Mechanical, Corrosion and Wear Characteristics of Cu-Based Composites Reinforced with Zirconium Diboride Consolidated by SPS

Abstract

This study aimed to investigate the physical, mechanical, corrosion, and tribological properties of Cu-based composites with varying zirconium diboride content. The composites were successfully consolidated using spark plasma sintering (SPS) at temperatures of 850 °C and 950 °C and a pressure of 35 MPa. The effect of the ZrB₂ content and the sintering temperature on the properties of the Cu-based composites was investigated. Scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and X-ray diffraction were used to analyse microstructure evolution in copper matrix composites. Microhardness tests were used to evaluate mechanical properties. Wear behaviour was evaluated using a ball-on-disc method. Corrosion properties were estimated on electrochemical tests, such as potentiodynamic polarisation. The results demonstrated an enhancement in the density and porosity of the composites as the sintering temperature increased. A uniform dispersion of ZrB₂ was observed in the copper matrix for all composites. With an increase in the content of the ZrB₂ reinforcement phase, there was an increase in microhardness and an improvement in the wear resistance of the sintered composites. A reduction in densification and corrosion resistance of Cu-based composites was observed with increasing ZrB₂ content.

1. Introduction

Pure copper is widely used for electrical and electronic equipment due to its excellent electrical conductivity and high resistance to corrosion [1]. Due to its low hardness, mechanical strength, and wear resistance, the expansion of the use of copper in demanding applications is limited [2]. On the one hand, pure copper is strengthened by introducing alloying additives. However, introducing an alloying additive reduces the electrical conductivity of copper [3,4]. Therefore, a second alternative is copper matrix composite materials. They are promising materials for various engineering applications in the electronics, energy, automotive, and manufacturing industries, where good electrical and thermal conductivity, good strength and corrosion properties, and microstructural stability are required [5,6,7]. The development of copper matrix composites is primarily based on using suitable ceramic as a reinforcing phase. The choice of reinforcing material is determined by its properties, such as hardness, wear resistance, corrosion and oxidation resistance, good chemical stability, price, and availability on the market [8,9,10]. The ceramic particles used in copper matrix composites are oxides (Al₂O₃, Y₂O₃, TiO₂, and ZrO₂ [7,11,12,13]), carbides (SiC, TiC, NbC, and B₄C [14,15,16,17]), nitrides (Si₃N₄, AlN, and TiN [18,19,20]), and borides (TiB₂, ZrB₂, and CrB₂ [21,22,23,24]). The consequence of introducing ceramic particles is to improve the strength and tribological properties of copper matrix composites. Cu-based composites have a wide range of applications in the automotive, military, electrical, and electronic industries and as heat exchangers in power plants. In particular, Cu-based composites are used to produce electrodes for electrical spot welding, electrical contacts and high-performance switches, heat exchangers, connecting wires for high-speed trains, and lead frames of semiconductors [7,10,15,18,25].

The corrosion properties of composite materials are a crucial aspect of their evaluation in technical applications. Metal-based composites with a ceramic reinforcing phase are characterised by a complex microstructure that may favour the formation of corrosion microcells between different phases. A significant problem in these materials is the presence of micropores and discontinuities at the boundary between the matrix and the reinforcement phase [26]. Electrolytes can penetrate micropores, which leads to increased reaction surface and acceleration of corrosion processes. The penetrating electrolyte increases the surface area susceptible to oxidation, and corrosion products can accumulate in the pores, causing crevice corrosion. The accumulation of corrosion products at these discontinuities can also promote the development of pitting and intergranular corrosion of the material used as the matrix. Due to these phenomena, the study of the corrosion properties of composite materials is critical. Considering the problems related to the decrease in the corrosion resistance of composites, it seems reasonable to use a metal or alloy with increased resistance as a matrix. As mentioned above, this role can be played by copper, a material with excellent corrosion resistance often used in corrosive environments [27].

