Inhibition of copper corrosion in cooling seawater under flowing conditions by novel pyrophosphate

Abstract

The inhibition of copper corrosion in cooling seawater by novel pyrophosphate SrNiP₂O₇ (SNP) was investigated under flowing conditions using mass-loss and electrochemical methods. The surface morphology was characterized by SEM coupled with EDX spectra. Comparable results show that SNP acts as a mixed-type inhibitor with predominantly cathodic effectiveness, suppressing the corrosive process by physical adsorption on the copper surface. The highest inhibition efficiency obtained from mass-loss, polarization and EIS measurements are 92.7%, 94.8% and 97.1%, respectively, at 120 mg L⁻¹ of SNP. The influence of increasing temperature on SNP inhibitor efficiency has been studied, and the activation energy has been calculated. Surface morphology observations evidenced the formation of a protective SNP film over the metal surface.

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1. Introduction

Cooling water systems are widely used in oil refineries and in chemical plants, as well as in homes, factories and public buildings.¹ These may be generally classified into two types.² One is the once-through cooling system, and the other is the recirculating cooling water system. Corrosion problems in the latter system are generally very high because seawater is a commonly used fluid in the cooling systems, particularly in coastal zones.³

Copper is widely used in cooling systems for domestic and industrial water utilities, including seawater, heat conductors, and heat exchangers.⁴ Despite the relatively corrosion resistant nature of copper, its corrosion takes place at a significantly accelerated rate in seawater.^5–7

The use of inorganic inhibitors is one of the most practical and effective methods for the protection of copper in cooling systems.^8,9 In general, chromate is accepted as a well-known corrosion inhibitor with the capability to passivate metals through the formation of a mono-atomic or poly-atomic oxide film on the metal surface. However, toxicity is the main disadvantage of using chromate.¹⁰

Environmental regulations drive researchers to increase their efforts in the development and evaluation of new non-toxic inorganic inhibitors to replace traditional inorganic inhibitors such as chromate. In view of these regulations, SNP was synthesized for the present study.

Recently, pyrophosphates have been considered to be of considerable industrial and biological importance because of their utility in various applications as catalysts, molecular sieves, or ion exchangers.¹¹

In general, various types of pyrophosphates such as tetrasodium pyrophosphate and phosphate glass have been used as corrosion inhibitors in various aggressive solutions^12–16 and were found to act successfully as steel and copper corrosion inhibitors. The corrosion inhibition efficiencies of these inhibitors are in the range of 80–95%.

This study focuses on the influence of the novel pyrophosphate SrNiP₂O₇ (SNP) on copper corrosion inhibition in recirculating cooling seawater. The novel pyrophosphate SrNiP₂O₇ could be considered to be an effective potential inhibitor owing to its electronegative O-heteroatom and its non-toxic nature. To the best of our knowledge, no published reports or research exists with regard to the use of SrNiP₂O₇ as a potential corrosion inhibitor.

The inhibiting performance has been evaluated through mass-loss, polarization and electrochemical impedance spectroscopy (EIS) measurements. The surface morphology of copper was examined using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) investigations.

2. Experimental

2.1. Materials and chemicals

Polycrystalline powder of SrNiP₂O₇ (SNP) has been prepared by a conventional solid-state reaction technique.¹¹ Stoichiometric quantities of SrCO₃, NiCO₃ and (NH₄)H₂PO₄ were well ground, mixed and progressively heated first to 473 K to expel NH₃ and H₂O and then to 1123 K for 10 h.

The crystalline structure of SNP is fully elucidated and presented in Fig. 1. While Ni⁺² is penta-coordinated by five oxygen atoms each belonging a pyrophosphate P₂O₇ group, NiO₅ groups are not directly connected in the structure; their connections are made through the O–P–O–P–O pyrophosphates bridges.

Fig. 1 Projection of the SrNiP₂O₇ structure. Dark green: [NiO₅], light green: [P₂O₇], blue circles: Sr²⁺, red circles: O.

