Degradation of Metal Guitar Strings: Corrosion-Induced Mechanical Damage and Electrochemical Analysis Under Accelerated Conditions

Abstract

Metal guitar strings are essential components whose acoustic performance and durability are compromised by surface degradation, mainly corrosion. This phenomenon directly affects their mechanical properties, limiting their useful life. This study investigated the corrosion behavior and associated mechanical degradation of commercial acoustic guitar strings under accelerated saline exposure. Electrochemical testing, accelerated ageing and frequency sweep testing were combined to assess the evolution of corrosion resistance and apparent dynamic stiffness. The results demonstrate a progressive and substantial loss of stiffness and corrosion resistance in the exposed guitar strings. This study contributes to the understanding and prediction of corrosion-induced degradation in metallic acoustic components.

1. Introduction

The manufacture of stringed musical instruments has evolved significantly over the centuries, moving from the use of natural materials to complex metal alloys designed to optimize sound, durability and resonance [1]. In the particular case of the acoustic guitar, metal strings are a critical component that directly influences essential parameters such as tonal quality, volume and sustain [2]. However, their exposure to ambient humidity, air and salinity and contact with the sweat of the musician’s hands makes them highly susceptible to corrosion processes, which compromises both their structural integrity and their acoustic properties [3].

Guitar string corrosion has begun to be a subject of heightened attention in recent years. Predictive models based on neural networks have been proposed to estimate the evolution of steel strings in corrosive media [4]. The microstructure and corrosion kinetics in artificial sweat solutions have also been analyzed, establishing degradation rates associated with direct contact with the performer [5].

Other studies have correlated acoustic measurements with electrochemical tests, evidencing the relationship between surface deterioration and loss of sound quality [6]. In addition, cathodic protection systems against human sweat and their effect on the harmonic response have been studied [7,8]. In the field of heritage conservation, the corrosion of metal strings in historical instruments, which constitutes a challenge for their preservation, has been documented [9].

Beyond studies focused on musical strings, the literature on copper-based alloys provides a solid framework. Comparative studies on copper, bronze and brass in salt solutions have characterized their electrochemical behavior and degradation mechanisms [10]. Recent research on Cu-Sn bronzes in 3.5% NaCl confirms the formation of oxide layers and their influence on charge-transfer resistance [11]. Recent reviews have also compiled the main protection strategies developed for copper and its alloys, including surface coatings and inhibitors [12]. Finally, nanoscale research on pure copper has described early intergranular corrosion phenomena [13], which opens the possibility of applying advanced characterization tools to the study of strings.

This study investigated corrosion-induced degradation in commercial metal guitar strings exposed to controlled salt-spray conditions. To this end, electrochemical tests were conducted in salt solution, accelerated exposure techniques were tested in a salt spray chamber, and characterization of their mechanical properties was completed via frequency sweep testing. The objective was to determine the nature of the relationships between the evolution of electrochemical parameters, surface morphology and loss of stiffness in order to understand the dominant mechanisms of corrosion and provide knowledge applicable to the improvement of string durability.

2. Materials and Methods

2.1. Tested Materials

Commercial acoustic guitar strings with a high-carbon steel core and metallic wound were used (see Figure 1) in this study.

Figure 1. Core and winding of a guitar string.

Two types of winding alloys were selected for their widespread use in the music industry: 80/20 bronze and phosphor bronze (PBr). Both alloys are tested in surface-coated and non-surface-coated versions.

The manufacturer-applied coatings consisted of micrometer-thick polymeric layers, described as polyamide/fluoropolymer-based, intended to reduce the adsorption of moisture, salts and contaminants on the winding surface.

The microstructure of the core was analyzed after conventional metallographic preparation and etching in a 2% Nital solution (2% nitric acid in ethanol). The microstructure of the guitar string core was examined using a JEOL JSM-5600 scanning electron microscope (SEM; JEOL Ltd., Tokyo, Japan) operated at an accelerating voltage of 20 kV and a working distance of approximately 20 mm. Energy-dispersive X-ray spectroscopy (EDS) was performed with an detector (Oxford Instruments, Abingdon, UK) to complete a semiquantitative chemical analysis of the corrosion products formed after the salt-spray chamber tests.