Tribological wear is caused by friction processes in which the properties of the contact areas’ mass, structure, and surface layer are altered. The intensity of tribological wear depends on the type of tribological pair and the type of interaction under operating conditions [28]. According to the literature [29,30,31,32,33,34], research was carried out on the influence of various ceramic particles (TiB₂, TiC, Al₂O₃, SiC, WS₂, and MoS₂) on the tribological properties of copper-based composites. The authors focused mainly on research on the influence of the reinforcing phase on the physical, mechanical, and tribological properties of composites. According to data from the literature, introducing a ceramic phase into the copper matrix improves the wear resistance of composites. This was confirmed by the research of Zhou et al. [30] on the tribological properties of Cu−WS₂ composites. It was shown that the friction coefficient increases gradually, and the wear rate increases dramatically as the particle size of WS₂ decreases from 5.0 to 0.6 µm. In turn, Fan et al. [35] showed that, as the ZrB₂ content increased, the hardness and abrasion resistance of the composites increased. At the same time, the electrical conductivity of the composites decreased. In another work [32], the influence of different amounts of TiC reinforcing phase on the wear resistance of sintered composites was determined using the pin-on-disc method. As research has shown, the wear resistance of composites improves with increasing volume fraction of the reinforcing phase. According to the authors, ceramic particles enhance the hardness of the matrix and consequently reduce the actual wear surface.

The above review illustrates that incorporating ceramic additives into copper can influence various properties. Equally important is the synthesis method used for Cu-based composites, as it plays a crucial role in determining the final material quality. In this context, our research focused on developing a synthesis technique for copper-based composites using the spark plasma sintering (SPS) method. We investigated the effects of varying ZrB₂ content (5–20 wt%) and sintering temperatures (850 °C and 950 °C) on the resulting Cu-based composites’ densification, mechanical, corrosion, and tribological properties.

2. Materials and Methods

Pure copper powder (approximately 10 μm in size; 99.9 wt% purity) and ZrB₂ powder (2.5–5.5 μm in size; 99.9 wt% purity) were used to produce Cu-based metal matrix composites. The ZrB₂ particles were added to the Cu powders by weight in 5, 10, 15, and 20%. The powder mixtures were prepared using a planetary ball mill (Fritsch Pulverisette 6, Birkenfeld, Germany) with a rotation speed of 200 rpm for 10 h. The composite powders were poured into a 20 mm diameter graphite die and sintered using an SPS system (LSP-100, Laboratory Sintering Press Dr. Fritsch GmbH, Fellbach, Germany) under an argon atmosphere. The sintering was carried out at temperatures of 850 °C and 950 °C, a pressure of 35 MPa, and a time of 300 s. After sintering, the samples were 7–8 mm in height and 20 mm in diameter (Figure 1).

The density of the sintered composites was determined using the Archimedes immersion method in water [36]. An analytical balance RADWAG AS 220/C/2 (Radwag, Radom, Poland) was used to measure the weight of the sinters. Apparent density (ρ) and open porosity (P_o) were calculated according to the relevant standards [36]. The apparent density was calculated according to the following:

$$\rho ={\displaystyle \frac{m_1}{m_1-m_2}}{\rho }_L$$

The open porosity was calculated according to the following:

$$P_o={\displaystyle \frac{m_3-m_1}{m_3-m_2}}100\%$$

where:

m₁—mass of the dry sample (g);

m₂—apparent mass of the immersed sample (g);

m₃—mass of the sample saturated with liquid (g);

ρ_L—density of water (g/m³).

Hardness measurements were made with a Vickers hardness tester (NEXUS 4000, Innovatest Europe BY., Maastricht, The Netherlands) under a load of 2.942 N and a hold time of 10 s. Each sample was tested with 10 points. The measurement error did not exceed 2%.

Samples for microstructural analysis were prepared using standard methods of mechanical grinding using SiC foil (400÷2000 gradation) and polishing using diamond suspension (3 and 1 μm) and MD-Mol discs. The examined samples were then chemically etched. Composites were characterised by scanning electron microscopy (SEM) using Jeol JSM 6610 LV (Akishima, Tokyo, Japan) with energy-dispersive spectroscopy (EDS). Additionally, the microstructure of the composites was characterised using high-resolution scanning electron microscopy (Inspect F50 FEI, Hillsboro, OR, USA) with electron backscatter diffraction (EBSD) analysis. The samples were tilted 70° from the horizontal to collect EBSD data at an accelerating voltage of 20 kV and a beam current of 200 mA. EBSD analysis was performed using TSL OIM software (Ametek, Berwyn, PE, USA) and a PDF ICCD 2011 database format. X-ray diffraction (XRD) measurements were carried out to identify the phases in the sintered samples using an Empyrean Panalytical (Malvern, United Kingdom) diffractometer equipped with Cu/Kα radiation.

