The synergistic mechanism of phytic acid monolayers and iodide ions for inhibition of copper corrosion in acidic media

Abstract

The adsorption behavior of phytic acid (PA) self-assembled monolayers (SAMs) at the copper surface, and its corrosion inhibition mechanism in 0.5 M H₂SO₄ solution, was studied by using electrochemical impedance spectroscopy (EIS), potentiodynamic polarization and surface-enhanced Raman scattering spectroscopy (SERS). Raman studies showed that PA SAMs on the copper surface formed via P–O bonding and the cyclohexyl ring. The adsorption of PA SAMs on copper surface followed Langmuir isotherm behavior and revealed chemical adsorption on the copper surface. The presence of I⁻ ions with PA SAMs at the copper surface showed a synergistic effect and increased the inhibition efficiency.

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1. Introduction

Copper and its alloys have been used in various industrial and microelectronic applications due to its good thermal, electrical conductivity, mechanical workability, and relatively noble properties.^1–5 However, copper and its alloys easily suffer from severe corrosion, especially in aggressive environments. Industrial acids are extensively employed in many industrial processes, including acid pickling and acid descaling as well as in oil well acidification, which cause the damage of copper. For example, sulfuric acid solution is one of the most commonly used industrial acids.⁶

Organic inhibitors containing nitrogen, oxygen or sulfur atoms are usually used to mitigate the electrochemical corrosion attack against metal materials.^7–11 However, the usage of these organic inhibitors not only results in economic cost but also causes the release of toxic substances into the environment.

From a green chemical view, we have introduced a kind of green molecule, phytic acid (PA) and its salts, which could be used as corrosion inhibitors in salt solution.¹² PA contains six phosphoric acid groups which can interact with the metal ions (see Scheme S1 in ESI†). The most attractive advantage of PA is its availability as a naturally polyphosphorylated carbohydrate, widely occurring in beans, brown rice, corn, sesame seeds and wheat bran. The use of PA as a cleansing agent, water treatment agent, food additive and cosmetic additive proves that it is non-toxic to humans and “green” to the environment.^13–19

Meanwhile, synergistic effect, which can highly improve the performance of the corrosion inhibitor compared with the inhibitor alone, has been proven to be an effective means to enhance the inhibitive ability and diversify the application of inhibitors in acidic media.^20–22 The synergistic effect of halides has been reported to be in the order: I⁻ ≫ Br⁻ > Cl⁻. The highest synergistic effect of I⁻ ions is due to the chemisorption onto metal surface with its larger size and ease of polarization.^23,24

In our previous work involving copper protected by PA in 3% NaCl solution, the inhibition efficiency was below 90%.¹² In this work, we attempted to improve the inhibition effect of PA self-assembled monolayers (SAMs) on copper in 0.5 M H₂SO₄ solution by the addition of I⁻ ions. The SAM technique is regarded as an easy method to form dense and ordered monolayers, acting as an effective barrier to protect the copper against corrosion.^25–27 The inhibition behavior was undertaken using electrochemical impedance spectroscopy (EIS), electrochemical polarization and surface-enhanced Raman scattering spectroscopy (SERS). SERS is a highly sensitive technique for probing the monolayer adsorption behavior of molecules on a metal surface.^28–30 Additionally, the adsorption behavior of PA SAMs was investigated by Langmuir adsorption isotherm studies.

2. Experimental section

2.1. Materials and chemicals

PA (50 wt%) solution was purchased from Sigma-Aldrich Corporation. Sulfuric acid, potassium iodide and ethanol were of analytical grade and purchased from Sinopharm Chemical Reagents Company. Electrochemical experiments were undertaken in 0.5 M H₂SO₄ solution in the absence and presence of different concentrations of PA, KI and mixed solution of 0.1 mM PA and KI, respectively. The KI concentration in the mixed solution was varied in the range of 1–5 mM. All the solutions were prepared with Milli-Q water (18 MΩ cm).

2.2. Apparatus

Raman spectroscopic measurements were obtained on a confocal microprobe Raman system (LabRam II, Dilor, France), with a holographic notch filter and a liquid-nitrogen-cooled CCD detector. A He–Ne laser at 632.8 nm with a power of ca. 5 mW was used as the excitation source for the Raman experiment. A 50× long-working-length objective was used to focus the laser spot onto the electrode surface. The slit and pinhole were set at 100 and 1000 μm, respectively. Each spectrum was measured three times and the acquisition time was 15 s. Calibration was done referring to the 519 cm⁻¹ line of silicon.

