Abstract
An investigation was conducted to improve the corrosion inhibition efficiency of molybdate-based inhibitors for mild steel which is the main construction material of cooling water systems, using nitroethane as an organic compound beside zinc. In this study a new molybdate-based inhibitor was introduced with the composition of 60 ppm molybdate, 20 ppm nitrite, 20 ppm nitroethane, and 10 ppm zinc. Inhibition efficiency of molybdate alone and with nitrite, nitroethane, and zinc on the uniform corrosion of mild steel in stimulated cooling water (SCW) was assessed by electrochemical techniques such as potentiodynamic polarization and electrochemical impedance (AC impedance) measurements. Weight loss measurements were made with coupon testing specimens in the room temperature for 48 h. Studies of electron microscopy, including scanning electron microscopy (SEM) photograph and X-ray energy dispersive spectrometry (EDS) microanalysis, were used. The results obtained from the polarization and AC impedance curves were in agreement with those from the corrosion weight loss results. The results indicate that the new inhibitor is as effective as molybdate alone, though at one-ninth of the concentration range of molybdate, which is economically favorable.
1. Introduction
The protection of cooling water systems as well as heat supply water has become one of the great important issues in the world economy. The application of corrosion inhibitors especially in closed systems holds a prominent place amongst other methods of corrosion control [1]. The actual trends in the environmental protection essentially have changed the traditional approach to corrosion inhibition. Since the toxicity of chromate-based inhibitor is a limiting factor in its use as a corrosion inhibitor, the changes in formulation of corrosion inhibitors are prompted primarily by an increasing demand to reduce environmental impact [2]. Molybdate-based inhibitor has long been known as an inorganic and anodic type of corrosion inhibitor, which is effective for protecting mild steel in the pH range 5.8–8.5 [3, 4]. Lizlovs has observed that in the aqueous system containing aggressive ions, molybdate has corrosion inhibition only in the presence of oxygen [5]. In fact, the presence of aggressive ions such as chloride (Cl−) and sulfate anions reduces the efficiency of , so higher concentrations are necessary for corrosion inhibition [6, 7], which is not economically favorable. In order to achieve better efficiency and reduce the quantity of molybdate‚ other oxidizing agents such as nitrite () and organic compounds have been employed. Recently, the best method to improve inhibitive capability is using inhibitors in combination with others [8–11]. As it has been observed previously, organic inhibitors usually designated as a film forming protect the metal by forming a hydrophobic film on the metal surface. Therefore, natural organic molecules containing one pair of electrons or associated with multiple specially triple bonds or organic rings can bond to metal surface by electron transfer to the metal to form a coordinate type of link, which ultimately produces a barrier to the dissolution of the metal in the electrolyte [12]. As nitroethane contains a carbon chain together with oxidizing agent (), it is predicted that it can improve inhibition efficiency as an organic compound. In addition, Jefferies and Bucher recently studied the addition of zinc as cathodic inhibitor to improve the corrosion inhibition behavior of molybdate-based inhibitors [13]. As a result, it is worth investigating the corrosion inhibition of mild steel in stimulated cooling water (SCW) by using new developed inhibitor containing molybdate, nitrite, nitroethane, and zinc.
2. Experimental Procedures
Coupon testing specimens with the dimensions of 5, 2, and 0.3 cm were used for weight loss measurements. The composition of mild steel specimens is shown in Table 1.
Mild steel of the same alloy composition with an exposed area of 1 cm2 was embedded in epoxy resin and used for electrochemical measurements. Tafel polarization measurements were carried out at open circuit potential (), potentiostat/galvanostat (Princeton Applied Research EG & G Model 263 A), using counter electrode (Pt) and a saturated calomel electrode (SCE) as reference electrode. All the quoted potentials are referred to this reference electrode. The potentiodynamic current-potential curves were carried out at a scan rate 1 mV/sec. For impedance measurement, the same equipment was used for the Tafel polarization measurement and combined with frequency response analyzer (Princeton Model 1020). The impedance measurement was carried out using AC signals of amplitude 10 mV peak to peak at the open circuit potential in the frequency range 100 KHz to 10 mHz. Before each test, the specimens were prepared with silicon carbide paper 220–1200 grit, degreased in acetone, and washed in distilled water. Test solutions with different inhibitors (Table 2) were prepared by dissolving analytical grade sodium molybdate (Na2MoO4), sodium nitrite (NaNO2), zinc sulfate (ZnSO4), and sodium nitroethane (C2H5NNaO2).
Figure 1 shows the molecular structure of sodium nitroethane. Distilled water with 500 ppm sodium chloride, 520 ppm sodium sulfate, 170 ppm sodium bicarbonate, and 25 ppm sodium carbonate was used as SCW [14].