For corrosion and mechanical tests, string segments of approximately 70 mm were cut from the selected string type (diameter of 0.012 inches [≈0.305 mm]) and mounted in the corresponding test fixtures, selected for their smaller cross-section, which makes them more sensitive to degradation and allows for the early detection of changes in their mechanical properties [14].

2.2. Salt-Spray Chamber

Guitar strings were exposed to a marine atmosphere in a salt-spray chamber, after which corrosion (electrochemical tests) and mechanical (frequency sweep) degradation were analyzed. Different exposure intervals were established with the aim of covering different stages of degradation. In electrochemical tests, short initial times (1 h and 10 h) were selected to characterize the transient phase of film formation and stabilization of the response, together with intermediate and long times (100 h, 170 h and 240 h) that allowed the evolution of corrosion to be evaluated towards more advanced stages. Longer intervals (up to 345 h) were also included in mechanical tests in order to analyze the cumulative effects on string stiffness and vibrational capacity. This combination of times followed criteria used in previous studies of corrosion in metal strings and copper-based alloys in salt media [5,6,7,12]. The corrosion and mechanical behavior of the pre-degraded strings were compared with those not exposed to the salt-spray chamber (guitar strings denoted as 0 h).

The chamber temperature was maintained at 35 ± 2 °C. The corrosive atmosphere was generated by atomizing a 5 wt.% NaCl solution in distilled water (pH = 7). The fog collection rate was maintained at 1–2 mL h−1 per 80 cm² of horizontal collecting area, following the general conditions of ASTM B117 [15]. The guitar strings were held in an inert sponge holder to prevent chemical or mechanical interference during the test.

2.3. Electrochemical Tests

Electrochemical tests were performed in order to quantify the corrosion behavior of strings in a saline environment [16]. A three-electrode cell was used, with the guitar string serving as the working electrode (WE), an Ag/AgCl electrode as the reference electrode (RE), and platinum as the counter electrode (CE). Electrochemical tests were carried out using an Ivium Vertex C potentiostat (Ivium Technologies B.V., Eindhoven, The Netherlands). The electrolyte solution used was a solution of 3.5% sodium chloride (NaCl) in distilled water (pH = 6.7) to simulate an aggressive marine or saline environment. A constant volume of 280 mL of electrolyte was maintained, and the immersion of the working electrode was limited to 3.5 cm (half its length), ensuring a constant exposure surface. The working electrode was positioned about 2 mm from the reference electrode, following technical recommendations for optimization of the measurement [17]. A new working electrode was used for each test. All the electrochemical experiments were performed in duplicate, and the reported values correspond to the average of the two measurements.

2.3.1. Linear Polarization Resistance (LPR)

The LPR technique is used to determine polarization resistance (${\mathrm{R}}_{\mathrm{p}}$), a parameter inversely proportional to the corrosion rate [18]. A symmetrical potential sweep of ±30 mV (−0.030 V to +0.030 V) was applied with respect to the open-circuit potential (OCP) at a sweep rate of 0.7 mV/s.

The ${\mathrm{R}}_{\mathrm{p}}$, as shown in Equation (1), was calculated as the derivative of the potential ($\mathrm{d}\mathrm{E}$) with respect to the current ($\mathrm{d}\mathrm{I}$), evaluated when the current ($\mathrm{I}$) tended to zero. When current density was used, R_p was obtained from the slope of the potential–current density curve near the corrosion potential and is expressed in Ω·cm².

$${\mathrm{R}}_{\mathrm{p}}={\left.{\displaystyle \frac{\mathrm{d}\mathrm{E}}{\mathrm{d}\mathrm{I}}}\right|}_{\mathrm{I}\to 0}$$

(1)

2.3.2. Electrochemical Impedance Spectroscopy (EIS)

EIS is a versatile, non-destructive analytical technique, ideal for studying the electrochemical processes that occur at the interface between metals and their environments. This technique provides valuable information about corrosion mechanisms and rates, as well as the effectiveness of protective coatings, making it an invaluable tool for corrosion research [19,20].