The electrochemical measurements were carried out by a CHI750c electrochemistry workstation (CH Instruments, Inc.).

2.3. Pretreatment for electrodes

The copper electrode was made from a polycrystalline copper rod (99.999%, Sigma-Aldrich) and the geometric area of the surface was ca. 0.0314 cm² (embedded in a Teflon sheath). Before all the experiments, the copper electrode was sequentially polished with emery paper and 0.3 μm alumina/water slurries until a shiny, mirror-like surface was obtained, and then was ultrasonically washed with Milli-Q water and pure ethanol to remove any alumina particles. For SERS detection, to obtain the necessary roughness of the copper surface, an oxidation reduction cycle treatment (ORC) was performed in a conventional three-electrode cell.³¹ The bare copper specimen or the PA SAMs modified copper specimen was used as the working electrode. A platinum electrode was used as the counter electrode and the reference electrode was a saturated calomel electrode (SCE). All potentials cited in this paper were converted to SCE.

2.4. Self-assembled monolayers

To form the monolayers, the polished copper electrode was subjected to ORC in 2 M H₂SO₄ by switching between −550 and +450 vs. mV/SCE to achieve a fresh and oxide-free surface, then rinsed with Milli-Q water and absolute ethanol as soon as possible. Then the copper electrode was immersed immediately into deoxygenated varying concentration of PA solutions, KI solutions and mixed solutions of PA and KI for different times at room temperature. Finally, the coated electrode was taken out of the solutions and rinsed carefully with ethanol and Milli-Q water, then subsequently dried by flowing nitrogen gas prior to further investigations.

2.5. Electrochemical measurements

The electrochemical studies were carried out in a three-electrode cell assembly. EIS measurements were performed at open circuit potentials with an AC voltage amplitude of 5 mV in the frequency range from 100 kHz to 0.01 Hz. The polarization curves were obtained from +200 to −300 mV vs. SCE with a scan rate of 10 mV s⁻¹.

3. Results and discussion

3.1. Inhibition by PA SAMs

3.1.1. Electrochemical impedance measurement. The EIS technique, a nondestructive, sensitive and informative method, has been extensively used for the evaluation of coatings on metal surface for corrosion inhibition, especially for analyzing the surface with SAMs. Nyquist plots for copper in 0.5 M H₂SO₄ solution (with and without PA SAMs) for the same assembled time (4 h) are presented in Fig. 1 and the corresponding impedance parameters are given in Table 1. Nyquist plots and data for copper electrodes without and with PA SAMs formed at different assembly times in 0.5 M H₂SO₄ solution are given in ESI.†

Fig. 1 Nyquist plots of copper electrodes in 0.5 M H₂SO₄ solution with PA films formed from different concentrations of PA solutions for 4 h. The inset is the Nyquist plots of blank copper.

Table 1 EIS parameters of copper electrodes in 0.5 M H₂SO₄ solution with PA SAMs formed from different concentrations of PA solutions for 4 h

c/mM	Rs/Ω cm^2	Qd^a/μY	n	Rct/kΩ cm^2	Qf^a/μYf	n	Rf/kΩ cm^2	W/μΩ cm^2	η (%)
a The dimensions are S; s^n cm^−2; if n = 1, they are F cm^−2.
0	489.3	1.101	0.80	112.28				0.1142
0.05	277.8	1.713	0.61	27.80	0.1155	0.67	247.4		59.2
0.075	374.5	0.4164	0.80	28.20	18.41	0.80	374.2		72.1
0.1	300	0.2017	0.77	72.41	2.692	0.58	652.0		84.5
0.25	410.5	0.2068	0.83	22.47	0.1339	0.55	389.51		72.7
0.5	594.9	0.520	0.80	39.92	5.047	0.80	379.0		73.2

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The equivalent circuits analyzed by ZsimpWin software are presented in Scheme 1. R(Q(RW)) and R(QR(QR)) are suitable for the Nyquist plots of copper without and with PA SAMs, respectively. R_s is the resistance of the solution, R_ct is the charge-transfer resistance at high frequency, R_f is the film resistance related to the adsorption of inhibitor molecules and all other accumulated species on the metal/solution interface in the low frequency region,^32–34 Q_dl and Q_f stand for the constant phase elements (CPE), representing double-layer capacitance and film capacitance, and W is the Warburg impedance. The impedance function of the CPE is described by the following equation:

