Electrochemical Investigation of Corrosion Behavior of CuFeP Alloy in Chloride Solution

Abstract

The corrosion behavior of copper and a Cu-Fe-P alloy in 3.5% NaCl solution was studied in this paper. This study focused on the influence of microalloying in the Cu-Fe-P alloy containing 0.003 wt% Fe and 0.014 wt% P on corrosion resistance in chloride media. Additionally, the effect of 2-mercapto-1-methylimidazole as an inhibitor was evaluated using electrochemical techniques, including potentiodynamic polarization, cyclic voltammetry, and electrochemical impedance spectroscopy. According to the potentiodynamic polarization results, 2-mercapto-1-methylimidazole can be classified as a mixed-type inhibitor. The inhibition efficiency also increases with increasing concentration. The results indicate that the Cu-Fe-P alloy has improved corrosion resistance compared to copper, and a higher inhibition efficiency of 2-mercapto-1-methylimidazole was observed for the Cu alloy.

1. Introduction

Copper is used in various natural and industrial environments because of its favorable properties, such as its good mechanical characteristics and excellent electrical and thermal conductivity [1,2,3]. However, the corrosion behavior of copper depends greatly on environmental factors, including pH, dissolved oxygen, temperature, and the presence of aggressive ions such as chlorides and sulfides. The presence of chlorides in the solution leads to the formation of soluble copper–chloride complexes and further degradation of the material [4]. In addition to pure copper, copper-based alloys are also recognized as suitable materials for various applications [5].

Copper-based alloys, particularly Cu-Fe-P alloys, have attracted considerable attention in recent years due to their favorable combination of high electrical conductivity, mechanical strength, and satisfactory corrosion performance. Copper alloys containing Fe and P have been increasingly studied in recent years, primarily because they offer a balanced combination of good electrical conductivity, improved mechanical properties, and acceptable corrosion resistance. Such alloys are used in various industries, including consumer electronics, automotive electronics, and aerospace systems, where electrical reliability and mechanical stability are critical requirements [6].

The alloying elements iron (Fe) and phosphorus (P) play distinct but complementary roles in tailoring the properties of copper. The addition of Fe enhances mechanical strength without significantly compromising electrical conductivity, while phosphorus is added as a deoxidizing agent and to mitigate hydrogen embrittlement, also contributing to improved microstructural stability [6].

In addition to mechanical integrity, corrosion resistance is a key consideration for Cu-Fe-P alloys, especially in applications involving humid, industrial, or marine environments. The corrosion behavior of Cu-based alloys alloyed with Fe is strongly governed by the microstructural distribution of the Fe phase and the characteristics of the surface corrosion products formed in chloride-containing environments. As previously observed in Cu-Fe alloys, an optimal Fe content promotes the formation of a dense and protective corrosion product layer, whereas excessive Fe leads to phase coarsening, non-uniform distribution, and a less compact surface film with reduced protective capability [7,8]. The presence of phosphorus has been shown to enhance resistance to oxidation and corrosion. However, the overall corrosion behavior of these alloys is strongly affected by processing conditions, such as the degree of plastic deformation and the recrystallisation temperature. These parameters affect the resulting microstructure, which in turn governs both electrochemical activity and degradation mechanisms. Despite significant progress, the relationship between processing routes, microstructural evolution, and corrosion performance in Cu-Fe-P alloys remains an active area of research [6].

Recent studies have emphasized that careful optimization of processing parameters is necessary to achieve a balanced combination of high mechanical strength, low residual stress, and adequate corrosion resistance [8,9,10]. Therefore, electrochemical techniques are essential tools for evaluating the corrosion behavior of copper-based alloys.

