The Influence of Rust Layers on Calcareous Deposits’ Performance and Protection Current Density in the Cathodic Protection Process

Abstract

Calcareous deposits are a consequential outcome of cathodic protection in marine environments, exerting significant influence on the cathodic protection process and current density prerequisites. This study investigates the process of calcium deposition and its impact on the cathodic protection current density of carbon steel under the influence of a rust layer in different corrosion periods. This was investigated using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The results demonstrate that the formation processes of calcareous deposits vary after exposure to the corrosive environment for 0, 7, and 30 days. While a longer corrosion period leads to thicker rust layers on the metal surface and a higher initial cathodic protection current, the presence of these rust layers facilitates the deposition of calcium and magnesium ions, resulting in a rapid decrease in cathodic protection current density after a certain period. Meanwhile, long-term cathodic protection facilitates the thickening and densification of the oxide layer, thereby enhancing its protective efficacy, effectively reducing the corrosion rate of the metal surface and stabilizing the cathodic protection current density at a lower level. This study provides theoretical data and experimental evidence to support the maintenance of corroded marine engineering equipment.

1. Introduction

The technique of cathodic protection is widely employed for corrosion prevention in current practice [1,2,3]. As early as 1948, Humble published a pioneering study on the application of constant current to provide protection for low-carbon steel [4]. The formation of calcareous deposits is commonly attributed to cathodic protection as a byproduct, yet they significantly contribute to the effectiveness of cathodic protection [5,6]. Firstly, the presence of calcareous deposits can impede the penetration of oxygen, thereby hindering the rate of oxygen reduction reaction at the cathode. This leads to a decrease in the required protection current density and allows for polarization and maintenance of the steel structure within the desired protection potential range using a relatively low current density. Consequently, this enhances the economic viability of cathodic protection [7]. Secondly, calcareous deposits can also play a role in the case of insufficient or interrupted cathodic protection, enhancing the durability of cathodic protection. Lastly, calcareous deposits can make the distribution of cathodic protection potential more uniform, thus enhancing the stability of cathodic protection.

Under neutral and alkaline conditions in the ocean, there are two cathodic reactions corresponding to anodic metal dissolution reactions [8]. When the potential is below −950 mV (vs. SCE), the primary cathodic reaction involves the reduction of dissolved oxygen (Equation (1)). Conversely, when the potential exceeds −1100 mV (vs. SCE), the main cathodic reaction involves the hydrogen evolution reaction, where water is reduced to produce hydrogen gas (Equation (2)).

$${\mathrm{O}}_2+2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}\to 4{\mathrm{OH}}^{-}$$

(1)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2+2\mathrm{O}{\mathrm{H}}^{-}$$

(2)

Both reactions produce hydroxide ions, which increase the pH value of the electrolyte near the metal surface [9,10], leading to the precipitation of insoluble salts, namely CaCO₃ and Mg (OH)₂:

$${\mathrm{OH}}^{-}+{\mathrm{HCO}}_3^{-}\to {\mathrm{CO}}_3^{2-}+{\mathrm{H}}_2\mathrm{O}$$

(3)

$${\mathrm{Mg}}^{2+}+2{\mathrm{OH}}^{-}\to {\mathrm{Mg}\left(\mathrm{OH}\right)}_2$$

(4)

$$\mathrm{C}{\mathrm{a}}^{2+}+\mathrm{C}{\mathrm{O}}_3^{2-}\to \mathrm{CaC}{\mathrm{O}}_3$$

(5)

The above reactions together lead to the formation of a calcium deposition layer on the steel surface.

