

Table 1: Polarization parameters for carbon steel corrosion in 1 M H2SO4 in the presence of different concentrations AS at different temperatures. 

 

Figure 2: Potentiodynamic polarization curves for carbon steel in 1 M H2SO4 in the absence and presence of AS at 313 K Click here to View figure

 

Weight Loss Measurements

Weight loss measurements were performed using different concentrations of AS at 313 K (the optimum temperature determined from the potentiodynamic polarization measurements); Table 2 shows the obtained results. It was found that the corrosion inhibition efficiency increases as the inhibitor’s concentration increases, this could be attributed to the increment of the adsorbed molecules of AS on the metal surface (higher surface coverage) forming most effective protecting film, the maximum inhibition efficiency (97.1%) was observed at the highest concentration of AS (0.08 mM).

Table 2: Inhibition efficiencies of AS according to weight loss measurements. 

AS Concentration (mM)	Inhibition efficiency Ƞw%	Surface coverage Ɵ
0.005	89.8	0.898
0.01	92.9	0.929
0.02	93.1	0.931
0.04	93.3	0.933
0.08	97.1	0.971

 

Adsorption Isotherm

A linear plot was obtained from plotting C_inh/Ɵ versus C_inh, indicating that the adsorption of AS on the metal surface obeys Langmuir isotherm, Figure 3 shows Langmuir isotherms of AS at 303 – 33 K. Equations (3) and (4) were used to calculate the equilibrium constant (K_ads) and Gibbs’ free energy (ΔG_ads) of the adsorption process^22,23

The standard entropy change (ΔH_ads) values were calculated according to equation (6)²⁵

Figure 3: Langmuir adsorption isotherms of AS at different temperatures 303 –33 K. Click here to View figure

 

Figure 4: log Kads versus 1/T for AS. Click here to View figure

 

The calculated thermodynamic parameters for the adsorption process are shown in Table 3. The high values of K_ads indicate that there is a high affinity for AS to be adsorbed on the metal surface forming the protective film and leading to the high inhibition efficiency. From the negative values of ΔG denote that the adsorption process is spontaneous, and confirms the stability of the adsorbed film on the carbon steel surface; furthermore, its value indicates the physisorption process.²⁶

Table 3: The calculated thermodynamic parameters for the adsorption of AS.

Scanning Electron Microscope (SEM) Analysis

To confirm the high corrosion inhibition efficiency of AS, SEM was used to study the effect of the aggressive acidic solution on the surface of the carbon steel in the presence and absence of AS by visualizing the damage caused as a result of the corrosion process, Figure 5 shows the obtained images with 10 µm scale. It is clear that the surface of the uninhibited sample (absence of AS, Figure 4 A) is severely damaged due to the acid corrosion, while the surface of the inhibited sample (presence of AS, , Figure 4 B) is smoother and no significant damage was observed, this confirms that AS is acting as excellent corrosion inhibitor.

Figure 5: SEM images of uninhibited (A) and inhibited (B) carbon steel surface Click here to View figure

 

Conclusions

A new azo-schiff compound (AS) has been synthesized, the inhibition efficiency of AS for carbon steel corrosion in 1 M H₂SO₄ was investigated, tween 80 was used as a surfactant to improve the solubility of the inhibitor. High inhibition efficiency was recorded (93.3% by potentiodynamic polarization and 97.1% by weight loss measurements) at 0.08 mM concentration. Thermodynamic studies proved that AS is spontaneously adsorbed on the carbon steel surface via physisorption interaction forming a stable protecting film in the aggressive acidic environment. Such high corrosion inhibition efficiency with such low concertation make AS an excellent carbon steel corrosion inhibitor with environmental and economic advantages.

Acknowledgments

The authors thank the University of Karbala and the University of Babylon for the financial support.

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