According to a literature review, many compounds have been investigated as corrosion inhibitors for copper in chloride media, including azoles [11,12], amino acids [13,14], plant extracts [15,16], and pharmaceutical compounds [17,18]. It is well known that the characteristics of a potential inhibitor, such as the presence of S, N, and O atoms, polar groups, and/or π-electrons in its molecular structure, lead to a decrease in the corrosion rate of metals. Despite the widespread industrial application of Cu-Fe-P alloys, insufficient attention has been given to their corrosion behavior in aggressive solutions. Furthermore, the inhibition mechanism of mercapto-imidazole derivatives in chloride media on these alloys remains insufficiently understood. With this in mind, the present work focuses on investigating the corrosion characteristics of a Cu-Fe-P alloy in chloride media without and with the addition of 2-mercapto-1-methylimidazole and to compare it with the corrosion behavior of copper under identical conditions. Special emphasis is placed on evaluating the inhibitor efficiency and elucidating the role of alloying elements in the corrosion and inhibition processes.

2. Materials and Methods

For microalloying Cu with Fe and P alloy, cathode copper with 99.99% purity (Grade A quality from AURUBIS, Hamburg, Germany) was used. The copper cathodes were melted in an electric resistance furnace under a protective atmosphere. The alloy was then cast using the “up-cast” method. During casting, the chemical composition of the alloy was continuously monitored by sampling every hour. The Fe and P content was measured using an optical emission spectrometer with spark (OES), model 4460, manufactured by ARL (Ecublens, Switzerland).

The electrochemical measurements were conducted using a potentiostat (IVIUM XRE, IVIUM Technologies, Eindhoven, The Netherlands) with the appropriate software. A three-electrode system was used, consisting of a working electrode, a reference electrode (saturated calomel electrode, SCE), and an auxiliary electrode (platinum wire). Copper and a copper alloy (CuFeP“0” with a composition of 0.003 wt% Fe, 0.014 wt% P and the remainder copper) served as the working electrodes. Copper is cold rolled with a reduction rate of 80%. The working electrode was polished before each experimental measurement with alumina paste (0.3 μm Al₂O₃, Buehler, Lake Bluff, IL, USA), then rinsed with distilled water and dried.

Electrochemical methods, including cyclic voltammetry, open circuit potential (OCP) measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS), were used in this investigation. The open circuit potential was measured for 30 min. Potentiodynamic polarization measurements were then conducted from the open circuit potential to +0.05 V (vs. SCE) in the anodic direction and from the open circuit potential to −0.60 V (vs. SCE) in the cathodic direction at a scan rate of 1 mV/s. Potentiodynamic polarization measurements were performed in three parallel experiments. Cyclic voltammetry curves were recorded over a potential range from −1 V (vs. SCE) to 1 V (vs. SCE) at a scan rate of 10 mV/s. EIS was performed using IVIUM software (IVIUMSOFT 2.411) under the following conditions: frequency range 100 kHz to 0.01 Hz, with a single-compartment perturbation of 10 mV.

Sodium chloride was used to prepare a blank solution with a concentration of 3.5%. 2-mercapto-1-methylimidazole (MMI) was used as a possible corrosion inhibitor at concentrations ranging from 1 × 10⁻⁴ M to 1 × 10⁻² M.

Equations (1)–(3) are used to calculate the inhibition efficiency (IE), anodic (f_a) and cathodic (f_b) interaction coefficients:

$$\%IE=\left({\displaystyle \frac{i_{corr}-i_{corr\left(inh\right)}}{i_{corr}}}\right)·100$$

(1)

$$f_a={\displaystyle \frac{i_{corr\left(inh\right)}}{i_{corr}}}\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{E_{corr}-E_{corr\left(inh\right)}}{b_a}}\right)$$

(2)

$$f_b={\displaystyle \frac{i_{corr\left(inh\right)}}{i_{corr}}}\mathrm{e}\mathrm{x}\mathrm{p}\left({\displaystyle \frac{E_{corr}-E_{corr\left(inh\right)}}{b_c}}\right)$$

(3)

Here, i_corr and i_corr(inh) are the corrosion current densities without and with the addition of an inhibitor, respectively. E_corr and E_corr(inh) are the corrosion potential values in the blank and in the inhibited solution, while b_a and b_c are the anodic and cathodic Tafel slopes, respectively.