The complex impedance of the system is expressed according to the Equation (2):

$$\mathrm{Z}\left(\mathsf{\omega }\right)={\mathrm{Z}}^{\prime }\left(\mathsf{\omega }\right)+\mathrm{j}·{\mathrm{Z}}^{″}\left(\mathsf{\omega }\right)$$

(2)

where ${\mathrm{Z}}^{\prime }\left(\mathsf{\omega }\right)$ represents the real (resistive) part and ${\mathrm{Z}}^{″}\left(\mathsf{\omega }\right)$ the imaginary (capacitive/inductive) part of the impedance.

EIS measurements were conducted over a frequency ranging from 0.05 Hz to 50,000 Hz with a ±10 mV sweep from the OCP. EIS measurements were conducted in 3.5% NaCl solution at room temperature by using a traditional three-electrode system. The data obtained were plotted in Nyquist and Bode diagrams for analysis, which allows for the modelling of the system with equivalent circuits and understanding the mechanisms of charge transfer and diffusion of species, as discussed below.

2.4. Mechanical Test: Frequency Sweep

The dynamic-mechanical analysis (DMA) was performed on RSA3 equipment (TA Instruments, New Castle, DE, USA) in order to characterize the viscoelastic response of the strings subjected to corrosion. This technique is based on the application of an oscillatory load of variable frequency and the measurement of the resulting deformation. The test provided the storage modulus (E′), loss modulus (E″) and loss factor (tan δ). In this study, these quantities should be interpreted as apparent dynamic parameters of the tested string configuration.

The strings were mounted on the device with a controlled pretension of 500 g to ensure repeatable boundary conditions during testing. The test consisted of the application of a harmonic excitation in the range of 2 to 80 Hz, with increments of 2 Hz, with the mechanical response in each exposure interval being recorded.

The choice of this technique was justified by its ability to provide the mechanical properties of the strings, so that through the test being repeated at different stages of corrosion, changes in stiffness (E′) could be identified and correlated with the observed surface damage.

3. Results and Discussion

The findings of each of the techniques used are detailed below, and an integrated discussion of their implications is presented, with the aim of specifying the underlying mechanisms of degradation.

3.1. Microstructural Characterization

Commercial acoustic guitar strings were characterized by SEM and EDS prior to the corrosion tests.

As shown in Figure 2a, the string consisted of a high-carbon steel core surrounded by a helically wound phosphor-bronze (PBr) wire. The winding geometry left narrow inter-turn gaps that could facilitate electrolyte penetration during exposure. The cross-sectional microstructure of the steel core (Figure 2b) exhibited a heavily cold-worked ferritic structure, consistent with the severe plastic deformation introduced during the wire drawing process. The outer winding (Figure 2c) had an average thickness of approximately 10 μm and was mainly composed of phosphor bronze, as confirmed by EDS analysis. This bimetallic configuration was expected to influence the corrosion behavior of the string, particularly once the electrolyte reached the steel core through the spaces between adjacent windings.

Figure 2. Microstructure of metal guitar strings with a PBr winding: (a) Cross-section of the guitar string. (b) Cold-worked ferrite in the core. (c) EDS (energy-dispersive X-ray spectroscopy analysis) in the winding.

3.2. Dynamic Stiffness (E′) Analysis

Frequency sweep testing provides an accurate method for obtaining the elastic mechanical behavior of strings, a fundamental aspect of their acoustic function. The measured E′ is used as an apparent indicator of the dynamic stiffness of the string. Changes in this parameter are expected to influence vibration behavior and may affect acoustic performance.

The evolution of the storage modulus (E′) as a function of frequency in four types of guitar strings is presented in Figure 3. PBr and 80/20 bronze strings were compared, both in their coated and uncoated versions. The results show that uncoated PBr strings had the highest dynamic stiffness across most of the frequency spectrum. Uncoated strings generally exhibited a more linear relationship between storage modulus and frequency, while coated strings exhibited a turning point starting at approximately 60 Hz.

Figure 3. Evolution of the storage modulus (E′) as a function of frequency for four types of strings in the as-received state: PBr without and with coating and 80/20 bronze without and with coating.