Currently, extensive research has been conducted on the formation mechanism of calcareous sedimentary layers and the factors influencing their growth. However, relevant research mainly focuses on the effects of temperature [5,11,12], pH [13,14], dissolved oxygen [15], flow rate [16,17,18], and other factors on the electrochemical properties of calcium deposition layers. Limited research has been conducted on the impact of rust layers on metal surfaces in relation to calcium deposition layers. Although W.H. Hartt [19] investigated the effects of rust on current density trends and cathodic protection, their research remained focused on the electrochemical data of current density changes without delving into the growth mechanism of rust layers in calcium deposition layers, and lacked evidence from other images and data; P. Refait conducted a six-year experiment to study the changes in the composition of a rust layer under cathodic protection [20], but they did not mention the impact of the rust layer on the calcium deposition layer, which plays an important role in cathodic protection. In fact, engineering equipment such as offshore platform jackets are already in a corrosive environment before service and have corrosion products generated on the surface [21,22]. So, what is the impact of the rust layer on the current density required for cathodic protection, and how does the rust layer affect the formation and growth of calcium deposition layers? These are all worthy of attention.

The objective of this article is to investigate the impact of rust layers on the formation, growth, and electrochemical properties of calcium deposition layers during the cathodic protection process. Based on previous research, we selected Q235 steel as the experimental material and subjected it to corrosion conditions for 0, 7, and 30 days before conducting a 90-day static seawater experiment under sacrificial anode cathodic protection. The degree of metal protection was studied by recording the changes in the potential and current density of the metal under cathodic conditions. Scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) were used to observe the surface morphology and elemental distribution of the calcium deposition layers under different rust layer conditions, and electrochemical impedance spectroscopy (EIS) was used to analyze the electrochemical properties of the calcium deposition layers under different corrosion states. The present article unveils the impact and mechanism of rust layers on the performance of calcium deposition layers from diverse perspectives through the utilization and analysis of the aforementioned techniques.

2. Experimental Section

2.1. Materials

In this experiment, the sacrificial anode material size was a round bar measuring Φ10 × 50 mm. The wire was soldered after drilling at the top, and the connection was sealed with silica resin once the digital multimeter confirmed that the sacrificial anode was properly connected to the wire. The composition of the sacrificial anodes is listed in

Table 1. Composition of sacrificial anodes.

Elements	Content (wt.%)
Al	94.5000
Zn	5.3100
In	0.0265
Si	0.0872
Fe	0.0467
Ca	0.0107
Other	0.0189

.

The experimental object was Q235 carbon steel with a working surface length of 1 cm. The surface of the Q235 carbon steel was polished step-by-step with 180~1000# sandpaper, and then moistened with alcohol and set aside after drying. The electrolyte solution was natural seawater (taken from Dalangwan, Jiangmen, China), and its conductivity was 49 mS/cm.

In this study, the experimental temperature was set to 25 °C. Initially, the Q235 carbon steel electrode was immersed in the electrolyte solution for durations of 0 days, 7 days, and 30 days, respectively, to induce varying degrees of corrosion on its surface. The sacrificial anode was subsequently connected to the Q235 carbon steel electrodes that had been immersed in the electrolyte solution for varying durations, resulting in the gradual formation of a calcareous deposit layer on their surfaces. The two subjects were both exposed to a 90-day immersion experiment in the electrolyte solution. The experiments were conducted in triplicate under identical conditions for each group, and the average results of each group were obtained.

2.2. Electrochemical Impedance Spectroscopy (EIS) Tests

EIS was tested using a Gamry electrochemical workstation. The standard three-electrode system was adopted. The working electrode was a carbon steel sample, the counter electrode was a platinum electrode, and the reference electrode was a saturated calomel electrode (SCE). The amplitude of the sine wave signal applied in the test was ±10 mV, and the frequency range was 10⁵~10^–2 Hz. The electrochemical impedance data obtained from the test were processed and analyzed by ZSimpWin 3.60 impedance analysis software.

2.3. SEM-EDS Tests

The surface morphology and composition of the sample were characterized using scanning electrochemical electron microscope (SEM, JEOL JSM-IT200A, Tokyo, Japan) and energy-dispersive X-ray spectrometer (EDS, JEOL JSM-IT200A series, Tokyo, Japan).