The test solution used in the study is a seawater solution (pH = 7.8) that was collected from Egyptian coastal red seawater, the composition of which is provided in Table 1.

Table 1 The composition of the seawater

Ions	Concentration mg L^−1
Na^+	12 477
Mg^2+	1621
Ca^2+	578
K^+	488
Cl^−	22 877
SO4^2−	2355
HCO3^−	255
Br^−	62

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The SNP is added to the test solution at concentrations from 20 mg L⁻¹ to 180 mg L⁻¹. SNP powder is dissolved in of 1.0 M nitric acid and then added to the test solution. The solution in the absence of SNP was taken as a blank. The temperature of the mixture was controlled by an aqueous thermostat.

For mass-loss experiments, the copper electrodes (99.999%) were mechanically cut into 2.2 cm × 1.2 cm × 0.2 cm dimensions. For electrochemical testing, the copper electrodes were embedded in epoxy resin with a geometrical surface area of 0.442 cm² exposed to the test solutions. Prior to all experiments, the copper electrode was abraded with emery paper of increasing fineness of up to 1200 grit. The copper electrode was then washed with distilled water, degreased with acetone and ethanol, washed again with distilled water and finally dried.

2.2. Mass-loss measurements

Mass loss was calculated by weighing the cleaned copper electrodes before and after immersion in seawater solution for 10 days at 298 K. Mass-loss experiments were carried out in triplicate, and the mean value of the mass loss is reported. The corrosion rate (C_R) in mg cm⁻² h⁻¹ has been calculated using the following equation:where W is the mass loss (mg), S is the electrode surface area (cm²), and t is the immersion time (h).

2.3. Electrochemical measurements

Polarization and electrochemical impedance spectroscopy (EIS) were carried out using an ACM instruments Potentiostat/Galvanostat (Gill AC Serial no. 947).

All electrochemical experiments were carried out in the newly designed water-jacketed electrolytic cell as previously reported.¹⁷ In this cell, a copper metal, a platinum sheet and a saturated calomel electrode (SCE) were used as the working, auxiliary and reference electrodes, respectively.

For polarization experiments, the potential was in the range of ±200 mV relative to the open-circuit potential with a scan rate of 2.5 mV s⁻¹.

EIS measurements were carried out in a frequency range of 10 mHz–100 kHz with an amplitude of 10 mV peak-to-peak using AC signals at an open-circuit potential. Before the EIS experiment, the copper electrode was immersed in test solutions for 1 h until it reaches a steady-state condition.

Each electrochemical experiment has been repeated three times under the same conditions, and the mean values and standard deviations of the results are reported.

In general, no significant variations in the repetitions for each concentration were observed.

2.4. Surface morphology investigations

The surface morphologies of copper specimens after immersion in seawater solution for 10 days at 298 K with and without SrNiP₂O₇ were characterized by a digital camera (KODAK EASYSHARE P850 zoom digital camera) and a JEOL-JEM 1200 EX II scanning electron microscope.

Energy-dispersive X-ray spectroscopy (EDX) investigations were carried out to identify the elemental composition of the species formed on the metal surface. EDX examinations were carried out using a Traktor TN-2000 energy dispersive spectrometer.

3. Results and discussion

3.1. Mass-loss measurements

The value of inhibition efficiency (η_W%) and corrosion rate (C_R) obtained from the mass-loss method at various SNP concentrations in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at 298 K are shown in Table 2. The η_W% values were calculated from the following relation:where C_R0 and C_R are the corrosion rate without (blank) and with the inhibitor (SNP), respectively.