Because the coating appeared to have a less significant effect on PBr strings compared to 80/20 bronze strings, PBr strings (Figure 3) were selected for future corrosion testing to ensure more consistent baseline properties.

As the strings were subjected to exposure in the salt-spray chamber, the DMA results showed a progressive and significant reduction in the storage modulus (E′) (see Figure 4). This decrease in stiffness is a clear indicator of material degradation. The data show that after 345 h of exposure, the strings show a 63.9% reduction in their original stiffness, decreasing from 2.02 × 10¹¹ Pa to 0.73 × 10¹¹ Pa. This phenomenon can be explained by surface corrosion, which generates microdefects, pitting and porosity on the surface of the metal. These defects act as stress concentrators and reduce the effective cross-section of the material and thus its ability to resist elastic deformation.

Figure 4. Evolution of the storage modulus (E′) at 60 Hz of metal guitar string with a PBr wound during salt-spray exposure.

This decrease is attributed to the combined effects of loss of effective load-bearing section, localized corrosion defects, surface roughening, possible damage to the winding/coating and reduced mechanical coupling between string components. The evolution of the storage modulus (E′) over time showed a decreasing trend, demonstrating the cumulative nature of corrosion damage. This finding underscores that deterioration in stiffness is an irreversible and progressive process that can affect string functionality and acoustic performance.

3.3. Electrochemical Results

3.3.1. Linear Polarization Resistance

The evolution of the polarization resistance (${\mathrm{R}}_{\mathrm{p}}$) with exposure time in the salt-spray chamber revealed a multi-stage corrosion process that reflects changes in the surface of the material. The results obtained from the linear polarization resistance (LPR) measurements are summarized in Table 1, while the corresponding linear polarization curves are shown in Figure 5a,b.

Table 1. R_p values obtained during different exposure time intervals in the salt-spray chamber of metal guitar strings with a PBr winding.

	Exposure Time (h)
	0	1	10	100	170	240
Rp (Ω∙cm2)	25,027 ± 600	15,445 ± 400	2037 ± 400	1956 ± 442	1387 ± 220	457 ± 160

Figure 5. (a) Linear polarization curves of PBr-wound guitar strings in 3.5 wt.% NaCl solution, (b) linear polarization curve in the original state (R_p = 25,027 Ω∙cm²), and (c) the same curves showing the evolution of potential versus the log of the current density.

Under the initial conditions (0 h), the value of ${\mathrm{R}}_{\mathrm{p}}$ was high (25,027 Ω∙cm²), which indicates a high resistance to corrosion, as expected for guitar strings in their original state. As an example, the calculation of the polarization resistance is illustrated in Figure 5b. After only one hour of exposure in the salt-spray chamber, ${\mathrm{R}}_{\mathrm{p}}$ underwent a drastic decrease (15,445 Ω∙cm²), which can be interpreted as the onset of corrosion. The saline medium, rich in chloride ions (Cl⁻), is highly aggressive and rapidly (within 1 h) destabilizes the thin oxide layer that naturally forms on the surface of copper alloys [21,22]. Polarization resistance notably decreased with increasing exposure time, indicating a continuous reduction in the corrosion resistance of the guitar strings as degradation proceeded. This trend is consistent with the increasing electrolyte penetration through the inter-turn gaps of the phosphor-bronze winding and the subsequent exposure of the steel core. Consequently, in the range from 10 to 100 h of exposure, the polarization resistance underwent a second marked decrease, reaching 2037–1956 Ω·cm². This behavior suggests that the initially formed surface film, composed mainly of metallic oxides and hydroxides, is not uniform and partially breaks down as a result of the continued penetration of chloride ions and the accumulation of porous, non-protective corrosion products. This interpretation is supported by the surface appearance of the corroded guitar strings and is further discussed in the following section based on the EIS results.

After 170 h of exposure, the progressive degradation of the phosphor-bronze winding was already clearly visible and allowed the electrolyte to reach the steel core. Consequently, the electrochemical response was especially expected to arise from the combined contribution of the phosphor-bronze winding and the steel substrate. Under these conditions, galvanic interaction between both metals may also have contributed to the observed corrosion behavior (R_p $\approx$ 1387 Ω·cm²).