All tests were performed at neutral pH and room temperature. Before measurements of polarization curves, a stabilization period of 20 min was observed which proved sufficient as indicated by open circuit potential (). Table 2 shows the composition of different inhibitors used in this work.
The surface morphology of the mild steel samples after immersion in SCW without inhibitor and with inhibitor 1000 M and the new optimized inhibitor was photographed using SEM and was analyzed by EDS (Cam Scan 2300 MV).
3. Results and Discussion
3.1. Polarization Curves
The anodic and cathodic polarization curves for mild steel in SCW without inhibitor and with different inhibitors are shown in Figures 2–5. Figure 2 shows polarization curves for mild steel without inhibitor and with inhibitor 1000 ppm molybdate, 40 ppm nitrite, and 20 ppm nitroethane.
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The addition of 1000 ppm molybdate, 40 ppm nitrite, and 20 ppm nitroethane shifted the corrosion potentials of the mild steel to more positive values and also decreased the corrosion current densities (), indicating that molybdate, nitrite, and nitroethane are anodic inhibitors.
Table 3 gives values of corrosion potentials (), corrosion current densities (), Tafel slopes (βc and βa), and inhibition efficiency () obtained from polarization measurements of mild steel for different inhibitors in SCW. The corrosion current densities were obtained from the polarization curves by linear extrapolation of Tafel curves at point of 50 mV more positive and 50 mV more negative than , respectively.
The inhibition efficiency is defined as where and are the corrosion current density values without inhibitor and with different inhibitors, respectively.
For the curve 1000 M, corrosion current density is 0.185 (μA/cm2), indicating a negligible corrosion rate. But it is not suitable because the concentration is high. According to the obtained results, molybdate was considered as a weak oxidizer, so other oxidizing agents such as nitrite and nitroethane were added to molybdate in order to achieve better efficiency and reduce the quantity of molybdate.
Figure 3 exhibits the polarization curves for mild steel exposed to mixture of 60 ppm molybdate and 40 ppm nitrite, 40 ppm nitrite and 20 ppm nitroethane, and 60 ppm molybdate and 20 ppm nitroethane.
According to the curves 60M40N, a concentration of 60 ppm and 40 ppm appeared to be more efficient than molybdate and nitrite alone at mentioned concentrations [11]. Also by considering curve 40N20Ne, one should point out that in solution containing both nitrite and nitroethane ions, anodic curves were shifted to more positive values compared with nitrite and nitroethane alone, indicating synergistic behavior and improvement in the inhibition efficiency. According to Figure 2, nitroethane with half of the concentration range of nitrite exhibits better inhibition corrosion efficiency. So the combination of 60 ppm molybdate and 20 ppm nitroethane was used in order to achieve better efficiency and reduce the corrosion current densities of Tafel curve 60M40N.
Considering the fact that cathodic inhibitors improve the corrosion inhibition efficiency and reduce the corrosion current densities, combination of zinc with mixture of 60 ppm molybdate and 40 ppm nitrite and mixture of 60 ppm molybdate and 20 ppm nitroethane was used to improve the corrosion inhibition efficiency of Tafel curves 60M40N and 60M20Ne, as shown in Figure 4.
According to the Tafel curves 60M40N10Zn and 60M20Ne10Zn, addition of zinc (with delay to cathodic reactions) to oxidizing agents such as molybdate, nitrite, and nitroethane improves the corrosion inhibition efficiency and reduce the corrosion current densities of mild steel specimens in SCW. Moreover, addition of zinc to Tafel curves 60M40N and 40N20Ne shifts the cathodic and anodic branches of the Tafel plots to less corrosion current values () at relatively the same corrosion potentials, indicating that zinc acts as cathodic inhibitor besides other anodic inhibitors.
Polarization curves for mild steel were exposed to SCW water by adding molybdate, nitrite, nitroethane, and zinc (60 ppm, 20 ppm, 20 ppm, and 10 ppm, resp.), and 1000 ppm molybdate alone is shown in Figure 5. For comparison polarization curves in SCW and without inhibitor are included.
The suitable combination of beside nitroethane and zinc ions provides the necessary oxidizing environment to support film formation and synergically lower the corrosion rate. This combination also overcomes the high concentration of , with relatively the same, which is economically favorable. Also for optimized inhibitor current density was on the order of 0.173 (μA/cm2), which was lowered by about 6.9% compared with 1000 ppm molybdate alone. It can be attributed to the absorption of other ions on the metal in conjunction with to produce an insoluble compound, providing passivity more readily than alone [11].
Regarding these results, it can be concluded that the value of corrosion of current density of mild steel in SCW with the addition of molybdate decreases, and with addition of nitrite, nitroethane, and zinc ions to molybdate it decreases more and its inhibition efficiency increases.
3.2. AC Impedance Curves
Figures 6 and 7 show the Bode and Bode phase plots of steel electrode without inhibitor and with different inhibitors. The main effect of different inhibitors is an increase in the impedance modulus, , below 10 mHz (an increase in polarization resistance ()) and also a higher phase angle.
