Evolution of surface state of 70/30 cupronickel tubes with stable protective films under cathodic polarisation

Abstract

Cathodic protection is an effective strategy for mitigating corrosion in marine engineering applications. This study systematically investigates the evolution of surface states and corrosion resistance of 70/30 cupronickel tubes with pre-formed stable protective films under cathodic polarisation in natural seawater. Electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) were employed to evaluate the electrochemical behaviour. Key findings reveal that a negative shift in polarisation potential (from −350 to −750 mV) reduces the barrier properties of the pre-existing protective film by nearly one order of magnitude, yet simultaneously accelerates the formation of a dense calcareous deposit layer. Notably, the innovation lies in the quantitative correlation between EIS-derived parameters and deposit growth: charge transfer resistance ( R _ct) increases significantly from 14,260 to 44,790 Ω·cm², while the power index value of constant phase element (CPE) suggests the involvement of O₂ diffusion in the electrochemical processes. The novelty is further highlighted by the identification of aragonite as the dominant phase in the calcareous deposits formed at −550 and −750 mV, which contributes to enhanced surface coverage.

Keywords

Copper alloys corrosion resistance electrochemical behaviour cathodic protection

Introduction

Cupronickel alloys, which contain 5–30% nickel, are widely used in marine applications, particularly in ship condensers, coolers and heat exchangers, owing to their high thermal conductivity, excellent corrosion resistance and ability to resist marine biofouling.^1,2 However, despite these advantages, these alloys can still undergo significant degradation when exposed to aggressive seawater environments.^3,4 Cathodic protection (CP), an electrochemical corrosion control technique, is highly effective in protecting metallic structures, including copper alloys, from seawater-induced corrosion.^5,6 By polarising the alloy to a more negative potential relative to its open-circuit potential (OCP), CP suppresses the thermodynamic tendency for corrosion reactions. As the potential shifts further in the negative direction, the driving force for cathodic reactions increases, which in turn promotes the formation of protective calcareous deposits on the alloy surface.⁷

Numerous studies have investigated the formation of calcareous deposits on the bare metals under cathodic polarisation in seawater. For example, Sarlak et al. demonstrated that both the substrate material and polarisation potential influence the composition of calcareous deposits. Specifically, aragonite was the exclusive component of deposits formed on copper across all potential ranges, whereas deposits on 316L stainless steel exhibited compositional variations with changing potentials.⁸ Sun et al. examined calcareous deposits on high-manganese aluminium bronze at polarisation potentials of −0.8 and −0.5 V, revealing that CaCO₃ was the primary constituent, with its structure becoming denser as the potential shifted more negatively.⁹ Li et al. reported that the Ca/Mg ratio in calcareous deposits varied with temperature under galvanostatic polarisation in natural seawater.¹⁰ Additionally, Du et al. observed that increased hydrostatic pressure reduced the Ca/Mg atomic molar ratio in calcareous deposits on carbon steel under cathodic protection in seawater.¹¹

In practical engineering applications, for ship condensers typically equipped with 70/30 cupronickel tubes, pre-treatment in seawater with a controlled flow rate is necessary prior to service to form a stable protective film on the tube surface, thereby improving the corrosion resistance of the 70/30 cupronickel alloy.¹² For 70/30 cupronickel tubes with a pre-formed stable protective film, limited research has focused on how cathodic protection affects their surface properties and corrosion resistance. Under cathodic polarisation, two competing processes – the reduction of the pre-formed stable protective film and the formation of calcareous deposit layers modulate the corrosion resistance of the 70/30 cupronickel tube. To address this issue, the present study investigates the corrosion behaviour of 70/30 cupronickel tubes with a pre-formed stable protective film under cathodic polarisation in natural seawater. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were utilised to characterise the surface morphology and chemical composition of the 70/30 cupronickel tube, while electrochemical impedance spectroscopy (EIS) and linear polarisation resistance (LPR) were employed to investigate the variations in their corrosion resistance.