3. Open Circuit Potential and Potentiodynamic Polarization Measurements

Before potentiodynamic polarization measurements, open circuit potential curves were obtained for both copper and its alloy. Measurements were performed in the blank solution as well as in the presence of MMI, and the results are shown in Figure 1. The continuous potential variation observed during OCP measurements suggests that complete steady-state conditions were not fully established within the investigated immersion period. According to the literature [19], a steady state is assumed to be reached when the E_OCP changes by less than 5 mV over 10 min. The absence of complete OCP stabilization indicates the presence of dynamic interfacial processes at the electrode/electrolyte interface. According to the obtained curves, the addition of an inhibitor causes a shift of E_ocp toward positive values, for both Cu and CuFeP“0”. However, the movement of the open circuit potential is less than 85 mV, indicating that MMI could be classified as a mixed-type inhibitor. The positive shift in open circuit potential observed after adding the inhibitor is attributed to the adsorption of inhibitor species onto the copper and alloy surfaces, resulting in greater suppression of the anodic metal dissolution process [20].

Potentiodynamic polarization measurements for Cu and the copper alloy were performed in 3.5% NaCl solution without and with the addition of various concentrations of MMI. Figure 2 illustrates the obtained results, and

Table 1. Mean and standard deviation values of electrochemical parameters of copper and alloys in 3.5% NaCl in the presence of MMI.

Alloy	Inhibitor Concentration, M	Ecorr, V vs. SCE ± SD	icorr, A/cm2 ± SD	ba, V/dec ± SD	bc, V/dec ± SD	fa	fb	IE, %
Cu		−0.296 ± 0.013	9.75 × 10−6 ± 1.28 × 10−7	0.108 ± 0.028	−0.102 ± 0.016	/	/	/
	1 × 10−4	−0.271 ± 0.011	4.60 × 10−6 ± 2.45 × 10−7	0.070 ± 3.05 × 10−3	−0.050 ± 0.024	0.258	0.224	52.8
	5 × 10−4	−0.195 ± 5.42 × 10−3	2.13 × 10−6 ± 1.77 × 10−7	0.074 ± 7.64 × 10−3	−0.123 ± 7.62 × 10−3	0.030	0.051	78.1
	1 × 10−3	−0.235 ± 6.78 × 10−3	1.05 × 10−6 ± 6.31 × 10−7	0.064 ± 1.30 × 10−3	−0.122 ± 5.73 × 10−3	0.038	0.059	89.2
	1 × 10−2	−0.245 ± 2.47 × 10−3	8.22 × 10−7 ± 7.55 × 10−8	0.054 ± 1.05 × 10−3	−0.112 ± 3.07 × 10−3	0.028	0.046	91.5
CuFeP“0”		−0.279 ± 0.019	6.87 × 10−6 ± 3.32 × 10−7	0.102 ± 4.18 × 10−3	−0.067 ± 0.011	/	/	/
	1 × 10−4	−0.259 ± 3.97 × 10−3	2.45 × 10−6 ± 5.78 × 10−7	0.054 ± 6.78 × 10−3	−0.044 ± 8.04 × 10−3	0.277	0.255	64.3
	5 × 10−4	−0.182 ± 4.26 × 10−3	1.38 × 10−6 ± 8.61 × 10−8	0.056 ± 2.58 × 10−3	−0.139 ± 2.68 × 10−3	0.040	0.113	79.9
	1 × 10−3	−0.241 ± 1.51 × 10−3	7.28 × 10−7 ± 9.02 × 10−8	0.077 ± 2.36 × 10−3	−0.103 ± 8.46 × 10−3	0.078	0.088	89.4
	1 × 10−2	−0.261 ± 6.34 × 10−3	5.60 × 10−7 ± 4.56 × 10−8	0.075 ± 3.93 × 10−3	−0.084 ± 9.55 × 10−4	0.072	0.074	91.8

summarizes the calculated kinetic parameters including corrosion potential (E_corr), corrosion current density (i_corr), anodic (b_a) and cathodic (b_c) Tafel slopes, anodic (f_a) and cathodic (f_c) interaction coefficients and inhibition efficiency (%IE) for both copper and the copper alloy.