Wear resistance tests of sintered Cu + ZrB₂ composites were carried out using the ball-on-disc method on a universal tribotester from ELBIT Innovation—Implementation Company (Tarnów, Poland) according to the standard [37]. Three tests were performed for each sintered material. The tests were conducted under dry friction conditions at 22 °C, with a load of 5 N, a sliding speed of 0.1 m/s, a sliding distance of 1000 m, and a time of 10,000 s. A counter-sample made of bearing steel (AISI52100; diameter of 3.175 mm) was used. During the test, each sample interacted with the new ball surface. Before the tests, balls and samples were washed in ethyl alcohol in an ultrasonic washer and weighed on a RADWAG AS 220/C/2 scale. The final mass after the ball-on-disc test was also weighed for each sample. Mass loss (Δm) was calculated based on the formula [38]:

$$\Delta m={\displaystyle \frac{m_o-m_k}{m_o}}100\%$$

(1)

where:

m_o—initial mass (g);

m_k—final mass (g).

The wear tracks were analysed in the next stage using an OLYMPUS LEXT OLS5100 (Tokyo, Japan) confocal microscope with OLS5100-Analysis Application software Ver. 2.2.3 dedicated to surface analysis. According to the standard [36], the wear profile was measured in four places and the wear volume of the disc sample was calculated (Figure 2). The wear rate (Wv) was calculated based on the formula:

$$W_{V\left(disc\right)}={\displaystyle \frac{V_{disc}}{F_n*L}}$$

(2)

where:

F_n—applied load (N);

V_disc—wear volume of disc specimen (mm³);

L—sliding distance (m).

The corrosion test was carried out using a 3.5 wt% NaCl solution. A conventional three-electrode system was applied with Ag/AgCl (3M KCl) as a reference electrode and a platinum sheet as a counter electrode. The samples were mounted horizontally and sealed with an 8 mm O-ring, and the sample mounted this way had an active surface of 0.503 cm². The examined surface was polished on a P800 grid of abrasive paper and cleaned in an ultrasonic water bath before each experiment. All measurements were conducted at room temperature (22 ± 3 °C) in a naturally aerated solution. The experiments were controlled with a potentiostat AUTOLAB PGSTAT128 (Metrohm AG, Herisau, Switzerland). The potentiodynamic polarisation scan was performed at a scan rate of 1 mV/s from −0.25 V vs. the open-circuit potential (OCP) to a potential where the current density reaches 5 mA/cm². Before each polarisation test, a sample was conditioned for 900 s in the solution to obtain the OCP value.

3. Results

3.1. Physical and Mechanical Properties

Figure 3 shows the apparent density and open porosity results for copper matrix composites with different weight percentages of ZrB₂ as a function of temperature. The apparent density gradually decreases as the amount of ZrB₂ increases. The apparent density gradually decreases with increasing ZrB₂ content. Wang et al. [39] observed a similar trend in their research. They produced Cu–ZrB₂ composites using a hot-pressing sintering process at 840 °C. A decrease in density was found (from 96.1 to 91.3%) when the ZrB₂ content was changed (1–9 wt%). Zang et al. [40] prepared Cu–5 wt.%ZrB₂ composites using hot-pressed sintering. They showed that relative density increased from 84 to 95% with the sintering temperature (760 °C to 920 °C). The difference in the apparent density of ZrB₂ and copper can explain this relationship. According to the literature [2,41], ZrB₂ ceramics have a lower apparent density (6.10 g/cm³) compared to copper (8.96 g/cm³). The results also indicated the dependence of density and open porosity on the sintering temperature. Composites produced at a temperature of 850 °C have a relative density of 87–95% of the theoretical density and open porosity in the range of 2.1–11.5% (for 5–20% ZrB₂). For higher ZrB₂ contents (from 10 wt%), the open porosity is greater than 9%. This result suggests that pores are not eliminated during sintering at 850 °C. This results in a decrease in relative density. The test results indicate that the temperature of 850 °C is inadequate to obtain a high degree of consolidation of Cu + ZrB₂ composites. Increasing the temperature by 100 °C improved the relative density and reduced open porosity. For the temperature 950 °C, the best relative density of 93–97% of the theoretical density was achieved. The open porosity is in the range of 1.2–3.8%. It should be noted that, regardless of the sintering temperature, the relative density of pure sintered copper is above 98% of the theoretical density.