Experimental section

Materials

The 70/30 cupronickel tube used in this study was supplied by Guilin Lijia Metals Co., Ltd, with an outer diameter of 16 mm and a wall thickness of 1.4 mm. The chemical composition of the alloy is listed in Table 1.

Table 1.

Chemical composition of the 70/30 cupronickel tube (wt-%).

Ni	Fe	Mn	Pb	S	C	Si	P	Zn	Sn	Cu
29.49	0.681	0.796	<0.005	0.001	0.020	<0.01	<0.005	0.012	<0.005	Bal.

Formation of a stable protective film

A 70/30 cupronickel tube (500 mm in length) was first sandblasted using 80-grit abrasive to remove surface oxides and contaminants, followed by erosion-corrosion testing in natural seawater with a custom-built pipe flow test apparatus. The testing procedure followed to China National Standard GB/T 43498–2023,¹³ and a schematic of the experimental setup is presented in Figure 1. Natural seawater, collected from the coastal waters of Qingdao, was circulated at a flow velocity of 2.5 m/s, which was selected to simulate the typical flow conditions in heat exchanger tubes. After pre-treatment in seawater with a regulated flow rate for 14 days, the tube was removed from the test apparatus, sectioned into smaller specimens and prepared for subsequent electrochemical measurements and surface characterisation. For electrochemical tests, each specimen was sealed with an organic silicone sealant, leaving only a 1 cm² area of the tube's inner wall exposed to serve as the working electrode (see Figure 2 for details).

Electrochemical tests

Electrochemical measurements were performed using a CS353 electrochemical workstation (Wuhan Corrtest Instrument Co., Ltd) with a conventional three-electrode cell configuration. The 70/30 cupronickel tube served as the working electrode, while a platinum electrode and a saturated calomel electrode (SCE) served as the counter electrode and the reference electrode, respectively. Prior to testing, the open circuit potential (OCP) was monitored for 60 min to ensure stabilisation. Subsequently, electrochemical impedance spectroscopy (EIS) measurements were performed at the stabilised OCP. The EIS tests adopted a sinusoidal perturbation amplitude of 10 mV (vs. OCP) and a frequency scanning range of 100 kHz to 10 mHz.

Only specimens with impedance values in the range of (1.0–1.1) × 10⁵ Ω·cm² (at 0.01 Hz) were selected for the subsequent cathodic polarisation tests, this screening process ensured that the pre-formed films on the surface of 70/30 cupronickel tubes exhibit stable protective performance, while reducing inter-sample variability.

Potentiostatic polarisation tests were carried out in natural seawater at four cathodic potentials: −350, −450, −550 and −750 mV (vs. SCE), with polarisation durations of 5 and 15 days, respectively. After completing the potentiostatic polarisation, the open circuit potential (OCP) of each specimen was monitored continuously for 30 min to confirm the restoration of electrochemical stability. This step is essential to prevent residual polarisation effects from interfering with subsequent measurements. Subsequently, post-polarisation electrochemical characterisations were performed in the following sequence: first, linear polarisation resistance (LPR) measurements were conducted, with a potential scan range of −10 to +10 mV (vs. the stabilised OCP) and a scan rate of 0.167 mV/s; then was followed by electrochemical impedance spectroscopy (EIS) measurements under the same stabilised OCP condition. To ensure the reproducibility of experimental results, triplicate specimens were tested. The electrochemical parameters were obtained by fitting the EIS data using ZSimpWin software.

Surface characterisation

Following the cathodic polarisation tests, the 70/30 cupronickel specimens were subjected to surface morphology observation and elemental composition analysis using a scanning electron microscope (SEM, model Ultra 55, ZEISS) equipped with an energy-dispersive spectroscope (EDS, model X-Max, Oxford Instruments). Phase composition analysis of the specimens was performed via X-ray diffraction (XRD, model Rigaku SmartLab). The XRD tests were conducted with a scanning rate of 10°·min⁻¹ (degree per minute) and a 2θ scan range of 10° to 90°.