Figure 2 shows that the copper alloy (CuFeP“0”) has a lower corrosion current density than copper in 3.5% NaCl solution. It is assumed that the addition of alloying elements Fe and P contributes to the improved corrosion resistance of CuFeP“0” compared to copper. Similar conclusions have been reported in the literature [7,8]. Zou et al. [7] observed that during corrosion, a Cu₂O passive film forms on the alloy surface and serves as the primary barrier against further dissolution. Additionally, Fe and P precipitates formed during aging contribute to alloy purification and corrosion resistance. Phosphorus preferentially segregates to grain boundaries and the alloy surface, where it reacts with oxygen and moisture under alternating wet–dry conditions to form a transient H₃PO₄ film. This film then reacts with Cu²⁺ ions to produce insoluble Cu₃(PO₄)₂ (reaction 4), which accumulates mainly along grain boundaries. The dense phosphate phase effectively reduces the electrochemically active area and restricts the access of oxygen and water to the alloy surface before the complete formation of the Cu₂O passive layer, thereby enhancing the overall corrosion resistance of the Cu-Fe-P system [7]. However, the improvement in corrosion resistance compared to Cu in the studied case can be attributed with certainty only to the presence of both alloying elements. To fully understand the effect of the specific alloying elements, Fe or P, as well as their combined influence on the corrosion behavior of the Cu-Fe-P alloy, alloys containing different amounts of these elements should be investigated. Further, an attempt was made to reduce the corrosion rate by adding MMI. In the presence of 1 × 10⁻² M MMI, the corrosion rate for both copper and its alloy decreased, and the inhibition efficiency was higher than 90%. The diminished current density observed in solutions containing the inhibitor indicates the formation of a layer that hinders further deterioration of the tested electrodes. The inhibitor exhibited both anodic and cathodic effects at all tested concentrations, although an increase in current density at more positive potentials was recorded at each concentration. Also, in aerated chloride media, oxygen reduction is considered the predominant cathodic reaction on copper-based materials. As a result, the adsorbed inhibitor layer may partially hinder oxygen transport to the electrode surface, causing deviations from ideal Tafel behavior and promoting mixed kinetic–diffusion control. This assumption is further supported by the EIS results, which show diffusion-related contributions associated with Warburg impedance at low frequencies. Although inhibitor adsorption is assumed to play the dominant role in corrosion mitigation, the contribution of oxygen diffusion limitations should also be considered [21]. According to Simonović and coauthors [22], quantum chemical calculations of MMI as a copper corrosion inhibitor indicate that it has low values of ΔE and dipole moment, suggesting high reactivity of MMI and its accumulation in the surface layer. This agrees with the obtained polarization curves, which indicate effective protection of Cu and CuFeP“0” against corrosion in the presence of MMI, especially at higher inhibitor concentrations.

Analysis of the results shows that increasing the MMI concentration shifts E_corr toward more positive values. Because this shift is less than 85 mV, MMI is classified as a mixed-type inhibitor [23,24]. Furthermore, the values in Table 1 indicate that the inhibitor affects both the anodic (b_a) and cathodic (b_c) Tafel slopes, while f_a and f_b values less than 1 support the conclusion that the inhibitor molecules influence both the anodic and cathodic reactions [25]. Such changes are commonly associated with the formation of a protective film on the copper surface [24].

$$\mathrm{3}{\mathrm{C}\mathrm{u}}^{\mathrm{2}\mathrm{+}}\mathrm{+}{\mathrm{H}}_{\mathrm{3}}\mathrm{P}{\mathrm{O}}_{\mathrm{4}}\mathrm{\to }{\mathrm{C}\mathrm{u}}_{\mathrm{3}}\mathrm{(}\mathrm{P}{\mathrm{O}}_{\mathrm{4}}{\mathrm{)}}_{\mathrm{2}}\mathrm{+}\mathrm{3}{\mathrm{H}}^{\mathrm{+}}$$

(4)