3. Results and Discussion

3.1. Variation of Cathodic Protection Current Density with Time

Figure 1 shows the change in cathodic protection current density with time under three corrosion conditions. For the corroded samples from 30 days, 7 days, and 0 days, the cathodic protection current density at the moment when the sacrificial anode was turned on reached 1800 mA/m², 720 mA/m², and 400 mA/m². Subsequently, the three groups of samples showed different current density changes, in which the current density of corrosion for 0 days kept a large value in the first five days after the start of polarization. The passage of time led to the gradual deposition of calcareous deposits on the sample surface, thereby impeding the diffusion of dissolved oxygen and diminishing the rate of cathodic reduction reactions. Therefore, the current density drops rapidly during 5~15 days after polarization. After 30 days, the current density tended to stabilize gradually, and the current density reached 1~2 mA/m² at the end of the 90-day experiment. The calcareous deposit currently offers adequate metal protection, but its compactness cannot rival that of organic coatings. Therefore, the use of a sacrificial anode is necessary to provide a current for protection. The slope of the current density curve suggests that in the initial stage, there is a relatively significant slope, indicating that the deposition of calcareous deposits provides noticeable protection for metals during the polarization initiation. However, as time progresses, the slope decreases, implying that the protective effect of these deposits does not exhibit a linear relationship. Therefore, the initial stage of cathodic protection for metals is very important, and the earlier a better calcareous deposit is formed, the earlier and better protection can be provided for the metal matrix.

The initial current density of the group corroded for 7 days was about 720 mA/m², which is nearly twice that of the group corroded for 0 days, which is consistent with the observed situation in the experiment. After 7 days of corrosion, despite the presence of some corrosion products on the metal surface, it failed to offer comparable protection to the metal substrate due to its inherent porosity and limited thickness. The corroded metal sample, therefore, needs to act as the sacrificial anode in order to provide a higher protection current and reduce its potential to fall within the protection potential range. The current density curve exhibited a similar trend to that of the 0-day corrosion group as the polarization time was prolonged. During the initial 15 days, there was a rapid decrease in current density. After 25 days, the calcareous deposit completely covered the metal surface and provided good protection for the metal, so the current density dropped below 15 mA/m² after 90 days.

Different from the other two groups, the current density of the samples corroded for 30 days at the beginning of polarization reached 1800 mA/m², which is much larger than that of the samples corroded for 0 days and 7 days. After that, the trend of the current density curve is basically the same as that of the other two groups; it also decreases with a steep slope in the first 10 days and the decreasing speed gradually slows down in 10–20 days. After 20 days, the curve changes in a small range, and the value finally drops to about 30 mA/m².

The cathodic protection current density of the three samples generally follows a similar trend, peaking after connecting to the sacrificial anode and subsequently experiencing a significant decrease within the following 10 days. During the next 10–25 days, although the current density is still decreasing, the decrease rate is much lower than that in the previous 10 days. Although the current density value fluctuates in 25–90 days, the trend of slow decline remains unchanged. Figure 1b and

Table 2. The changes in the ratio of cathodic protection current density to initial cathodic protection current density under different corrosion conditions.

	200 mA/m2	100 mA/m2	50 mA/m2
0	10	22	49
7 d	9	14	36
30 d	9	17	56

show the change in the ratio of current density to initial current density at different time points. This figure clearly reflects that the current density values of the three groups decreased in the first 70 days with the following order: the 30-day corrosion group, >7-day corrosion group, and >0-day corrosion group. At the end of the 90-day experiment, the final i/i₀ values of the 7-day corrosion group and the 30-day corrosion group remained at 0.02, while the i/i₀ values of the 0-day corrosion group were all lower than those of the other two groups after the 90-day experiment.

3.2. Comparison of Surface Morphology, Structure and Composition

In order to investigate the coverage, morphology, and elemental distribution of calcareous deposits on the surface of steel blocks, high-resolution photographs were captured before and after 90 days of cathodic protection treatment. Subsequently, the calcareous deposits from the three systems were characterized using SEM and EDS.