The results of the Vickers microhardness measurements of the sintered copper and Cu + ZrB₂ composites are shown in Figure 4. Microhardness increases with increasing ZrB₂ content in the copper matrix. The highest microhardness values were obtained for composites containing 20% ZrB₂. In comparison, the microhardness of pure copper is 61.4 HV0.3. Adding a 20% reinforcement phase resulted in an almost 3-fold increase in microhardness to values of 162 HV0.3 and 179 HV0.3 for sintering temperatures of 850 °C and 950 °C, respectively. Improvement in microhardness can be attributed to the high hardness of ZrB₂ ceramics (above 22 GPa [41]) and the uniform distribution of ZrB₂ in the copper matrix. Shaik [6] and Pouyani [42] observed a similar relationship between an improvement in hardness and an increase in the reinforcement-phase ZrB₂. The analysis of the results (Figure 4) shows a negligible effect of the sintering temperature on the microhardness of the composites.

In general, higher microhardness values were obtained at 950 °C. This increase is approximately 10% compared to the microhardness of all Cu + ZrB₂ composites sintered at a lower temperature, i.e., 850 °C. Wang et al. [42] produced Cu composites with various ZrB₂ contents (1–9 wt%) by hot sintering. They showed that the microhardness of pure copper (57.5 HV0.2) was increased to 100.8 HV0.2 with an increase in the ZrB₂ content to 7 wt%. The addition of 9 wt% ZrB₂ to the copper matrix resulted in a decrease in microhardness to 82 HV0.2. Similarly, Zang et al. [40]

] prepared Cu–5 wt.%ZrB₂ composites using hot-pressed sintering. They reported that, by increasing the sintering temperature to 840 °C, the microhardness of the composite continuously increased to the maximum value of 92 HV0.2 and then decreased to 85 HV0.2 with further increase in the sintering temperature (to 920 °C). In turn, Bagheri [32] showed that the hardness of the composites improved with an increase in the percentage prepared of TiC particles. Hardness enhanced from 43 to 285 HV1 for the copper and copper-36 vol% TiC composite, respectively. The increase in hardness is explained by the formation of TiC nanoparticles during the in situ process, their uniform distribution in the copper matrix, and the Orowan strengthening mechanism.

3.2. Microstructures

Figure 5 and Figure 6 show the results of the microstructural tests of Cu + ZrB₂ composites sintered at 950 °C with EDS chemical composition analysis. The EDS profiles (point 1, Figure 4 and Figure 5) show peaks of copper (Cu), zirconium (Zr), and boron (B), confirming the presence of the ZrB₂ reinforcing phase in the copper matrix. ZrB₂ particles are uniformly distributed in the matrix, and their agglomeration was not observed. Furthermore, no micro- or macrocracks were observed in the Cu + ZrB₂ composites. The results of the EDS analyses were confirmed by phase composition studies using the X-ray (Figure 7) and EBSD technique (Figure 8 and Figure 9). Based on EBSD phase analyses, the presence of the following phases in the microstructure of the sintered composites was confirmed: Cu and ZrB₂.

3.3. Wear Properties

Table 1. Wear results of Cu + ZrB₂ composites after the wear test.

Sintered Materials	Sintering Temperature (°C)	Friction Coefficient μ (-)	Specific Wear Rate Wv⋅10−4 (mm3/Nm)	Loss of Weight Δm (%)
Cu	850	0.61	8.16	0.36
	950	0.60	8.08	0.29
Cu + 5% ZrB2	850	0.64	6.56	0.27
	950	0.62	6.26	0.24
Cu + 10% ZrB2	850	0.62	4.42	0.18
	950	0.58	4.28	0.17
Cu + 15% ZrB2	850	0.56	3.98	0.12
	950	0.54	3.61	0.11
Cu + 20% ZrB2	850	0.49	2.59	0.09
	950	0.48	2.37	0.09