4. Cyclic Voltammetry Measurements

Cyclic voltammograms were recorded to characterize the corrosion of copper and its alloy in 3.5% NaCl, both without and with the addition of MMI. The resulting voltammograms, shown in Figure 3, indicate a decrease in the anodic current density of CuFeP“0” compared to that of copper. According to several investigations [7,26,27], the anodic polarization of copper and copper-based alloys in sodium chloride solutions leads to the formation of characteristic corrosion products. These products mainly consist of copper oxides and chloride-containing phases, such as CuCl₂·3CuO·3H₂O, Cu₂Cl(OH)₃, Cu₂O, and CuCl. As reported by several research groups, namely, Tasić et al. [4], Petrović Mihajlović et al. [11], and Milošev et al. [28], in a chloride-containing solution without inhibitors, the dominant cathodic process on copper is the reduction in dissolved oxygen (reaction (5)). On the anodic side, copper primarily forms cuprous chloride species (CuCl₂⁻). These complexes may form in two ways: either through direct interaction between metallic copper and chloride ions, involving the transient formation of CuCl as an intermediate (reactions (6)–(8)), or through initial dissolution of copper into cuprous ions, which then combine with chloride ions to produce the same complexes (reactions (9) and (10)). During the cathodic scan of the polarization curve, a distinct cathodic peak is typically observed. This peak is generally attributed to the reduction in cupric corrosion products formed on the surface during the preceding anodic sweep, as consistently reported in the literature [29,30,31,32].

O₂ + 2H₂O + 4e⁻ → 4OH⁻

(5)

Cu + 2Cl⁻ ⇌ CuCl₂⁻ + e⁻

(6)

Cu + Cl⁻ ⇌ CuCl + e⁻

(7)

CuCl + Cl⁻ ⇌ CuCl₂⁻

(8)

Cu ⇌ Cu⁺ + e⁻

(9)

Cu⁺ + 2Cl⁻ ⇌ CuCl₂⁻

(10)

The presence of MMI results in the reduction in current density for both copper and its alloy. At higher inhibitor concentration, a significant reduction in current density is observed. It is already confirmed [22,33] that MMI acts via adsorption on the surface of both copper and alloys, so it is assumed, based on microscopic images (Figure 4), that at lower tested concentrations, the inhibitor layer formed on the Cu and CuFeP“0” surfaces has defects or provides incomplete coverage. In either case, this allows Cl− ions to further damage the electrodes. During the reverse scan, a small anodic activation peak is observed in the presence of MMI (1 × 10⁻⁴ M to 1 × 10⁻³ M), indicating the occurrence of the reactivation process for both Cu and CuFeP“0”. It is proposed that a porous protective layer forms or that there is competition between the dissolution and precipitation of the layer on the electrode surfaces [34]. At the highest MMI concentration, there is a clear reduction in anodic current density for both Cu and its alloy, suggesting the formation of a stable protective film that prevents further deterioration. Similar observations were reported for MMI as a brass corrosion inhibitor [33].

5. Electrochemical Impedance Spectroscopy—EIS

Tests included electrochemical impedance spectroscopy (EIS) measurements to enhance our understanding of the processes occurring at the metal/solution interface. Only the highest tested concentration of the corrosion inhibitor MMI was used to evaluate its effect. The data obtained are presented in Figure 5 and

Table 2. Electrochemical impedance spectroscopy parameters for copper and CuFeP in NaCl solution without and with the addition of MMI.

3.5% NaCl Solution	Rs, Ω cm2	Rf, Ω cm2	Rct, Ω cm2	Cf, μF cm−2	n1	Cdl, μF cm−2	n2	W, Ω−1 cm−2 s0.5	IE, %
Cu	7.181	391.3	109.2	3.84	0.803	279	0.725	130.0	/
CuFeP“0”	6.013	391.8	342.2	4.72	0.807	31.3	0.801	114.4	/
Cu + 1 × 10−2 M MMI	9.558	2457	962.6	19.7	0.900	20.7	0.800	25.1	85.4
CuFeP“0” + 1 × 10−2 M MMI	7.102	2862	5425	0.286	0.835	0.987	0.810	23.5	91.1

. The equivalent circuit used for data fitting, shown in Figure 6, is commonly used to describe copper corrosion [4,11,21,35,36,37,38,39,40,41]. To select the appropriate equivalent circuit, we evaluated two models: Rs(Q(RW)) and Rs(Q(R(QRW))). The Rs(Q(RW)) circuit produced higher χ² values (on the order of 10⁻²), while Rs(Q(R(QRW))) yielded lower χ² values (on the order of 10⁻³) and better agreement with the experimental spectra. Therefore, Rs(Q(R(QRW))) was chosen as the most suitable model for interpreting the EIS data, which is consistent with the literature [35,42,43].