Table 2 Mass loss data of copper in absence and presence of various concentrations of SNP in sea water with solution flow rate of 0.8 m s⁻¹ at 298 K

SNP conc. (mg L^−1)	CR (μg cm^−2 h^−1)	ηw%
Blank	521.5 ± 8.5	—
20	426.5 ± 7.2	18.2
40	371.8 ± 5.2	28.7
60	260.2 ± 4.8	50.1
80	168.4 ± 3.3	67.7
100	77.1 ± 2.4	85.2
120	37.6 ± 1.9	92.7
140	37.7 ± 1.8	92.7
160	38.9 ± 1.9	92.5
180	38.6 ± 1.6	92.6

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It is apparent that the corrosion rate of copper in seawater clearly decreases as SNP is added into seawater, and the inhibition efficiency increases with an increase in seawater concentration. The maximum inhibition efficiency from mass-loss data (92.7%) is at 120 mg L⁻¹. No significant changes were observed in η_W% values at higher SNP concentrations (>120 mg L⁻¹). These results confirm that this inhibitor exhibited good corrosion inhibition even at low concentrations, further suggest that SNP acts as a fairly efficient inhibitor of copper corrosion in seawater.

3.2. Polarization measurements

3.2.1. Effect of SNP concentration. Fig. 2 shows typical polarization curves (Tafel plots) for copper in both the absence and presence of various SNP concentrations in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at 298 K.

Fig. 2 Polarization curves for copper in the absence and presence of various concentrations of SNP in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at 298 K.

Electrochemical parameters, such as corrosion potential (E_corr) and corrosion current density (j_corr), are presented in Table 3. The corrosion current densities (j_corr) decrease significantly once SNP is added into the blank solution.

Table 3 Electrochemical parameters and the corresponding inhibition efficiency of copper in absence and presence of various concentrations of SNP in sea water with solution flow rate of 0.8 m s⁻¹ at 298 K

SNP conc. (mg L^−1)	Ecorr mV (SCE)	jcorr μA cm^−2	ηj%
Blank	−245 ± 3.5	8.31 ± 0.52	—
20	−251 ± 3.2	6.52 ± 0.33	21.5
40	−260 ± 3.2	5.58 ± 0.32	32.8
60	−268 ± 2.9	3.72 ± 0.25	55.2
80	−272 ± 3.3	2.26 ± 0.18	72.8
100	−277 ± 3.6	0.98 ± 0.04	88.2
120	−294 ± 3.8	0.43 ± 0.02	94.8

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The presence of SNP remarkably shifts the E_corr towards cathodic potentials. Furthermore, displacement in ΔE_corr(E_corr(Blank) − E_{corr(in the presence of inhibitor)}) is less than 85 mV. Therefore, SNP can be defined as a mixed-type inhibitor with predominantly cathodic effectiveness.¹⁸ SNP (η_j%) inhibitor efficiency was evaluated from the polarization measurements using the following equation:¹⁹

where j_corr(0) and j_corr are the corrosion current densities in the absence and presence of SNP, respectively.

The inhibition efficiencies obtained from the polarization data (Table 3) increase with SNP concentration. Maximum SNP inhibition performance (η_j% = 94.8) was achieved at 120 mg L⁻¹.

3.2.2. Effect of temperature. The effect of temperature on SNP inhibitor efficiency for copper in seawater containing 120 mg L⁻¹ SNP in the aforementioned conditions was studied using polarization data at temperatures ranging from 303 to 333 K.

The results are presented in Fig. 3 and Table 4. It is clear that the rate of copper corrosion (j_corr) in seawater in a recirculating system containing SNP increases with increasing temperature, whereas the inhibitor efficiency of SNP decreases. Increasing the temperature leads to SNP desorption from the copper metal surface, which causes a decrease in inhibitor efficiency.²⁰

Fig. 3 Polarization curves for copper in the presence of 120 mg L⁻¹ of SNP in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at various temperatures.

Table 4 Electrochemical parameters and the corresponding inhibition efficiency of copper in sea water containing 120 mg L⁻¹ SNP with solution flow rate of 0.8 m s⁻¹ at different temperatures

Temperature K	Ecorr mV (SCE)	jcorr μA cm^−2	ηj%
303	−294 ± 3.8	0.43 ± 0.02	94.8
313	−286 ± 3.9	0.79 ± 0.03	90.4
323	−301 ± 3.9	1.06 ± 0.05	87.2
333	−318 ± 4.1	1.18 ± 0.09	85.7

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The Arrhenius equation provides the quantitative basis of the relationship between the activation energy and the corrosion rate.²¹ From the Arrhenius equation, the activation energy can be expressed as²²

where R is the molar gas constant, T is the absolute temperature, and A is the frequency factor.