The corrosion test specimens before cathodic protection were vertically suspended in seawater with the upper part closer to the air/seawater interface, resulting in a higher oxygen concentration, while the lower part of the samples had a lower oxygen concentration compared to the upper part. The difference in oxygen concentrations formed an oxygen concentration cell, with the upper part of the specimen near the air/seawater interface as the cathodic area, and the lower part as the anodic area (as shown in Figure 2b,c). In the cathodic area, an oxygen reduction reaction occurred, while in the anodic area, a dissolution reaction of iron took place, leading to the formation of a rust layer primarily in the anodic region [23]. After cathodic protection was applied by connecting the sacrificial anode, the calcium deposition layer was directly deposited on the metal surface since there was no rust layer present in the cathodic area. In the anodic area, the inner layer consisted of ferrous sulfate green rust (Fe⁻²₄Fe⁻³₂(OH)₁₂SO₄·8H₂O (GR(SO₄²⁻)) and FeOOH, partially due to the reduction product Fe₃O₄ that resulted from the presence of more rust layers, while the outer layer mainly consisted of FeOOH [24] with a small amount of GR(SO₄²⁻). FeOOH underwent a reduction reaction, forming a denser Fe₃O₄. After connecting the sacrificial anode for cathodic protection, the initial difference in current density was because the more extensive the reduction reactions in the rust layer, the larger the cathodic current needed. This led to the inner layer not only containing GR(SO₄²⁻) and FeOOH, but also the reduction product Fe₃O₄ [25]. The rust layer provided a certain degree of protection to the substrate, while the reduction product Fe₃O₄ was denser compared to FeOOH and GR(SO₄²⁻). The rust layer in the early stages of cathodic protection partially hindered the occurrence of corrosion reactions, resulting in a decline in current density. Consequently, the group subjected to 30 days of rust corrosion exhibited the most rapid decrease, followed by the group exposed to 7 days of rust corrosion.

Figure 3 shows images of the three groups of samples after 90 days of cathodic protection. The calcium–magnesium deposition layer formed on the surface of the metal without corrosion is the densest. However, the density of the calcium–magnesium deposition layer on the surface of the 7-day corrosion sample is poor. Black metal substrate and a yellow rust layer can even be seen. The samples of the 30-day corrosion group have the worst degree of protection, with large pieces of the rust layer falling off. After the rust layer falls off, the exposed metal surface has locally excessive cathodic protection current density. Therefore, the OH⁻ concentration is higher in those areas, which can lead to the formation of a relatively good calcium–magnesium deposition layer. However, this may make it difficult for other areas to meet the conditions for deposition. Consequently, the calcium–magnesium deposition layer formed on the surface of the rust layer is very rare. Therefore, the magnitude of the current density in the last 10 days in Figure 1 is consistent with the macroscopic appearance.

From Figure 4, it can be seen that after 90 days of cathodic protection, the calcium deposition layer of the specimens with 0 days of corrosion exhibits the highest density, and the interior calcite crystals have essentially grown completely. The thickness of the deposition layers for the three groups are 425 μm, 250 μm, and 300 μm, respectively. The thicker structure in the group with 0 days of corrosion is identified as a crucial factor in protecting the metal substrate from corrosion. In Figure 4b, only the distribution of Ca and Mg elements is detected, with no presence of Fe elements, indicating that corrosion reactions did not occur. Compared to other samples, the proportion of Ca elements in the calcium deposition layer of the specimens with 0 days of corrosion is the highest. Since the Ca/Mg ratio is an important evaluation criterion for the quality of calcium–magnesium deposition layers, this suggests that the specimens with 0 days of corrosion, under the action of no corrosion products, can form higher-quality calcium–magnesium deposition layers, thereby better protecting the metal substrate. Therefore, at the end of the 90-day experiment, a cathodic protection current density of 1–2 mA/m² is sufficient to maintain the protected metal in a cathodic protection state.