Z_CPE = Y₀⁻¹(jω)⁻ⁿ (1)

where Y₀ is the modulus, j is the imaginary root, ω is the angular frequency and n is the phase. Depending on the value of the exponent n, Q may be a resistance, R (n = 0); a capacitance, C (n = 1); Warburg impedance, W (n = 0.5), or an inductance, L (n = −1). The value range of a real electrode of n is often between 0 and 1, showing the phase shift that can be explained as the degree of surface inhomogeneity.³⁵ In the present fitting, all the χ² values of the blank and modified electrodes are around 1 × 10⁻³ and the acceptable errors of EIS elements in fitting mode are below 10%.

Scheme 1 Electrical equivalent circuit models for impedance data: (a) in the presence of PA SAMs; (b) bare copper and copper with KI adsorption from different concentrations of KI solutions; (c) in the presence of PA film formed in 0.1 mM PA solution in combination of different KI solutions.

The Nyquist plot of bare copper (the inset picture) displays a straight line at low frequencies (Warburg impedance) and a small semicircle (R_ct) in the region of high frequency. The Warburg impedance in the low frequency region is due to the diffusion process of soluble copper species from the electrode surface to the bulk solution.³⁶

However, the Nyquist plots of PA SAMs covered copper electrodes have two semicircles, which are different from that of bare copper. The small semicircle is related to R_ct in the region of high frequency. The appearance of R_f in the region of low frequency means that the PA SAMs have formed on the copper surface, which can obviously increase the resistance of the copper surface and protect the copper from corrosion. In the low-frequency region, compared with the blank copper, the Warburg impedance disappears in the PA SAMs coated copper surface, indicating the formation of PA SAMs on copper electrode can significantly inhibit the diffusion process of soluble copper species from the electrode surface to the solution. The semicircles in Nyquist plots show that the rise in concentrations results in an increase in diameter and it reaches a maximum at a concentration of 0.1 mM.

The inhibition efficiency (η) obtained from Table 1 can be calculated according to the following equation:³⁷

where R_p⁰ is the polarization resistance of the naked copper, and R_p is the polarization resistance of the PA SAMs-covered copper electrode. The polarization resistance is the sum of R_ct and R_f. The calculated η as an optimum value is 84.5% at a concentration of 0.1 mM and assembly time of about 4 h in 0.5 M H₂SO₄ solution (see ESI†).

3.1.2. Potentiodynamic polarization studies. The electrochemical polarization method can provide the corrosion rate from linear polarization and determine the efficiency of the corrosion inhibitor. Fig. 2 shows polarization curves recorded in 0.5 M H₂SO₄ solution in the absence and presence of PA SAMs on the copper surface formed at different immersion times in 0.1 mM PA solution. The corresponding electrochemical parameters such as the corrosion potential (E_corr) and corrosion current density (I_corr) along with the inhibition efficiency (η) are listed in Table 2. Both the anodic and cathodic Tafel regions are used to determine the I_corr and E_corr values.³⁸ The polarization curves and corresponding parameters the copper surfaces in the absence and presence of PA SAMs formed at different concentrations of PA solutions, recorded in 0.5 M H₂SO₄ solution, are displayed as ESI.†

Fig. 2 Potentiodynamic polarization curves in 0.5 M H₂SO₄ solution for blank copper electrode and the presence of PA SAMs formed in 0.1 mM PA solution for different times.

Table 2 Potentiodynamic polarization parameters for copper in 0.5 M H₂SO₄ solution for blank copper electrode and the presence of PA SAMs formed in 0.1 mM PA solution for different times

t/h	−Ecorr/mV	Icorr/μA cm^−2	η (%)
0	59	166.8
3	115	34.3	79.5
4	173	27.9	83.3
5	76	54.1	67.6
6	91	70.1	58.0