The parameters presented in Table 2 have following the definitions: R_s—solution resistance; R_f—film resistance; R_ct—charge transfer resistance; Q_f and Q_dl—constant phase elements (CPE) indicating non-ideal double-layer capacitance; C_f—film capacitance; C_dl—double layer capacitance; W—Warburg impedance; n—deviation parameter [4,37,39,44,45].

$$C_f={\left(Q_f{R}_f^{1-n_1}\right)}^{1/n_1}$$

(11)

$$C_{dl}={\left(Q_{dl}{R}_{ct}^{1-n_2}\right)}^{1/n_2}$$

(12)

Inhibition efficiency is calculated according to the following equation:

$$IE={\displaystyle \frac{R_p-R_{p0}}{R_p}}·100$$

(13)

where R_p0 is the total polarization resistance of the electrode in NaCl solution, and R_p is the total polarization resistance of the copper electrode in NaCl solution with MMI.

Similar R_s values, presented in Table 2, indicate that the test conditions were comparable [9]. The value of n indicates the CPE characteristics: n = 0 denotes a pure resistor, and n = 1 is a pure capacitor; it can also represent Warburg impedance if n = 0.5, or an inductor when n = −1 [21,46]. In the presence of an inhibitor, the electrode surface changes, becoming more homogeneous and smoother, while porosity and defects decrease, which can be observed as an increase in the value of n [42,45].

The high-frequency semicircle, observed in the Nyquist plot, can be correlated to charge transfer resistance and double layer capacitance [47]. The straight part of the plot, visible in the low-frequency region, is presented via Warburg impedance in the applied circuit and appears due to the diffusion of different species to or from the surfaces of the copper and the alloy [36,42,44,45,47,48]. This could be a result of the diffusion of soluble copper species, dissolved oxygen, or other corrosive species [35,42,44,47,48]. The combination of large semicircle and Warburg impedance indicates that the process is under the influence of diffusion and charge-transfer [35,49]. The decrease in Warburg impedance correlates more with the alloy than with the copper, and the more substantial decrease that appears in MMI-containing solutions indicates the inhibition of diffusion processes and the presence of a surface film that affects mass transport [38,45]. The diameter of the Nyquist plot semicircle can be correlated to the electron transition through the surface layer. As it becomes more difficult and corrosion resistance improves, this is illustrated as an increase in the Nyquist plot semicircle diameter. It can be further correlated with the calculated values of polarization resistance (Rp = R_f + R_ct) increasing and C_dl decreasing. The semicircle diameter is greater for the alloy compared to pure copper and also increases in the presence of MMI, which can be attributed to the corrosion reduction effect [4,8,21,35,41,48,50]. The values of n, which are between 0.5 and 1, indicate the presence of some imperfections, pores, and roughness on the surface [21]. The analysis of all these parameters leads to the conclusion that the corrosion inhibition effect can be attributed to the adsorption of MMI molecules on the surface. Adsorbed molecules reduce the available electrode surface, form a protective barrier, increase the thickness of the electrical double layer, and prevent transfer to the electrode surface [38,41,45,47,49].

The impedance values included in the Bode plots (Figure 5) are higher—in the whole frequency range—in the MMI-containing solutions in comparison to blank solutions. Also, slightly higher values are recorded for the alloy than for copper. The maximum value of log|Z| recorded in the low-frequency region for the copper electrode is 2.90, whereas in the presence of MMI, it reaches ~3.80. For the copper alloy in the NaCl solution without and with MMI, the values of log|Z| are 3.10 and ~4.00, respectively.