The samples corroded for 7 days and those rusted for 30 days exhibit similar characteristics. Figure 4d,f depict primarily Fe-containing rust layers in contact with the metal substrate at the bottom, while the middle-to-upper parts consist of calcium–magnesium layers. Combining the results from Figure 2 and Figure 3, it can be inferred that the original rust layer adhering to the metal surface after corrosion is reduced to Fe₃O₄, forming a thinner inner rust layer. The relatively thicker Fe layer observed in the EDS spectra is attributed to the outer rust layer. Additionally, it is evident from the EDS spectra that regions rich in Fe elements also contain significant amounts of Ca and Mg elements. This proves that the four ions—Ca²⁺, Mg²⁺, CO₃²⁻, and OH⁻—involved in forming the calcium–magnesium deposition layer can enter the metal surface through larger pores in the rust layer to deposit and form the calcium deposition layer [26]. Furthermore, the deposition layer gradually densifies the rust layer through surface deposition, thereby enhancing the protection capability of the metal substrate. Therefore, in the early stages of cathodic protection, calcium and magnesium ions deposit around rust grain centers, filling the internal pores of the relatively loose and porous rust layer, thus forming a more protective mixture of a calcium–magnesium deposition layer and a rust layer. Despite the higher initial current density in the early-stage rusting group, the rapid formation of a protective layer of a certain thickness led to a swift reduction in the required cathodic protection current density.

From the EDS image in Figure 4b, it can also be observed that the calcium layer and magnesium layer in the calcium–magnesium deposition layer are not deposited in the same plane, but rather exhibit an intertwined growth state. The reason for this phenomenon may be the formation of small pores connecting the metal matrix surface during the growth of CaCO₃. When O₂ passes through the reduction reaction to form OH⁻, due to the difficulty of diffusion, the OH- concentration inside the pores increases, promoting the nucleation and growth of Mg(OH)₂. After Mg(OH)₂ deposition reaches a certain degree, the pH value decreases, which favors the deposition of CaCO₃. The two deposition reactions proceed repeatedly, resulting in the occurrence of a small amount of Mg(OH)₂ interspersed in the calcium deposition layer [26].

3.3. The Variation of Electrochemical Impedance Spectroscopy Over Time

The Nyquist diagram of the impedance spectrum is depicted in Figure 5. Figure 5a shows the Nyquist diagram of the 0-day corrosion group. In the initial stage, the impedance spectrum exhibits a semi-arc shape, reflecting the electrochemical process of base metal corrosion while also indicating deposits on the metal surface. However, these calcium and magnesium deposits exhibit relatively low impedance values that do not independently manifest another characteristic time constant. As time progresses, a small capacitive reactance arc gradually emerges in the high-frequency range of the impedance spectrum, with its radius expanding proportionally to elapsed time. The Nyquist diagrams of the 7-day and 30-day corrosion groups are illustrated in Figure 5b,c. In the initial stage, the impedance spectrum exhibits two time constants due to the presence of rust. Over time, a distinct diffusion tail appears at 55 days for the former and at 42 days for the latter, which can be attributed to either the challenging diffusion of dissolved oxygen through the dense sediment layer to reach the substrate metal surface or the outward diffusion process of OH⁻ ions, which are reduction reaction products [27].

The impedance of different samples during the cathodic protection processes was fitted using the circuit diagram shown in Figure 6. For samples where the calcium deposition layer had not yet formed on the surface, the equivalent circuit diagram shown in Figure 6a was employed for fitting, including the solution resistance R_s, constant phase element CPE₁, and charge transfer resistance R_ct. When the metal surface was fully covered by the calcium deposition layer, the circuit included an additional calcium deposition layer resistance R_f and constant phase element CPE₂, as depicted in Figure 6b. As the deposition layer reached a certain thickness, diffusion limitations led to the addition of a Warburg impedance W_c in the metal interface circuit, as shown in Figure 6c.

The analysis of equivalent circuit components in different corrosion states, as shown in Figure 7 and Figure 8, indicates that with the passage of time, both the resistance R_f of the calcium deposition layer and the charge transfer resistance R_ct exhibit an overall increasing trend. Additionally, in all three corrosion states, the R_f and R_ct values of the samples in the 0-day corrosion group are the highest, while the difference between the R_f and R_ct values of the samples in the 7-day corrosion group and the 30-day rust group is not significant. The presence of rust facilitates the nucleation and growth of a calcium deposition layer on its surface. As the deposition layer gradually fills the pores in the rust layer, it becomes increasingly challenging for ions to diffuse into the interior of the rust layer for further deposition. Moreover, the density of the rust layer itself cannot compare to that of the calcium deposition layer, resulting in inferior metal protection. Consequently, there is a gradual increase in resistance values for both the 7-day and 30-day rust groups.