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It can be seen from Fig. 2 that, both anodic and cathodic current densities of the copper electrodes with PA SAMs decrease obviously at the same potential compared with the bare electrode, demonstrating that the formation of PA SAMs inhibit anodic and cathodic reaction processes simultaneously. Nevertheless, the cathodic reaction is inhibited to a larger extent compared with the anodic reaction. An inhibitor can be classified as an anodic or cathodic type inhibitor based on the difference between corrosion potential (E_corr) values of copper electrodes with and without film, which is larger than 85 mV or not.³⁹ Herein, most of the displacement in E_corr value is below 85 mV. Therefore, PA might act as a mixed type inhibitor with predominantly cathodic inhibitor. The negative shift of E_corr could be explained by the fact that the inhibitor has a stronger influence on the oxygen reduction (cathodic reaction) than on the copper dissolution reaction (anodic reaction). Therefore, PA SAMs suppress the transfer of O₂ to the cathodic sites of the copper surface, decreasing the rate of oxygen reduction. The corresponding inhibition efficiency is calculated using the following equation:⁴⁰

where I_corr(a) and I_corr(b) are the corrosion current densities in the absence and presence of PA SAMs on the copper surface, respectively.

After immersion in 0.5 M H₂SO₄ solution, all the corrosion current density values of the coated copper electrodes are lower than that of the bare copper electrode. The inhibition efficiency (83.3%) of the copper electrode with PA SAMs formed for 4 h assembly time is higher than the others. The SAMs can only be formed under the proper assembled time and the concentration of inhibitor. In this work, the optimized assembly time is 4 h.

3.1.3. Raman study. The presence of PA SAMs can be detected by SERS spectroscopy in Fig. 3a and their assignments for PA SAMs observed vibrations are also summarized in Table 3, based on the vibrational calculation of PA using the B3LYP/LANL2DZ method in Gaussian 03 programs.¹⁵ Because of its good chemical activity and SERS-active substrate treatment under the ORC conditions, the copper oxide layers are inevitably formed. The bands around 532 and 594 cm⁻¹ are due to oxide species of copper.¹³

Fig. 3 SERS spectrum of copper electrode with (a) PA film formed in 0.1 mM PA solution for 4 h; (b) PA film formed in 0.1 mM PA solution in combination with 5 mM KI.

Table 3 Assignments for SERS spectrum of PA SAMs and PA SAMs with I⁻ ions

PA (cm^−1)	PA + KI (cm^−1)	PA calc. (cm^−1)	Approximate^a assignment
a Abbreviations: s, strong; m, medium; w, weak; br, broad; bend, bending; str., stretching; ip, in-plane; op, out plane; as, asymmetric.
810w		808	P^43–O^42–C^41 rock. C^31–H, C^8–H^ip.bend.
917w		924	Cycl. ring. breath.
983w	979s	989	C^8–C^41–C^33 as.
	1003w		P^43–O^42–C^41 str.
1044m	1039w	1025	P^43–O^42–C^41 as.
			C^41–C^8–C^9 ip bend.
1105w		1132	Cycl. ring. bend.
1181w		1204	C^25–H, C^17–H, C^9–H, C^8–H^op.bend.
1278w		1279	P^35–O^36 str., C^33–H, C^25–H, C^17–H^ip.bend.
1353s	1355s	1318	P^27–O^28 str.
1474m	1479w	1482	C^41–H,C^8–H^op.bend.
1547s	1545w	1529	C^25–H, C^17–H, C^9–H^op.bend.

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Some bands at 810, 983, 1044, 1278 and 1353 cm⁻¹ are ascribed to P⁴³–O⁴²–C⁴¹ rock stretching, P⁴³–O⁴²–C⁴¹ stretching, P³⁵–O³⁶ stretching, P⁴³–O⁴²–C⁴¹ asymmetrical stretching and P²⁷–O²⁸ stretching, respectively. The peaks at 917 and 1105 cm⁻¹ are all relative to the vibrations of the cyclohexyl ring. It has been generally accepted that, for the SERS mechanism^41,42 and the surface selection rule,^43,44 the vibrational modes of groups that attach to or are very close to the surface should be more enhanced in the SERS spectrum and the vibrational modes with parallel polarizability components with respect to the surface will not be enhanced. Therefore, the SERS peaks relating to P–O are enhanced by the surface, meaning the P–O groups attach onto the surface and their vibration modes of stretching are with respect to the surface. The occurrence of carbon skeleton stretching besides P⁴³, P³⁵ and P²⁷ and two peaks at 917 and 1105 cm⁻¹ from the vibrations relating to the cyclohexyl ring hint that the cyclohexyl ring approaches the copper surface in a tilted manner. The above SERS observations suggested that the PA molecule is adsorbed at the surface via the P²⁷, P³⁵ and P⁴³ atoms and the cyclohexyl ring lies close to the surface. As a result, PA SAMs are able to show a relatively high corrosion efficiency over 80%.