Finally, at 240 h, ${\mathrm{R}}_{\mathrm{p}}$ exhibited a dramatic, marked decrease, indicating a highly advanced stage of metal surface degradation and an advanced degradation of electrochemical resistance. This behaviour reflects the progressive disruption of the winding and the continuous exposure of fresh metal to the aggressive saline environment [23,24]. Although the present study did not include full potentiodynamic polarization measurements, the linear polarization curves recorded within ±30 mV of the OCP reveal a progressive shift of the open circuit potential towards more negative values, especially after 1 h of exposure, as can be seen in Figure 5c. Together with the decrease in the experimentally measured polarization resistance, this behavior indicates the progressive degradation of the phosphor-bronze winding and the increasing accessibility of the electrolyte to the steel core.

3.3.2. Electrochemical Impedance Spectroscopy Analysis (EIS)

As previously mentioned, the guitar strings were placed in the salt-spray chamber and, after exposure times of 1, 10, 100, 170 and 240 h, were also examined using the electrochemical impedance spectroscopy (EIS) technique. The Nyquist and Bode plots are shown in Figure 6.

Figure 6. EIS analysis of metal guitar strings with a PBr wound: (a) Nyquist plots, (b) the impedance–frequency curves for all the exposure times, and (c) the phase angle–frequency curves.

The impedance spectra indicate the presence of two time constants. The first time constant represents the surface layer behavior and is indicative of the formation of hydroxide- or oxy-hydroxide-type corrosion products at frequencies above 1000 Hz [25]. The second time constant represents the charge transfer process of the metal dissolution. The impedance response is dominated by a very complex interfacial process based on the guitar string structure. As mentioned, the wound geometry of the string contains inter-turn gaps through which the electrolyte progressively penetrates. As exposure proceeds, the electrolyte reaches the steel core, and the electrochemical responses of the phosphor-bronze winding and the steel substrate become partially superimposed. Consequently, the permanent electrical contact between phosphor bronze and steel makes galvanic interaction between both metals a plausible contributing mechanism; however, its contribution is difficult to isolate because the electrochemical responses of both metals are superimposed. In the second process, the contribution of the steel core becomes progressively more significant as the electrolyte penetrates through the inter-turn gaps of the phosphor-bronze winding, allowing the electrochemical response of the steel substrate to contribute increasingly to the overall impedance.

The equivalent circuit shown in Figure 7 is proposed to fit the EIS data, reflecting the corrosion mechanisms identified after different exposure times [26,27,28]. The electrochemical parameters obtained from the mentioned EIS measurements are summarized in Table 2.

Figure 7. Equivalent circuit for metal guitar strings with a PBr winding in 3.5% NaCl at room temperature.

Table 2. The parameters of equivalent circuits for the exposure times of metal guitar strings with a PBr winding.

	Exposure Time (h)
	0	1	10	100	170	240
Rs (Ω)	2	5	4	4	5	5
Qfilm (sn/Ω)	2 × 10−4	5 × 10−4	2 × 10−4	3 × 10−4	6 × 10−4	-
nfilm	0.77	0.73	0.65	0.58	0.54	-
Rfilm (Ω·cm2)	120	80	52	30	14	-
Cfilm (µF/cm2)	66	152	16	10	10	-
Qdl (sn/Ω)	3 × 10−5	4 × 10−5	3.3 × 10−4	3.8 × 10−4	9 × 10−4	3 × 10−3
ndl	0.78	0.75	0.72	0.70	0.65	0.60
Rct (Ω·cm2)	29,000	15,850	2300	2200	1800	800
Cdl (µF/cm2)	26	34	296	352	1167	5378
${\chi }^2$ (error)	2.3 × 10−3	4.0 × 10−3	3.0 × 10−3	1.0 × 10−2	2.0 × 10−2	5.0 × 10−2