In Figure 7, the R_f values of the three samples gradually increase with time, indicating that regardless of the corrosion state, the calcium deposition layer gradually accumulates at the metal interface, increasing its thickness and enhancing its protective ability on the metal substrate [27]. The R_f value of the 30-day corrosion group is initially larger than the other two groups in the early days of cathodic protection, demonstrating that the thicker rust layer mentioned above can provide a “rudimentary barrier” for the metal substrate in the initial stage. The idea that the calcium–magnesium deposition layer can quickly nucleate and deposit with the help of rust grains is correct. However, as the duration of cathodic protection increases, the protective performance of the pure calcium–magnesium deposition layer gradually improves. The R_f value of the 0-day group increases rapidly, and at 90 days, its R_f value is three times that of the others, demonstrating that the growth status and protective ability of the calcium–magnesium deposition layer on the surface of samples without a rust layer are significantly superior to other samples. The cathodic protection current density curve reveals that the 30-day rust group demonstrates the most significant decrease, while the 0-day rust group exhibits the least substantial decrease. However, in the later stages of the experiment, it is observed that the 0-day rust group possesses the lowest protective current density, whereas the 30-day rust group displays the highest.

From the impedance fitting results in

Table 3. The fitting values of R_f, R_ct, and CPE₁ for the sample with 0 days of corrosion.

Time/d			1	2	3	4	5	8	9
Rf/Ω·cm2			-	-	-	20	17	40	50
Rct/kΩ·cm2			13.8	12.8	12	11.9	12.3	14.2	16
CPE1/μF·cm−2			230	247	255	266	271	270	266
Time/d	15	26	42	55	62	67	74	83	90
Rf/Ω·cm2	354	538	743	680	924	909	1191	1051	1426
Rct/kΩ·cm2	16.7	32	43.8	40.8	60	81.5	73.6	58	102
CPE1/μF·cm−2	376	200	190	410	345	315	350	400	290

,

Table 4. The fitting values of R_f, Rct, and CPE₁ for the sample with 7 days of corrosion.

Time/h		1	2		3	5	15		26
Rf/Ω·cm2		-	-		-	-	100		302
Rct/kΩ·cm2		1.2	2.9		8.6	8.9	28.5		16
CPE1/μF·cm−2		700	1100		580	290	336		280
Time/h	34	42	48	55	62	68	77	83	90
Rf/Ω·cm2	335	359	332	418	391	448	473	491	490
Rct/kΩ·cm2	15.5	18	17.8	21	18.3	17.6	15.6	20.1	18
CPE1/μF·cm−2	330	360	340	220	170	280	150	200	144

and

Table 5. The fitting values of R_f, Rct, and CPE₁ for the sample with 30 days of corrosion.

Time/d	1	2	3	4		5		6	7	8	9
Rf/Ω·cm2	45	55	56	75		80		107	94	113	165
Rct/kΩ·cm2	27.6	14.7	19.5	29.3		12.3		32	36.8	40	47.9
CPE1/μF·cm−2	725	714	676	572		537		520	526	483	445
Time/d	15	26	34	42	48		55	62	77	83	90
Rf/Ω·cm2	193	211	247	342	388		485	471	616	556	545
Rct/kΩ·cm2	50.6	54.9	24	30	34		30	28	29	19.3	30
CPE1/μF·cm−2	391	380	380	240	300		280	260	280	330	427

, it can be observed that the resistance value R_f of the calcium deposition layer contributes less than 1% to the system impedance. Therefore, besides analyzing the R_f value, the variation of R_ct should also be discussed.