Results and discussion

Analysis of pre-formed stable protective film

Figure 3 shows the EIS results of the 70/30 cupronickel specimens after pre-treatment in seawater for 14 days. The impedance modulus at 0.01 Hz is widely recognised as a key indicator for assessing the protective capability of corrosion product films,¹⁴ as it directly reflects the film's ability to inhibit mass/charge transfer during corrosion processes. Previous studies have reported that the impedance values of 70/30 cupronickel alloy in static seawater are typically below 10⁵ Ω·cm².^15,16 Notably, under the dynamic seawater conditions employed in this study (simulating heat exchanger service environments), the 70/30 cupronickel specimen achieved an impedance modulus of ∼10⁵ Ω·cm² after 14 days of erosion-corrosion test. EIS results indicate that a stable protective film has formed on the surface of 70/30 cupronickel tubes.

To further verify the film's structural and chemical characteristics, macroscopic morphology, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) were utilised to characterise the pre-formed protective film, with representative results shown in Figure 4. Macroscopic morphology revealed a continuous, tightly adherent film covered the entire surface of the 70/30 cupronickel specimen. SEM micrographs demonstrated no obvious cracks, voids, or delamination were detected in the film, indicating excellent structural integrity and morphological homogeneity. EDS elemental analysis confirmed that the protective film was predominantly composed of copper (Cu) and nickel (Ni) oxides and hydroxides. These findings confirm that a stable protective film was formed on 70/30 cupronickel tube after pre-treatment in seawater under a controlled flow rate.

Cathodic polarisation studies

The 70/30 cupronickel tubes with stable protective films were selected for cathodic polarisation tests. Figure 5 presents the potentiostatic polarisation curves of the 70/30 cupronickel specimens under different cathodic polarisation potentials. As shown in Figure 5(a), the polarisation current density exhibits distinct behaviours depending on the applied potential. While the current density generally increases with a more negative shift in polarisation potential, its time-dependent evolution varies significantly. For most applied potentials, the current density shows relatively stable or only slight variations over time. However, a notable exception is observed at −750 mV, where the current density decreases significantly as the polarisation time is prolonged. Figure 5(b) further illustrates this contrast by comparing the time-dependent evolution of the polarisation current density over the full 15-day period. After 15 days of polarisation at −750 mV, the current density stabilises at a low value of 4.26 μA/cm², which is even lower than that measured at −550 mV. This distinct behaviour at −750 mV indicates the deposition of a dense calcareous layer on the 70/30 cupronickel tube surface, which acts as a physical barrier to prevented dissolved oxygen and Cl⁻ ions in the solution from reaching the substrate, thus lowering the corrosion rate.^17,18

Figure 6 presents the relationship between linear polarisation resistance (LPR) and polarisation potential for 70/30 cupronickel specimens immersed in natural seawater. The results demonstrate that LPR increases progressively with a more negative shift in polarisation potential and prolonged polarisation time. Specifically, the highest LPR value is observed after 15 days of cathodic polarisation at −750 mV. To explain this trend, the electrochemical mechanism underlying cathodic polarisation is given. Under a fixed cathodic polarisation potential, the oxygen reduction reaction generates hydroxide ions (OH⁻). These OH⁻ ions locally elevate the surface pH, creating a favourable environment for the nucleation and growth of calcareous deposits (e.g., CaCO₃, Mg(OH)₂). As the polarisation time extends, these calcareous deposits gradually adhere to the alloy surface, forming a dense physical barrier that isolates the metal substrate from the corrosive seawater environment. This barrier not only reduces the direct contact between the alloy and electrolyte but also lowers the demand for cathodic protection current, since the film itself provides supplementary corrosion resistance. Once the calcareous deposit layer is fully developed and stabilised, the required protection current decreases significantly, and the alloy's corrosion resistance is further enhanced.⁷ Consistent with the polarisation current density results in Figure 5, the 70/30 cupronickel specimens subjected to 15 days of cathodic polarisation at −750 mV exhibited both the highest LPR and the low required protective current density. This result confirms that the formed calcareous deposit layer under −750 mV condition offers the good protective performance.