summarises the test results for the friction coefficient, mass loss, and specific wear rate for sintered materials at various temperatures. The results show that the tribological parameters slightly depend on the sintering temperature. When a higher sintering temperature of 950 °C was used, lower friction coefficient values, weight loss, and specific wear rate were obtained for sintered copper and each Cu + ZrB₂ composite. The test results (Figure 10) showed that the friction coefficient depends on the content of ZrB₂. The friction coefficient of pure copper (0.60–0.61) increased to 0.62–0.64 when the ZrB₂ content increased to 5 wt%. Adding more strengthening phase (above 10 wt%) gradually decreased the friction coefficient. The lowest friction coefficient (0.48–0.49) was obtained for the composite containing 20% ZrB₂. A different trend was observed for the specific wear rate, which was determined by analysing the change in the amount of ZrB₂ in the copper matrix. It was found that the abrasion resistance of Cu + ZrB₂ composites improves as a function of the change in the ZrB₂ content. According to the determined data (Table 1), the specific wear rate gradually decreases from 8.08 × 10⁻⁴ (mm³/Nm) to 2.37 × 10⁻⁴ (mm³/Nm) for copper and the Cu + 20% ZrB₂ composite, respectively. Using the pin-on-disc method, Shaik and Golla [6] examined the effect of different amounts (1, 3, 5, and 10 wt%) of ZrB₂ on the abrasive properties of copper-based composites. The friction coefficient decreased from 0.56 to 0.16 for pure copper Cu and Cu + 3wt% ZrB₂, respectively. Introducing a harder recovery phase (10 wt%) slightly increased the factor to 0.22. Pure copper showed a high wear coefficient of 17.33 × 10⁻², which decreased to 2.4 × 10⁻² for Cu + 3 wt% ZrB₂ composites. The further addition of ZrB₂ reinforcement reduced the wear resistance of the tested composites.

Figure 11, Figure 12, Figure 13 and Figure 14 show wear tracks and their profiles measured in four places for the Cu + 5% ZrB₂ composite according to the diagram presented in Figure 2 and the standard [37]. Analogous measurements were made for other materials tested using the ball-on-disc method (Figure 14). For all Cu + ZrB₂ composites, a very similar nature of material removal was observed by the ball from the worn zone. During the tests, the material was permanently deformed and abraded to the edge of the wear track. A characteristic layering of the material was observed at the edge of the wear track (Figure 11, Figure 12, Figure 13, Figure 14 and Figure 15). Profile analyses of composites with different amounts of ZrB₂ (5–20 wt%) showed differences in the dimensions of the wear track. An increase in the width and depth of the wear track was observed, along with an increase in the reinforcing phase in the copper matrix. During the test, hard ZrB₂ ceramic particles are torn out of the matrix and move in the contact zone of the ball with the sample surface. It is an additional factor that accelerates the wear of the composite.

Microstructural observations (SEM) confirmed the results of analysing the worn areas using confocal microscopy. Examples of microstructures of the worn surfaces of copper-based composites with different contents of the ZrB₂ phase are shown in Figure 16. The analysis of the results showed that the worn surfaces of the composites are subject to plastic deformation. As a result, delaminations were observed. In addition, abrasive scratches and grooves were observed in the worn surface area. It suggests a mixed nature of the wear of the tested materials. The wear mechanism of composites is a combination of abrasive and adhesive wear.

3.4. Corrosion Properties

Uniform corrosion of copper in a neutral pH environment and in the presence of chloride ions proceeds according to the reaction [27]:

$${\mathrm{C}\mathrm{u}}^{+}+{\mathrm{C}\mathrm{l}}^{-}\to \mathrm{C}\mathrm{u}\mathrm{C}\mathrm{l}$$

The reaction produces an initial corrosion product which, depending on the conditions, can evolve to Cu₂O, Cu(OH)₂, CuO, Cu₂(OH)₃Cl, or CuCO₃∙Cu(OH)₂CuCO₃∙Cu(OH)₂ [43].

The electrochemical behaviour of copper and the Cu + ZrB₂ composites immersed in a 3.5% NaCl solution was studied by potentiodynamic polarisation after monitoring the open-circuit potential. Figure 16 shows the result of the potentiodynamic polarisation measurements. Polarisation curves reveal that adding ZrB₂ to copper increases the measured corrosion current, equivalent to deterioration of the corrosion resistance of the composite. The most probable reason is the increased porosity from adding ZrB₂, leading to a larger specific surface area of Cu exposed to NaCl and crevice corrosion processes. It should also be noted that sinters containing up to 5% ZrB₂ exhibit a less noble potential than Cu.