The maximum phase angle also increases with the addition of MMI compared to the blank solution, indicating corrosion inhibition [48]. The phase angle of an ideal, intact layer without any defects on the electrode surface would be 90°; hence, an obtained phase angle value of approximately 70° corresponds to a layer with high inhibition efficiency [44]. Overall, an observed increase in impedance, the width of the phase angle, and the maximum phase angle together indicate improved corrosion protection via the MMI inhibitor [4,42,45,48,51]. The maximum phase angle values of approximately 70–80° indicate pronounced capacitive behavior and the formation of a protective adsorbed inhibitor layer. Similar Bode phase angle responses have been reported for azole- and imidazole-based inhibitors on copper and copper alloys in chloride media [52,53,54,55].

6. Adsorption Isotherm

The adsorption process occurs due to interactions between inhibitor molecules and the metal surface. These interactions may occur as electrostatic attraction, donor–acceptor interactions, or the formation of coordination bonds between heteroatoms in the inhibitor molecule and vacant orbitals of the metal atoms. In this study, the experimental data were fitted to the Langmuir adsorption isotherm model, as represented by Equation (14).

$${\displaystyle \frac{C_{inh}}{\theta }}={\displaystyle \frac{1}{K_{ads}}}+C_{inh}$$

(14)

$$-∆G_{ads}=(ln{K}_{ads}-ln\left(1/55.55\right))·R·T$$

(15)

According to the correlation coefficient values (R²), MMI adsorbs on the copper and CuFeP surfaces and thus suppresses corrosion. Based on the linear plots (Figure 7), the slope values are close to 1, indicating that the adsorption fits the Langmuir isotherm. Additionally, the free Gibbs energy was calculated according to Equation (15), and the values are presented in

Table 3. Langmuir adsorption parameters.

Electrode	Slope	R2	ΔG, kJ/mol
Cu	1.08	0.9999	−34.0
CuFeP “0”	1.07	0.9999	−35.0

, indicating strong adsorption of MMI on both the copper and CuFeP surfaces. MMI contains heteroatoms (N and S) in its molecular structure, which are crucial for its adsorption on the metal surface. Zhu et al. [56] explained that coordination between Cu and MMI occurs through the interaction of the N atom from the imidazole ring and the S atom, thereby providing corrosion protection. Based on the calculated Fukui indices by Larbi and coauthors [57], the sulfur atom in the MMI molecule is predicted to be more reactive toward electrophilic attack than the nitrogen atoms. However, Zhu et al. [56] suggested that the nitrogen atoms in the imidazole ring also contribute significantly to the adsorption process. Their theoretical results indicate that MMI adsorption on the metal surface may occur through both the sulfhydryl group and the π-electron system of the imidazole ring, resulting in a strong interaction between the inhibitor molecule and the metal surface.

7. Conclusions

This paper investigates the corrosion behavior of a CuFeP alloy in 3.5% NaCl solution and compares it with copper. Potentiodynamic polarization measurements show that CuFeP exhibits a lower corrosion current density than copper under the same conditions. The addition of alloying elements is assumed to enhance corrosion resistance. The effect of MMI as a corrosion inhibitor was also tested, and results from cyclic voltammetry, potentiodynamic polarization, and EIS confirm its ability to reduce the corrosion rate for both CuFeP and copper in 3.5% NaCl solution. MMI acts as a mixed-type inhibitor, affecting both anodic and cathodic reactions. EIS results indicate that the mechanism of the corrosion processes is not altered; rather, inhibitor molecules adsorb onto the electrode surfaces according to the Langmuir adsorption isotherm. This study provides insight into the influence of alloying elements (0.003 wt% Fe and 0.014 wt% P) and imidazole compounds on the corrosion behavior of a Cu–Fe–P alloy in 3.5% NaCl solution. The obtained results highlight its potential for use in applications where corrosion resistance is essential for long-term performance; however, further research should focus on assessing the oxidation behavior and corrosion resistance of the Cu–Fe–P alloy in acidic solutions. Such investigations would provide deeper insight into the structure–performance relationship and contribute to the development of more efficient corrosion protection strategies. Additionally, future studies should include Cu-based alloys with systematically varied Fe and P contents to clarify the individual contributions of these alloying elements to corrosion resistance and inhibition behavior.