Fig. 3b shows the SERS spectrum of the copper surface in the presence of PA film together with 5 mM KI. Compared with the SERS spectrum of the copper surface with PA film only, the two bands at 979 and 1335 cm⁻¹ are enhanced and blue shifted. Some bands, such as at 810, 917, 1105 and 1278 cm⁻¹ disappear. All these phenomena result from the effect of I⁻ on the PA film. The PA molecules adsorbed at the copper only adopt P⁴³ and P²⁷ atoms.

3.1.4. Adsorption isotherm. The measurement of the adsorption isotherm can provide valuable information on the nature and strength of adsorption of organic compounds on metal surfaces. Therefore, the investigation of relationship between the adsorption and corrosion inhibition is important.⁴⁵ Assuming a direct relationship between inhibition efficiency (η (%)) and surface coverage (θ) [η (%) = 100 × θ] for different inhibitor concentrations, data obtained from EIS measurements^46,47 are used to determine the adsorption characteristics of PA SAMs on copper surface in 0.5 M H₂SO₄ solution. To clarify the nature of the adsorption, theoretical fitting to different isotherms was undertaken and the correlation coefficients (r²) used to determine the best fit, which was obtained with the Langmuir isotherm (r² = 0.999). The Langmuir isotherm can be described by the following equation:^48,49where C is the inhibitor concentration, θ is the degree of surface coverage and K_ads is the equilibrium constant related to the free energy of adsorption ΔG_ads⁰ by the equation:

ΔG_ads⁰ = −RT  ln(55.5K_ads) (5)

where R is the universal gas constant and T is the absolute temperature. The value of 55.5 is the concentration of water in solution expressed in moles.

The plot of C/θ vs. C is shown in Fig. 4 to be linear. The adsorption equilibrium constant (K_ads) obtained is 8.33 × 10⁵ M⁻¹, indicating that the PA SAMs possesses strong adsorption ability onto the copper surface.⁵⁰ The value of ΔG_ads⁰ up to −38.02 kJ mol⁻¹ also indicates the strong interaction between inhibitor molecules and the copper surface, spontaneity of the adsorption process and stability of the adsorbed layer on the copper surface. It is well known that values of ΔG_ads⁰ of the order of −20 kJ mol⁻¹ or less negative are associated with an electrostatic interaction between the charged inhibitor molecules and the charged metal surface (physical adsorption);⁵¹ those of −40 kJ mol⁻¹ or more negative involves charge sharing or transferring from the inhibitor molecules to the metal surface to form a coordinate covalent bond (chemical adsorption).^52,53 The calculated value of ΔG_ads⁰ close to −40 kJ mol⁻¹ suggests that the adsorption mechanism of PA SAMs on the copper surface is a chemical adsorption. Chemical adsorption might be due to the interaction of unshared electron pairs or p-electrons of the adsorbate with the metal in order to form a coordinate type of bond.

Fig. 4 Langmuir adsorption isotherm for PA SAMs on the copper in 0.5 M H₂SO₄ solution at 25 °C.

3.2. Synergistic effect of iodide ions

3.2.1. Effect of iodide ions. Halide ions are known to enhance the adsorption of some organic cationic inhibitors on metal surfaces, thereby improving their inhibition efficiency considerably. This phenomenon, ascribed to a synergistic effect, is often most pronounced with I⁻ ions.

Fig. 5 and 6 show the electrochemical experiments undertaken to assess the effect of addition of I⁻ ions on the corrosion inhibition performance in 0.5 M H₂SO₄ for 2 h. The corresponding parameters are listed in Tables 4 and 5. The impedance data are analyzed using the equivalent circuit shown in Scheme 1(b). The values of inhibition efficiencies obtained are used (eqn (2) and (3)). The potentiodynamic polarization curves in Fig. 5 indicate that the presence of I⁻ ions slightly shifts the corrosion potential to more positive direction and reduces both the rate of anodic and cathodic reactions. The bigger semicircle of 5 mM KI in Fig. 6 shows that higher concentration of KI has better corrosion inhibition.