Figure 7 presents the electrochemical impedance spectroscopy (EIS) spectra of 70/30 cupronickel specimens following 15 days of potentiostatic polarisation in natural seawater. EIS results clearly demonstrate that the corrosion resistance of the specimens increase with a more negative shift in polarisation potential, with the maximum corrosion resistance achieved at −750 mV. This is directly evidenced by the largest capacitive arc radius in the Nyquist plot. In the corresponding Bode plots, quantitative differences in impedance further confirm this trend. The impedance modulus at 0.01 Hz for the specimen polarised at −750 mV is 2.33 × 10⁴ Ω·cm², which is nearly twice that of the specimen polarised at −350 mV (1.26 × 10⁴ Ω·cm²). This observation aligns with the well-established principle in corrosion electrochemistry that a higher low-frequency impedance modulus (|Z|) indicates a more effective surface film with superior barrier properties against corrosive species.¹⁹ Additionally, a notable trend is observed in the phase angle curves. The maximum phase angle shifts progressively towards lower frequencies as the cathodic polarisation potential becomes more negative. This frequency-dependent phase angle shift is typically attributed to the development of a thicker, more homogeneous calcareous deposit layer on the alloy surface.²⁰ It is noteworthy that although cathodic polarisation initially reduces the barrier performance of the pre-formed protective film by nearly one order of magnitude, it concurrently promotes the nucleation and growth of a calcareous deposit layer, which offsets the initially protective film degradation.²¹

Figure 8 depicts the equivalent electric circuit (EEC) used for fitting the EIS data, which was employed to extract a series of key electrochemical corrosion parameters. In the EEC shown, a constant phase element (CPE) is introduced to account for the inevitable surface heterogeneity of the electrode.²² R_s represents the solution resistance, wihile R_f and R_ct denote the film resistance and charge transfer resistance, respectively. Q_f is the CPE associated with the interface capacitance between the film and the electrolyte, and Q_dl is the double-layer capacitance. The extracted electrochemical parameters are summarised in Table 2. The quality of EIS fitting is evaluated using the Chi-square (χ²) value. Table 2 clearly illustrates that R_f of the 70/30 cupronickel tube gradually increases as the polarisation potential shifts more negative. This trend directly implies the growth and thickening of the calcareous deposit layer. Notably, The R_ct value follows a similar trend to the R_f value. It increases from 14,260 Ω·cm² at −350 mV to 44,790 Ω·cm² at −750 mV. This concurrent increase in R_f and R_ct further confirms that the growth rate of the calcareous deposits accelerates with a more negative polarisation potential. In addition to the resistance values, the CPE parameters provide further insights into the electrochemical processes. As shown in Table 2, the Q_dl increases from 1.30 × 10⁻⁴ Ω⁻¹·sⁿ·cm⁻² at −350 mV to 2.91 × 10⁻⁴ Ω⁻¹·sⁿ·cm⁻² at −750 mV, which indicates that the calcareous deposit layer may be changed with the negative shift of the potential.²³ Furthermore, the power index value of CPE-n (Table 2), which reﬂected the deviation from ideal capacitive behaviour (when n = 1, Q is the ideal capacitance; when 0 < n < 1, and ≠ 0.5, Q is a capacitor with dispersion effect).²⁴ The n_ct decreases from 0.67 at −350 mV to 0.61 at −550 mV before recovering to 0.79 at −750 mV. The lower n values in the range 0.6∼0.7 is associated with the influence of mass transport limitations, such as O₂ diffusion, in the electrochemical processes. The subsequent recovery of n_ct to 0.79 at −750 mV indicates that the calcareous deposit layer becomes more compact and uniform at the most cathodic potential, reducing the diffusion influence and restoring more ideal interfacial characteristics. Therefore, the enhanced corrosion resistance of the calcareous deposit layer is driven not only by the increase in thickness (reflected in R values) but also by improvements in morphological uniformity and density, as evidenced by the evolution of the CPE parameters.

Table 2.