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In Bode phase plots, only one peak is in the phase angle () versus frequency () plot, indicating that there is only one time constant (a single relaxation time constant).
Figure 8 shows Nyquist plots for the steel electrode in SCW solutions without inhibitor and with different inhibitors. The impedance loops measurements were depressed semicircles with their center below the axis. This phenomenon is known as the dispersing effect [15]. In Nyquist plots, the corrosion resistance of each of the samples was determined by . is given by [16] where represents the real part of the complex faradic impedance, and correspond to the angular velocity of the AC signal (, where is frequency, (Hz)). values were obtained by fitting the experimental Nyquist data to a simple semicircle and extrapolating to .
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By considering AC impedance curves, it was found that polarization resistance increases in the following order.
BLANK < 40N < 20Ne < 40N20Ne ≈ 60M40N < 60M40N10Zn < 60M20Ne < 60M20Ne10Zn < 1000 M ≤ 60M20N20Ne10Zn.
Table 4 gives the values of inhibition efficiency () and polarization resistance () obtained from the AC impedance measurements. The inhibition efficiency is defined as where and are the polarization resistance without inhibitor and with different inhibitors, respectively.
The results of the AC impedance test exhibit a good correlation with data obtained from the polarization curves.
3.3. Weight Loss Measurements
The weight loss of samples after 48 h exposure to solution SCW without inhibitor and with different inhibitors was measured by coupon testing methods [17]. Regarding experimental weight loss data, inhibition efficiency () was calculated classically as follows: where and are the weight loss observed in the absence and in the presence of inhibitor, respectively. Values of inhibition efficiency for mild steel in SCW without inhibitor and with different inhibitors are summarized in Table 5.
For comparison, the inhibition efficiency of polarization, AC impedance, and weight loss results were summarized in Figure 9. According to Figure 9, the results obtained from the polarization and AC impedance curves are in agreement with those from the corrosion weight loss tests.
3.4. Results of Surface Analysis Techniques
3.4.1. SEM Analysis
The presence of corrosion inhibitor could be more clearly investigated by means of surface analysis techniques. SEM micrographs of samples after 48 h exposure to (a) solution SCW without inhibitor, (b) solution SCW with inhibitor 1000 M, and (c) solution SCW with new optimized inhibitor are shown in Figure 10. Figures 10(b) and 10(c) exhibit a layer with uniform, adherent, and continuous structure, while Figure 10(a) shows crystal growth of iron oxide on the surface and its noncontinuous structure. In fact, the decrease in corrosion current densities and increase in polarization resistance when exposed to the previous inhibitors were due to the coverage of metal surface with more protective films [18] as shown in Figure 10. Also the surface roughness of mild steel exposed to solution SCW is much higher than mild steel exposed to solution SCW with the inhibitor 1000 M and optimized inhibitor.
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3.4.2. EDS Analysis
EDS analysis of samples after 48 h exposure to solution SCW without inhibitor and with inhibitor 1000 ppm molybdate and new optimized inhibitor is shown in Figure 11. The presence of C, Si, Mn, and Fe element can be observed in all figures. But the presence of Mo is only in Figures 11(b) and 11(c), indicating that molybdate oxide is formed on the surface of mild steel sample as stabilizer through its incorporation into the oxide film [18].
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In addition, as shown in Figure 11(c), the carbon peak is much higher than (1000 ppm) molybdate alone, indicating that nitroethane ions are adsorbed on the surface besides molybdate ions to improve its inhibition efficiency synergically.
Finally, a glance at the electrochemical (polarization, AC impedance and weight loss method) and surface analysis (SEM photograph and EDS microanalysis) results indicates that new optimized inhibitor has relatively better efficiency than 1000 ppm molybdate alone at one ninth of the concentration range of molybdate, which is economically favorable.
4. Conclusions
(1) was proved to be a weak oxidizer, so another oxidizing agents such as and nitroethane beside cathodic inhibitor zinc were shown to be required to provide adequate protection for mild steel in stimulated cooling water.(2)Results obtained from polarization curves showed that new optimized inhibitor with more than 98 percent inhibition efficiency noticeably lowers the corrosion rate of mild steel in stimulated cooling water.(3)AC impedance curves showed that the combination of molybdate, nitrite, and nitroethane, beside cathodic inhibitor zinc in optimized range can be a suitable replacement for high concentration of 1000 ppm molybdate.(4)The results of weight loss measurements obtained from coupon testing specimens exhibit a good correlation with data obtained from the polarization and AC impedance curves.(5)The presence of molybdate as stabilizer of oxide film is observed through the EDS analyzer.(6)SEM images indicate that new optimized inhibitor could form a relatively steady, compact, and uniform film on the surface of mild steel.