Fig. 5 Potentiodynamic polarization curves for copper electrode with and without KI adsorption formed in different concentrations of KI solutions for 2 h in 0.5 mM H₂SO₄.

Fig. 6 EIS curves for copper electrode with and without KI adsorption formed in different concentrations of KI solutions for 2 h in 0.5 mM H₂SO₄.

Table 4 Potentiodynamic polarization parameters for copper in 0.5 M H₂SO₄ solutions containing different concentrations of KI and in combination of 0.1 mM PA SAMs formed in 0.1 mM PA solution

c/mM	−Ecorr/mV	Icorr/μA cm^−2	η (%)
0	59	166.8
1	55	83.9	49.7
2.5	54	81.8	50.9
5	35	75.8	54.6
1 + 0.1 PA	78	55.3	66.9
2.5 + 0.1 PA	136	23.4	86.0
5 + 0.1 PA	151	16.8	89.9

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Table 5 EIS data for copper electrode with and without KI adsorption formed in different concentrations of KI solutions for 2 h in 0.5 mM H₂SO₄

c/mM	Rs/Ω cm^2	Qd^a/μY	n	Rct/kΩ cm^2	W/kΩ cm^2	η (%)
a The dimensions are S; s^n cm^−2; if n = 1, they are F cm^−2.
1	181.4	4.44	0.49	200.7	0.3916	44.1
2.5	326.6	4.72	0.80	227.23	0.3416	50.6
5	316.4	0.14	0.80	237.52	0.265	52.7

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3.2.2. Synergistic effect and inhibition mechanism. In 0.5 M H₂SO₄ solution, the impedance behavior of copper with the mixed film formed in solutions containing 0.1 mM PA with different concentrations of KI for 4 h are shown in Fig. 7. The impedance data are analyzed using the equivalent circuit simulation shown in Scheme 1(c) and the corresponding electrochemical parameters are given in Table 6. The results show that both R_ct and R_f increase with increasing concentration of KI. The inhibition efficiency increased from 86.0 to 89.9% in the presence of 0.1 mM PA on addition of 2.5 and 5 mM KI, which are higher than that of the PA SAMs alone.

Fig. 7 EIS plots of copper in 0.5 M H₂SO₄ solution in the absence and presence of PA SAMs formed in 0.1 mM PA solution in combination with different concentrations of KI.

Table 6 EIS data for copper in 0.5 M H₂SO₄ solution with the presence of various concentrations of KI + 0.1 mM PA SAMs

c/mM	Rs/Ω cm^2	Qd^a/μY	n	Rct/kΩ cm^2	Qf^a/μYf	n	Rf/kΩ cm^2	η (%)
a The dimensions are S; s^n cm^−2; if n = 1, they are F cm^−2.
1 + 0.1 PA	251.3	6.881	0.52	18.74	1.262	0.79	277.3	62.6
2.5 + 0.1 PA	557	7.124	0.81	343.7	1.387	0.61	520.0	87.0
5 + 0.1 PA	128.3	4.544	0.73	105.21	1.113	0.74	947.3	90.0

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The effect of I⁻ ions on the polarization behavior of copper in the presence of different concentrations of KI in combination with 0.1 mM PA are depicted in Fig. 8 and Table 4. From Fig. 8, it is clearly seen that the combination of PA SAMs and KI reduce remarkably the anodic and cathodic reactions and decrease considerably the corrosion current densities compared to the blank copper and 0.1 mM PA SAMs alone, leading to higher inhibition efficiency (89.9%). The results obtained from polarization and EIS techniques follow the same trend and are in close agreement. The enhanced inhibition efficiency noted for PA SAMs on addition of I⁻ ions is due to the synergistic effect.⁵⁴

Fig. 8 Potentiodynamic polarization curves for copper in 0.5 M H₂SO₄ solution in the absence and presence of PA SAMs formed in 0.1 mM PA solution in combination with different concentrations of KI.