EIS fitting parameters of 70/30 cupronickel specimens after polarisation under different cathodic polarisation potentials.

Potential (mV)	R_s (Ω·cm²)	R_f (Ω·cm²)	Y₀ (Q_f) (Ω⁻¹·sⁿ· cm⁻²)	n_f	R_ct (Ω·cm²)	Y₀ (Q_dl) (Ω⁻¹·sⁿ·cm⁻²)	n_ct	χ²
−350	15.05	336.9	1.58E-04	0.70	14,260	1.30E-04	0.67	0.001
−450	15.45	458.5	1.92E-04	0.80	17,600	1.55E-04	0.68	0.003
−550	18.56	590.7	2.82E-04	0.80	19,380	2.35E-04	0.61	0.002
−750	19.87	1743	1.59E-04	0.67	44,790	2.91E-04	0.79	0.001

Surface analysis

To investigate the effect of cathodic polarisation on the surface characteristics of the 70/30 cupronickel tube, comprehensive surface analyses were performed using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS) and X-ray diffraction (XRD). Figure 9 presents the SEM micrographs and corresponding EDS results of the 70/30 cupronickel tube after 5 days of cathodic polarisation at different potentials. When the polarisation potentials were set to −350 and −450 mV (Figures 9(a) and 9(c), respectively), the tube surfaces maintained their smooth morphology, with no distinguishable calcareous deposits observed. This observation was further confirmed by EDS analysis, which failed to detect signals corresponding to elements characteristic of calcareous deposits (e.g., Ca, Mg). In contrast, as the cathodic polarisation potential shifted to more negative values (Figure 9(e) and Figure 9(g)), the tube surfaces exhibited a noticeable increase in roughness, accompanied by the formation of sparsely distributed calcareous deposits. Notably, the calcareous deposits formed on the tube surface after 5 days of polarisation remained incomplete and non-uniform. EDS analysis revealed that calcium (Ca) was enriched in the calcareous deposits, with its content showing a clear upward trend with a negative shift of polarisation potential. The Ca content increased from 16.26 wt% at −550 mV to 21.58 wt% at −750 mV.

To further investigate the effect of polarisation time on the formation and evolution of calcareous deposits, the surface of the 70/30 cupronickel tube was characterised after an extended cathodic polarisation period of 15 days. As shown in Figure 10(a) and (c), when the polarisation potentials were set to −350 and −450 mV, the tube's surface morphology and chemical composition remained essentially unchanged even following 15 days of prolonged polarisation. This demonstrates that moderately negative potentials fail to induce substantial formation of calcareous deposits, even under extended polarisation conditions. In contrast, after 15 days of cathodic polarisation at more negative potentials (−550 and −750 mV), the entire surface of the 70/30 cupronickel tube was fully covered by a calcareous deposit layer. At these potentials, the calcareous deposits exhibited a cluster-like morphology, a feature consistent with the aragonite polymorph of CaCO₃, as reported in previous studies.^25,26 Notably, the tube surface polarised at −750 mV showed a higher coverage density of calcareous deposits than that polarised at −550 mV, indicating the formation of a dense calcareous deposit layer under more negative potentials.

EDS elemental analysis further confirmed the time-dependent growth of the calcareous deposits. The calcium (Ca) content in the calcareous deposits increased with prolonged polarisation time, reaching 34.55 wt% at −750 mV after 15 days. This value is substantially higher than the Ca content (21.58 wt%) measured at the same potential (−750 mV) after only 5 days of polarisation. During cathodic polarisation, the oxygen reduction reaction at the alloy surface generates hydroxide ions (OH⁻), which locally elevates the pH of the seawater adjacent to the electrode. This increased pH alters the carbonate equilibrium in seawater, promoting the precipitation of calcareous deposits through reactions involving dissolved carbonate (CO₃²⁻), bicarbonate (HCO₃⁻), and magnesium ions (Mg²⁺).⁷ Prolonged polarisation at more negative potentials sustains this high-pH environment, allowing for continuous growth and densification of the calcareous deposit layer.