The Arrhenius plots in the absence (blank) and presence of 120 mg L⁻¹ of SNP are presented in Fig. 4. The extracted values of E_a are 15.18 and 28.27 kJ mol⁻¹ in the absence and presence 120 mg L⁻¹ of SNP, respectively. The higher E_a value in the presence of 120 mg L⁻¹ of SNP in comparison to that obtained in the blank solution can be correlated with a physical adsorption phenomenon by SNP molecules on the copper surface. This indicates that a larger energy barrier for the corrosion reaction in the presence of SNP is obtained.²³

Fig. 4 The Arrhenius plots in the absence (blank) and presence of 120 mg L⁻¹ of SNP.

3.3. ElS measurements

Typical Nyquist plots obtained for copper in the absence and presence of various SNP concentrations in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at 298 K are shown in Fig. 5. It has been determined that the impedance spectra obtained show a single depressed capacitive loop, which is related to charge transfer in the corrosion process.^24,25 The capacitive loop diameters increase with increasing SNP concentration. The roughness and in-homogeneities of the copper surface cause non perfect semicircles.²⁶ EIS data are commonly analyzed by fitting it to an equivalent electrical circuit model, which is used to fit the EIS results shown in Fig. 6. In this circuit, R_s is the resistance of the solution, R_ct is the charge-transfer resistance, and CPE_dl is the constant phase element. Capacitors in EIS experiments often do not behave in an ideal fashion; rather, they perform similar to a constant phase element (CPE_dl). Impedance parameters (R_ct and CPE_dl) in the absence and presence of various SNP concentrations were analyzed using Sequencer software and are shown in Fig. 7. As shown in Fig. 7, the addition of SNP increases the R_ct values, and this effect appears to be enhanced upon an increase in SNP concentration. On the other hand, the CPE_dl values decrease with an increase in SNP concentration. This behavior may be due to the replacement of water molecules by SNP molecules adsorption at the copper/seawater solution interface, which leads to the adsorption of a protective film on the metal surface.²⁷

Fig. 5 Nyquist plots for copper in the absence and presence of various concentrations of SNP in seawater in a recirculating system with a solution flow rate of 0.8 m s⁻¹ at 298 K.

Fig. 6 The equivalent circuit model used to fit EIS results.

Fig. 7 Relation between R_ct and CPE_dl vs. SNP concentration.

Because the reciprocal of the charge-transfer resistance (R_ct⁻¹) corresponds to the corrosion rate of a metal in corrosive solutions, the inhibition efficiency (η_R%) of SNP can be calculated using the following equation:²⁸

where R_ct and R_ct0 are the charge-transfer resistances in the presence and absence of SNP, respectively.

Fig. 8 illustrates plot of η_R% versus SNP concentration. The data clearly show that the inhibition efficiency increases with an increase in SNP concentration, reaching a maximum value (97.1%) at a 120 mg L⁻¹ concentration.

Fig. 8 Plot of η_R% versus SNP concentration.

This data corroborates the data obtained by mass-loss and polarization measurements and provides further evidence of SNP's ability as a good corrosion inhibitor.

By comparing the results, it is observed that the inhibition efficiencies calculated from EIS measurements show the same trend as that obtained from both polarization and mass-loss measurements.