The existence of synergism phenomenon between PA SAMs and iodide ions was further evaluated by estimating the synergism parameter (S₁) from the inhibition efficiency values from the two techniques employed according to the following equation:^55–57

where I₁₊₂ = I₁ + I₂; I₁ is the inhibition efficiency of the iodide ions, I₂ is the inhibition efficiency of PA SAMs alone and I₁₊₂′ is the measured inhibition efficiency for the PA SAMs in the presence of I⁻ ions. If there is no effect between PA SAMs and I⁻ ions on each other and they are adsorbed at the metal/solution interface independently, then S₁ is equal to 1. Alternatively, a synergistic effect manifests when S₁ > 1 and a antagonistic effect prevails at S₁ < 1.⁵⁸ The values of the S₁ for 2.5 and 5 mM KI with PA SAMs in Table 7 are greater than unity, indicating that there is a synergistic effect between PA SAMs and I⁻ ions in 0.5 M H₂SO₄ solution. However, it is worth mentioning that the case of PA SAMs with 1 mM KI may be an exception. The inhibition efficiency in potentiodynamic polarization and EIS methods of 1 mM KI are both lower than those of 0.1 mM PA SAMs alone and the synergistic parameter is less than unity. This phenomenon could be ascribed to an antagonistic effect due to the low concentration of KI solution.

Table 7 Synergistic parameters (S₁) for different concentrations of KI from EIS and potentiodynamic polarization methods

KI concentration/mM	Synergism parameter (S1)
	EIS method	Polarization method
1	0.766	0.997
2.5	2.70	2.45
5	3.72	3.77

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In literature,^59–61 the authors explained the synergistic effect by initial contact adsorption of iodide ions on the copper surface, followed by a decrease in the positive charge on the metal, which facilitates the adsorption of protonated inhibitors. In other words, the presence of I⁻ ions on the copper surface could form interconnecting bridge between the copper surface and positively charged inhibitor molecules, improving the corrosion inhibition.

It was reported^47,62 that the copper surface was positively charged in H₂SO₄ solution while PA molecule can be expected to be protonated in equilibrium with the corresponding neutral form in strong acid solutions.

PA + H⁺ ↔ PAH⁺ (7)

Cu ↔ Cu_ads⁺ + e⁻ (fast step) (8)

Cu_ads⁺ ↔ Cu²⁺ + e⁻ (slow) (9)

where Cu_ads⁺ is an adsorbed species at the copper surface. The reaction eqn (9) is controlled by diffusion of soluble Cu²⁺ species from the electrode surface to the bulk solution, which is generally the rate-determining step.⁴⁷

Due to the electrostatic repulsion, it is difficult for the positively charged inhibitor molecules to approach the positively charged metal surface. In the case of PA SAMs alone, SO₄²⁻ ions should be first adsorbed onto the positively charged copper surface to create an excess negative charge towards the metal to form a protective layer, reducing the corrosion rate of copper.⁶³

Cu⁺ + ySO₄²⁻ + xPAH⁺ ↔ [Cu–ySO₄²⁻–xPAH⁺] (10)

where x and y are the numbers of PA molecules and SO₄²⁻ ions adsorbed on the copper surface, respectively.

Addition of KI to the PA SAMs in H₂SO₄ solution can significantly reduce the corrosion rate of copper due to the higher adsorption ability of I⁻, which enhances the adsorption of the positively charged inhibitor molecules and reduces the dissolution of copper.

Cu + I⁻ ↔ CuI_ads⁻ (11)

CuI_ads⁻ + PAH⁺ ↔ CuI_ads⁻PAH_ads⁺ (12)

Table 8 shows that both the inhibition abilities of PA alone and PA with I⁻ are quite good relative to other systems.

Table 8 Comparison of different inhibition efficiencies (η)

Inhibitor	η (%)			Ref.
	EIS	Potentiodynamic polarization	Mixed with I^−
Phytic acid	84.5	83.3	90	This work
Phytic acid	82.2	87.4		12
Cysteamine	69.2	77.9		1
Phytic acid	82.6	90.6		17
Rhodamine	98.8		99.9	47
1-[(1′,2′-Dicarboxy)ethyl]benzotriazole	63	62	85	60

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4. Conclusions

 (1) The addition of PA SAMs on the copper surface in 0.5 M H₂SO₄ solution leads to limited inhibition properties.

(2) PA molecules act as a kind of mixed type inhibitor adsorbed at the surface via P–O groups and the cyclohexyl ring.

(3) Adsorption of PA SAMs on copper surface obeys the Langmuir isotherm and is a chemical interaction.

(4) There is a synergistic effect from PA SAMs and I⁻ due to the addition of I⁻ enhancing the inhibition efficiency significantly.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant no. 21073121), and PCSIRT (IRT1269).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra47291d

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