{OH}^{-} + {HCO}_{3}^{-} \to H_{2} O + {CO}_{3}^{-}

(1)

{CO}_{3}^{-} + {Ca}^{2 +} \to {CaCO}_{3} ↓

(2)

{Mg}^{2 +} 2 {OH}^{-} \to Mg (OH)_{2} ↓

(3)

Previous studies have reported that the precipitation of calcium carbonate in aqueous environments typically requires a pH of approximately 8.7,²⁷ whereas the critical pH for magnesium hydroxide precipitation is above 9.5.^28,29 They also demonstrated that low cathodic current densities favour the formation of calcium-based salts (e.g., CaCO₃), while higher current densities promote the precipitation of magnesium-based salts (e.g., Mg(OH)₂).⁸ In the present study, no Mg(OH)₂ was detected on the surface of the 70/30 cupronickel tube following cathodic polarisation at −550 and −750 mV in natural seawater. This observation suggests that the alloy-seawater interfacial pH is inadequate to induce the nucleation and precipitation of Mg(OH)₂. Combined with the electrochemical test results, the 70/30 cupronickel tube exhibited a cathodic current density below 5 μA/cm² after 15 days of polarisation at potentials of −550 and −750 mV (vs. SCE). This relatively low cathodic current density also accounts for the fact that the deposits formed on the surface of the 70/30 cupronickel alloy are composed of calcium carbonate.

X-ray diffraction (XRD) was employed to determine the phase composition of the calcareous deposits formed on the 70/30 cupronickel alloy after 15 days of cathodic polarisation. As shown in Figure 11, when the polarisation potentials were set to −350 and −450 mV (vs. SCE), only diffraction peaks corresponding to metallic copper (Cu) were detected. These peaks originate from the 70/30 cupronickel alloy substrate, consistent with previous reports.⁹ In contrast, at more negative polarisation potentials (−550 and −750 mV), distinct and intense diffraction peaks attributed to aragonite emerged. These XRD results are in agreement with the earlier energy-dispersive spectroscopy (EDS) findings, providing conclusive evidence that the calcareous deposits formed at these potentials are composed exclusively of aragonite. As the polarisation potential shifted further negative, the diffraction peaks of aragonite became intense, which is consistent with the SEM observations of dense calcareous deposits. According to literature reports, aragonite provides superior surface coverage on metals, effectively inhibiting oxygen diffusion and reducing corrosion rates.¹⁷

Conclusions

In this study, we investigated the influence of cathodic polarisation on the microstructure and corrosion resistance of 70/30 cupronickel alloy with a pre-formed stable protective film. The main conclusions are as follows.

Electrochemical results revealed distinct time-dependent trends in cathodic current density. Within the polarisation potential range of −350 to −750 mV, the current density increases as the polarisation potential shifts to more negative values after 5 days of polarisation. When the polarisation duration is extended to 15 days, the current density at −750 mV decreases to a value lower than that recorded at −550 mV.

Cathodic polarisation was found to reduce the barrier properties of the pre-formed stable protective film by nearly one order of magnitude. With the negative shift of polarisation potential and the increase of polarisation time, the calcium deposition layer gradually forms and becomes denser. The initially decreased impedance value due to cathodic polarisation gradually increases, and the impedance value increases with the negative shift of potential.

Surface characterisation results showed a clear potential-dependent pattern in calcareous deposit formation. No calcareous deposits were detected on the tube surface after polarisation at −350 and −450 mV. In contrast, at more negative potentials (−550 and −750 mV), the 70/30 cupronickel tube surface was covered with calcareous deposits. XRD and EDS analyses confirmed that these deposits predominantly consisted of aragonite, a polymorph of calcium carbonate (CaCO₃). Additionally, the calcium content in the calcareous deposits increased with both the negative shift in polarisation potential and the extension of polarisation time.

Footnotes

Acknowledgements

The authors are grateful to the Luoyang Ship Material Research Institute for the financial support for this research work and permission to publish this paper.

ORCID iD

Xuehui Liu

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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