In this equivalent circuit model, ${\mathrm{R}}_{\mathrm{s}}$ represents the electrolyte resistance, ${\mathrm{Q}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ (${\mathrm{C}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$, ${\mathrm{n}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$) is the surface layer capacitance, and ${\mathrm{R}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ represents the resistance associated with the surface layer formed on the phosphor-bronze winding, including the native oxide film and the initial corrosion products. ${\mathrm{Q}}_{\mathrm{d}\mathrm{l}}$ (${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$, ${\mathrm{n}}_{\mathrm{d}\mathrm{l}}$) is the double-layer capacitance at the substrate–electrolyte interface, and ${\mathrm{R}}_{\mathrm{c}\mathrm{t}}$ is the charge-transfer resistance. The effective capacitance (${\mathrm{C}}_{\mathrm{i}}$) can be calculated according to Equation (3). The capacitance (${\mathrm{Q}}_{\mathrm{i}}$) is used to describe the non-ideal capacitive behaviour typically observed in electrochemical systems, and the parameter $\mathrm{n}$ is an exponent (0 < $\mathrm{n}$ < 1) that quantifies the deviation from ideal capacitive behaviour.

$${\mathrm{C}}_{\mathrm{i}}={\mathrm{Q}}_{\mathrm{i}}·({{\displaystyle \frac{{\mathrm{R}}_{\mathrm{s}}·{\mathrm{R}}_{\mathrm{i}}}{{\mathrm{R}}_{\mathrm{s}}+{\mathrm{R}}_{\mathrm{i}}}})}^{{\displaystyle \frac{1-\mathrm{n}}{\mathrm{n}}}}$$

(3)

The fitted parameters provide further insight into the corrosion mechanism. ${\mathrm{R}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ values are lower than those calculated for the ${\mathrm{R}}_{\mathrm{c}\mathrm{t}}$. This fact is unsurprising since ${\mathrm{R}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ is the resistance of the surface layer through which the metal ions diffuse. Values of ${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$ are higher than those of ${\mathrm{C}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$, especially for the longer exposure times. The much higher values of ${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$ compared with ${\mathrm{C}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ are physically consistent with the different electrochemical processes represented by these elements [25,29]. Whereas ${\mathrm{C}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ is associated with the native oxide/surface film formed on the phosphor-bronze winding, ${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$ represents the electrochemical double layer at the metal–electrolyte interface.

The progressive decrease in ${\mathrm{R}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$, together with the reduction in ${\mathrm{n}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$, indicates the gradual degradation of the native oxide layer and the surface film formed on the phosphor-bronze winding. Simultaneously, the marked decrease in ${\mathrm{R}}_{\mathrm{c}\mathrm{t}}$ and the continuous increase in ${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$ suggest an increasing electrochemically active area as the electrolyte progressively penetrates through the inter-turn gaps of the winding and reaches the steel core. Consequently, the electrochemical response becomes increasingly dominated by charge-transfer processes occurring at the exposed metallic interfaces. At the longest exposure time (240 h), the disappearance of the ${\mathrm{R}}_{\mathrm{f}\mathrm{i}\mathrm{l}\mathrm{m}}$ element indicates that the surface film no longer provides a distinguishable contribution to the impedance response.

These results support our hypothesis that the electrolyte progressively penetrates through the inter-turn gaps of the phosphor-bronze winding, eventually reaching the steel core. This interpretation is consistent with the marked increase in the double-layer capacitance (${\mathrm{C}}_{\mathrm{d}\mathrm{l}}$) and the simultaneous decrease in the charge-transfer resistance (${\mathrm{R}}_{\mathrm{c}\mathrm{t}}$), indicating an increase in the electrochemically active surface area.

Prior to exposure in the salt-spray chamber, the guitar strings exhibited excellent corrosion resistance. The largest diameter observed in the Nyquist plot (Figure 6a) is indicative of good corrosion performance. Similarly, in the Bode plot (Figure 6c), the phase angle close to −70° indicates that the winding was practically intact and the guitar string was well protected [30,31]. This observation is also consistent with the linear polarization results obtained at 0 h. With no exposure time (Figure 8a), the strings had a smooth and uniform surface, with the characteristic texture of winding, as can be seen in Figure 8a.