3.4. Adsorption isotherm of SNP

The data obtained from adsorption isotherms are very important in the description the SNP adsorption process on the copper surface. A wide variety of adsorption isotherms have been formulated to describe how adsorbed molecules interact with adsorbent materials.²⁹ The Freundlich isotherm is the earliest known relationship describing non-ideal and reversible adsorption that is not restricted to monolayer formation. We adopted it in this study to describe the SNP adsorption process on the copper surface. This empirical model can be applied to multilayer adsorption on a heterogeneous surface.³⁰ According to this model, there is a relation between the surface coverage (θ) and inhibitor concentration (C_inh), expressed as follows:

θ = K_F(C_inh)^1/n, (6)

The linear form of the Freundlich isotherm is represented by the following equation:

where K_F is the Freundlich constant indicative of the relative adsorption capacity of the adsorbent. The Freundlich slope (1/n), ranging between 0 and 1, is a measure of adsorption intensity or surface heterogeneity. In fact, a value close to zero means a heterogeneous surface; however, a value below unity implies chemisorption processes and above is indicative of cooperative adsorption.³¹ The experimental data from mass-loss, polarization and EIS measurements were used for presenting the linear Freundlich isotherm (Fig. 9). The surface coverage degree θ can be calculated from mass-loss, polarization and EIS measurements using the relations (η_w%/100), (η_j%/100) and (η_R%/100), respectively.

Fig. 9 Freundlich isotherm plots for SNP adsorption on the copper surface.

The linear Freundlich isothermic parameters for SNP adsorption on the copper surface are listed in Table 5. The regression coefficient (r²) is employed to analyze the fitting degree of the isotherm with the experimental data, where its values vary from 0 to 1.³² The high correlation coefficients of 0.9837, 0.9786 and 0.9842 confirm that SNP adsorption on the copper surface is consistent with the Freundlich isotherm. The value of 1/n located between 0 and 1 confirms the favorable adsorption conditions.

Table 5 Freundlich isotherm parameters for the adsorption of SNP on the copper at 298 K

Methods	r^2	KF (L mg^−1)	1/n	ΔG^0ads (kJ mol^−1)
Polarization	0.9837	14.3 × 10^−3	0.8839	−30.7
EIS	0.9786	21.6 × 10^−3	0.8078	−30.8
Mass loss	0.9842	11.2 × 10^−3	0.9569	−30.1

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The Freundlich constant K_F can be used to calculate the standard free energy of SNP adsorption (ΔG⁰_ads) on the copper surface using the following equation:³³

ΔG⁰_ads = −RT ln(55.5 K_F), (8)

where R is the molar gas constant (J K⁻¹ mol⁻¹), T is the absolute temperature (K), and 55.5 is the concentration of water in solution expressed in moles.

The ΔG⁰_ads values calculated from mass-loss, polarization and EIS data are −30.1, −30.7, and −30.8 kJ mol⁻¹, respectively.

It is clear that ΔG⁰_ads values have negative signs and values of less than −40 kJ mol⁻¹, which indicate that SNP adsorption on the copper surface will be favored and will release energy. Furthermore, this type of adsorption is regarded as a physical adsorption.³⁴

3.5. Surface morphology observation

The formation of a complex layer by inhibitor adsorption on the metal surface was confirmed by surface morphology observation.

Macroscopic images of copper specimens before and after immersion in a seawater solution for 10 days at 298 K with and without SNP are shown in Fig. 10. Compared with the copper specimen before immersion (Fig. 10a), the surface in the absence of SNP (Fig. 10b) is severely corroded, and the copper surface contains several pits in addition to a yellow-brown corrosion product (anhydrous CuCl₂⁻). However, in the presence of SNP (Fig. 10c), the specimen surface is well protected, in which the surface characterized by a red-orange film is formed on the copper surface. This film is mainly attributed to SNP adsorption.

Fig. 10 Macroscopic images of (a) abraded copper, (b) copper immersed in seawater, and (c) copper immersed in seawater containing 120 mg L⁻¹ SNP.

Further surface morphology observation was carried out using SEM micrographs of copper specimens before and after immersion in seawater solution for 10 days at 298 K with and without SNP (Fig. 11). It is clearly observed that the copper surface morphology was severely damaged through exposure to seawater without SNP (Fig. 11b), whereas in the case of an inhibited solution (seawater + 120 mg L⁻¹ SNP; Fig. 11c), the surface of copper specimen was smoother with fewer pits. This clearly proves that the corrosion process was suppressed via the formation of an adsorbed SNP layer on the copper surface.