In comparison, those with more than 10% ZrB₂ exhibit a more noble potential than Cu. The observed relationship occurs regardless of the sintering temperature. The shift of the potential to a more negative direction for copper may be caused by an increase in the concentration of chloride ions or an increase in temperature [27]. As a result of ongoing corrosion processes, this can be attributed to the increased concentration of chloride ions in the pores and free space. This theory appears to be confirmed by the shape of the polarisation curves, where a flattening appears on the anode side, suggesting diffusion effects before the Tafel region (η approximately 100 to 200 mV). This flattening indicates limited mass transport, likely due to the narrow micropores. The discussed effect does not appear for ZrB₂ contents of 10% and more. The addition of ZrB₂ is probably already large enough to change the electrode potential according to mixed potential theory [44]. In this case, the values of the anode branch slope are close to the standard value of 120 mV/decade. The ignition of the anodic curve at 120 mV/decade is typically due to the Tafel behaviour in the active region of the corrosion process, indicating a well-defined electrochemical reaction mechanism.

Table 2. Potentiodynamic polarisation parameters for copper and Cu + ZrB₂ composites in 3.5% NaCl with different content of ZrB₂ and sintering temperature.

Samples	Sintering Temperature	Erest (mV)	Ecorr (mV)	Icorr (µA/cm2)	βa (mV/dec)	βc (mV/dec)
Cu solid	-----	−0.183	−0.171	3.33	26	201
Cu (sintered)	850 °C	−0.190	−0.194	4.62	67	222
Cu + 5% ZrB2		−0.187	−0.245	6.33	86	133
Cu + 10% ZrB2		−0.136	−0.168	16.68	71	160
Cu + 15% ZrB2		−0.136	−0.184	34.51	101	199
Cu + 20% ZrB2		−0.133	−0.172	27.67	121	161
Cu (sintered)	950 °C	−0.183	−0.253	3.30	179	163
Cu + 5% ZrB2		−0.201	−0.244	4.48	74	189
Cu + 10% ZrB2		−0.149	−0.208	22.70	106	166
Cu + 15% ZrB2		−0.118	−0.208	52.60	145	136
Cu + 20% ZrB2		−0.094	−0.177	124.45	110	123

presents parameters obtained from polarisation curves such as E_rest, E_corr, I_corr, β_a, and β_c. The E_rest potential result is the last OCP value detected before the polarisation measurement. However, the corrosion potential of E_corr was estimated using Tafel extrapolation. In some cases, significant differences between these values result from the lack of straightness in the Tafel region, which may suggest complex electrode reactions. The values obtained from the corrosion currents allow the samples to be divided into two groups. The first group of composites contains up to 5% ZrB₂. They do not differ significantly from the reference sample of pure solid copper and are characterised by corrosion currents below 10 uA/cm². The second group consists of samples with a 10% ZrB₂ content, with much higher corrosion current values. Based on the results, it is difficult to find a significant impact of temperature on corrosion resistance for samples containing up to 5% ZrB₂. However, for composites with a ZrB₂ content greater than 10%, samples sintered at 950 °C show significantly worse corrosion resistance despite lower open porosity (approximately 3% vs. 9%). One can try to explain this by saying that, with a sufficiently high open porosity value, there is no accumulation of corrosion products in the pores and no local pH changes, so corrosion occurs mainly uniformly.

4. Conclusions

Based on the experimental results obtained for Cu + ZrB₂ composites fabricated by SPS, the following conclusions can be drawn:

• Using a temperature of 950 °C in the SPS process is beneficial for obtaining a high degree of densification of Cu + ZrB₂ composites, for which the apparent density is 93–97% of the theoretical density. The densification of the composites was observed to decrease with increasing ZrB₂ phase.
• The microhardness increased significantly with the change in the content of ZrB₂. Among all the composites, Cu + 20% ZrB₂ showed the maximum hardness (179 HV0.3).
• The wear resistance of the Cu + ZrB₂ composites increases with the increasing content of ZrB₂.
• Adding ZrB₂ above 5% significantly reduces the resistance of the composite to corrosion in chloride solutions. Regardless of sintering temperature, composites with a ZrB₂ content of up to 5% do not show a significant difference in corrosion resistance compared to pure copper.