Figure 8. Surface appearance of the guitar strings after salt-spray exposure: (a) no exposure time, (b) 1-h exposure time, (c) 10-h exposure time, (d) 170-h exposure time.

Likewise, after 1 h of exposure (Figure 8b), the phosphor-bronze winding still dominated the electrochemical response, indicating that the steel core remained largely shielded from the electrolyte. Nevertheless, the decrease in the phase angle (Figure 6c) suggests the onset of surface film degradation and the initial penetration of the electrolyte through the inter-turn gaps. This interpretation is consistent with the decrease in R_film from 120 to 80 Ω·cm², together with the reduction in R_ct from 29,000 to 15,850 Ω·cm², reflecting a progressive deterioration of the surface film and the increasing accessibility of the metal–electrolyte interface.

After 10 h of exposure (Figure 8c), the corrosion resistance was significantly lower than that at 0 h, in agreement with the linear polarization results. The first relaxation, with a phase angle of approximately −40°, in the high-to-mid frequency range, indicates significant degradation of the surface. The fitted parameters (Table 2) show a decrease in R_film from 120 (0 h) to 52 Ω·cm² (10 h) together with an effective film capacitance of 16 µF/cm² and a reduction in n_film to 0.65, indicating an increasingly heterogeneous and degraded surface film. In the lower frequency range, a second relaxation process, with a phase angle of approximately −50°, is more evident. This process is mainly associated with charge-transfer reactions occurring at the exposed metal–electrolyte interface. As the electrolyte progressively penetrates through the inter-turn gaps of the phosphor-bronze winding, the contribution of the steel core is expected to increase, although the electrochemical response of the phosphor-bronze winding may still contribute to the overall impedance. The effective double-layer capacitance increases to approximately 296 µF/cm², while the charge-transfer resistance decreases markedly to about 2300 Ω·cm².

These changes indicate a substantial increase in the electrochemically active surface area and support the proposed mechanism of progressive electrolyte penetration. This analysis agrees with the surface morphology observed after 10 h of exposure, characterized by a granular layer of corrosion products, as can be seen in Figure 8c. The corrosion products detected after exposure are consistent with iron-rich oxides/oxyhydroxides, such as $\gamma -FeOOH$ and ${Fe}_3{O}_4$ [32,33].

Similarly, after 100 h of exposure, the same trend could be observed, confirming the progressive degradation of the surface film and the phosphor-bronze winding. The low values of R_film and R_ct, together with the increase in C_dl, indicate a larger electrochemically active surface area as the electrolyte progressively penetrates through the inter-turn gaps.

After 170 h of exposure, the phosphor-bronze winding was severely degraded, and the electrochemical response was dominated by charge-transfer processes. The effective double-layer capacitance increased markedly to approximately 1167 µF/cm², while R_ct decreased to 1800 Ω·cm², indicating a substantial increase in the electrochemically active surface area as the electrolyte progressively reached the steel core. The increase in capacitance can be attributed to the formation of a more reactive surface resulting from localized corrosion beneath the wound and an increase in surface roughness [34,35]. The surface exhibited pronounced irregularity and porosity, with cracks, fissures and loss of the original winding structure. Corrosion products concentrated on the grooves, resulting in a non-uniform layer. The surface appearance shown in Figure 8d aligns with the EIS results, confirming that corrosion was highly active.

Finally, after 240 h of exposure, this trend continued, with the lowest charge-transfer resistance (R_ct = 800 Ω·cm²) being obtained, confirming the advanced stage of corrosion.

Finally, it is important to highlight that the electrochemical parameters presented in Table 2 show a good fit for the different interval times, with low chi-square (${\chi }^2$) values below 0.1 [36]. Additionally, the trends observed in the impedance tests (AC) show the same overall trend as those observed in the polarization tests (DC) previously presented, as also shown in Figure 9.

Figure 9. Variation of R_p and |Z|_{0.05 Hz} with exposure time in the salt -spray chamber.