Fig. 11 SEM images of (a) abraded copper, (b) copper immersed in seawater, and (c) copper immersed in seawater containing 120 mg L⁻¹ SNP.

The EDX spectra after immersion in the blank solution (seawater; Fig. 12a) show several peaks of copper and chloride. This indicates that the corrosion products on the copper surface are mainly CuCl and/or CuCl₂⁻. On the other hand, the EDX spectra (Fig. 12b) obtained for copper in the presence of 120 mg L⁻¹ SNP show that the peak attributed to the chloride decreased dramatically, which suggests that there is considerably less corrosion product at the metal surface. Compared with the blank (without SNP), the EDX spectra (Fig. 12b) show additional signals characteristic to the O, P, Ni, and Sr. It confirms that SNP has been directly adsorbed onto the copper surface to form a protective film.

Fig. 12 EDX spectra of (a) copper immersed in seawater and (b) copper immersed in seawater containing 120 mg L⁻¹ SNP.

3.6. The mechanism of corrosion inhibition by SNP

The corrosion mechanisms of copper in seawater can be described as follows:^35,36

(i) the cathodic reaction,

O₂ + 2H₂O + 4e → 4OH⁻ and (9)

(ii) the anodic reactions,

Cu → Cu⁺ + e and (10)

Cu⁺ + Cl⁻ → CuCl. (11)

Because of the very weak adhesion and stability of the CuCl layer formed on the copper surface, it reacts with chloride ions to form a soluble cuprous chloride complex CuCl₂⁻ (eqn (12)):

CuCl + Cl⁻ → CuCl₂⁻. (12)

Finally, the chloride complex CuCl₂⁻ moves from the copper surface to the bulk solution and forms Cu⁺² and Cl⁻ ions (eqn (13)):

CuCl₂⁻ → Cu⁺² + 2Cl⁻ + e (13)

The data obtained from mass-loss, polarization and EIS measurements indicate that SNP is one of the most effective corrosion inhibitors for copper metal in seawater. SNP acts through adsorption on a copper surface, blocking the active sites by displacing water molecules and forming a barrier film to decrease the corrosion rate.^37,38 The mechanism appears to be relatively well resolved when the inhibiting molecule reacts on a clean surface such as polyphosphate. The inhibitor adsorption on the corroded metal is affected by several parameters such as the type of metal, metal surface charge, electrolyte pH, temperature, ions present in electrolytic and molecular geometry, and electronic and crystal structures of the inhibitors molecules.^39,40

From the chemical structure of the SNP molecule, we could propose that Sr²⁺ atoms in SNP can interact with CuCl₂⁻ species, which are formed in eqn (12) through electrostatic force, and then prevent an oxidative reaction (eqn (13)). Simultaneously, owing to lone-pair electrons of O atoms, SNP molecules may combine with Cu⁺² ions to form metal inhibitor complexes. These complexes might get adsorbed onto the copper surface by the van der Waals force to form a protective barrier that separates the copper surface and seawater solution.

4. Conclusion

SNP was synthesized as a powder and tested as a new inorganic inhibitor for the corrosion of copper metal in recirculating cooling seawater. The inhibition performance obtained from mass-loss data exhibits the same trend as observed in the electrochemical data. The highest inhibition efficiency obtained from mass-loss, polarization and EIS measurements are 92.7%, 94.8% and 97.1%, respectively, at a 120 mg L⁻¹ concentration. The polarization curves suggest that SNP is classified as a mixed-type inhibitor with predominantly cathodic effectiveness. The rise in temperature suppresses SNP adsorption on the copper surface and accelerates the corrosion process. The best correlation between the experimental results and the isotherms was obtained using the Freundlich isotherm. Surface morphology observations supported that SNP formed a protective layer on the copper surface during the inhibition process.

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