3.4. Trend Between Mechanical Properties and Corrosion

One of the main objectives of this study was to establish a trend between corrosion degradation and loss of mechanical properties. The results show a trend between the evolution of the polarization resistance (${\mathrm{R}}_{\mathrm{p}}$) and that of the storage modulus (E′) over exposure time. It is important to note that polarization resistance (DC) and EIS (AC) results are in good agreement.

The initial drop in corrosion resistance (${\mathrm{R}}_{\mathrm{p}}$) preceded the more gradual and continuous reduction of the storage modulus (E′). This suggests that corrosion acts as the trigger for degradation and that the loss of mechanical properties is a direct consequence of accumulated surface damage. As corrosion progresses, pitting and fissures form, which act as stress concentrators. These defects, even on a microscopic scale, weaken the structure of the material, reducing its ability to withstand loads and vibrations. This hypothesis is supported by the observation that the reduction of the storage modulus (E′) was progressive, even when the rate of electrochemical corrosion appeared to stabilize. This is because structural damage, once initiated, is cumulative and does not recover, even if the rate of corrosion decreases.

Based on Figure 10, the evolution of the polarization resistance (${\mathrm{R}}_{\mathrm{p}}$) with exposure time is similar to that reported by López reported [37]. Both curves show a significant initial decline in resistance in the first hours or days, suggesting a rapid deterioration of the protective properties of the string surface. After this initial drop, the rate of decline in ${\mathrm{R}}_{\mathrm{p}}$ becomes more stable. Similar to López et al. [6], during the early stages of exposure, the phosphor-bronze winding and its native surface film provide the main barrier against electrolyte ingress.

Figure 10. R_p variation with time of exposure to corrosive agents. Experimental data is compared with the reference model from J. L. López Muelas (2017) [37].

As exposure progresses, degradation of the surface film and progressive electrolyte penetration through the inter-turn gaps reduce this protective effect, increasing the contribution of the steel core to the overall electrochemical response, resulting in only minor variations in the overall electrochemical response.

Figure 11 shows that while the resistance to polarization decreased sharply in the first hours and then tended to stabilize, the E′ reduced gradually over time. This discrepancy in behavior suggests that, although the electrochemical corrosion process accelerates after one hour and becomes quasi-stable after 10 h, the underlying structural damage (such as cracks or microdefects) continues to develop and affect the mechanical properties of the material. According to López [6], electrochemical methods can be used to assess how controlled corrosion may affect acoustic performance of musical instrument strings. Building on this approach, this study shows that integrating electrochemical techniques with accelerated mechanical aging offers a powerful and efficient approach to assessing corrosion degradation in metal guitar strings. This research shows that corrosion leads to a loss of stiffness, which in turn causes a reduction in the acoustic properties of the material. The methodology proposed in this study opens new scenarios for the selection of materials applicable to the musical instrument industry, accounting for both corrosive and mechanical degradation.

Figure 11. Temporal evolution of E′ and R_p during the salt-spray chamber test of metal guitar strings with a PBr winding.

4. Conclusions

The combined electrochemical and mechanical approach provides a useful framework for assessing corrosion-induced degradation in metal guitar strings. The agreement between R_p, EIS and DMA supports the proposed interpretation of coupled electrochemical and mechanical deterioration.

The results show a monotonic, but non-linear, decrease in apparent dynamic stiffness with increasing salt-spray exposure time. The loss of stiffness, which exceeded 60% in the strings after 345 h of exposure, is clear evidence of the irreversible deterioration of the material. This finding is of great practical importance, as stiffness is a determining factor for the tone, tuning and lifespan of the strings.

The degradation process can be broadly described in three stages: an initial activation stage (0–1 h wound intact), a pseudo-stationary state phase (10–100 h metal partially exposed) and a final collapse phase (170–240 h).

Electrochemical measurements revealed a drastic drop in corrosion resistance in the first hours of exposure, attributed to the rapid dissolution of the surface oxides and the early degradation of the wound structure as the electrolyte penetrated the inter-turn gaps. The final decrease in electrochemical resistance indicates advanced surface degradation and extensive exposure of electrochemically active metal areas. SEM and EDS analyses provided visual and compositional evidence of this degradation, confirming the formation of corrosion products such as oxides and chlorides that destroy the original surface morphology.