In Figure 8, it can be observed that the R_ct values of the 7-day rust group and the 30-day rust group experienced significant decreases at 15 and 19 days, respectively. There were also slight decreasing trends at other time points. The reason for this phenomenon can be analyzed from the perspective of rust layer composition. When steels are under cathodic protection, both oxygen reduction reactions and hydrogen evolution reactions occur simultaneously. The release of hydrogen is a significant detriment to loosely adhered rust layers. Referring to Figure 3, it can be inferred that the detachment of rust layers is relatively easy. Therefore, when large rust layers detach, the protection of the metal sharply decreases. This leads to an accelerated reaction rate at the detachment points of the rust layer, resulting in a significant decrease in R_ct. Small declines at other time points may be attributed to reduction reactions of the rust layer. When steel specimens start to corrode, the primary corrosion product formed on their surface is GR(SO₄²⁻), which is one of the main corrosion products of carbon steel in seawater [20,28,29]. When oxygen is abundant, GR(SO₄²⁻) will oxidize to form the yellow corrosion product—goethite (γ-FeOOH). Therefore, γ-FeOOH mainly appears in the outer layer of the rust layer. Its crystal structure is orthorhombic. Thermodynamically, γ-FeOOH is a metastable crystal form. Reduction reactions begin to occur at around −760 mV (vs. SCE) [30]. As the corrosion reaction continues and the surface potential of the metal gradually shifts positively, the rust layer divides into an outer rust layer and an inner rust layer. The composition of the outer rust layer remains as γ-FeOOH produced during the initial stages of corrosion. However, besides GR(SO42-) and γ-FeOOH, the inner rust layer also contains lepidocrocite (α-FeOOH), maghemite (β-FeOOH), and magnetite (Fe₃O₄). Lepidocrocite (α-FeOOH) also possesses an orthorhombic crystal structure [31]. However, it is formed from GR(SO₄²⁻) under conditions with relatively low oxygen availability. Moreover, thermodynamically, α-FeOOH is considered a stable form of FeOOH [32,33,34]. It undergoes reduction at around −900 mV (vs. SCE) [35]. On the other hand, β-FeOOH, compared to the former two, is a more reactive crystal form. It has a tetragonal-crystal system structure and exhibits higher electrochemical activity, initiating reduction at around −690 mV (vs. SCE). In summary, throughout the entire process of cathodic protection, various substances within the rust layer can undergo reduction reactions. Moreover, as the calcium deposit layer gradually thickens and covers the surface of the rust layer, making it difficult for oxygen to penetrate and initiate oxygen reduction reactions on the metal surface, the proportion of reduction within the metal surface rust layer gradually increases. The resulting cathodic current will lead to a decrease in R_ct.

The electrochemical activity and reduction efficiency of different FeOOH compounds can be ranked as follows: β-FeOOH > γ-FeOOH > α-FeOOH. Among these, β-FeOOH is the most readily reducible [36,37]. The generation of corrosion products leads to an increase in the thickness of the inner rust layer, accompanied by a higher concentration of FeOOH. After 30 days of corrosion, the inner rust layer formed is thicker compared to that after 7 days. With more FeOOH present inside, more reduction reactions can occur. Consequently, the decrease in R_ct for the samples corroded for 30 days, as depicted in Figure 8, is greater than that for the samples corroded for 7 days. Among the three components within the rust layer, only Fe₃O₄ exhibits good conductivity, while the other two are nearly non-conductive. Therefore, oxygen does not undergo reduction reactions at the outer rust layer but reaches the surface of the metal or the Fe₃O₄ within the inner rust layer to accept electrons released from the metal dissolution reaction, leading to the generation of OH⁻. As a result, the pH at the metal surface and the Fe₃O₄ location is relatively high. Hence, calcium deposits preferentially nucleate and grow on the surfaces of these two entities. During the growth process, the calcium deposition layer exerts two effects on the relatively loose outer rust layer—coverage and displacement. These actions scatter the rust layer within the calcium deposition layer. As the deposition layer gradually thickens, the originally oxygen-accessible rust layer becomes gradually deprived of oxygen. This condition favors the transformation of FeOOH and GR(SO₄²⁻) into Fe₃O₄. At this point, the outer rust layer contains materials with good conductivity. Seawater can penetrate from the pores of the calcium deposition layer into the areas containing Fe₃O₄. The conductivity of Fe₃O₄ allows oxygen to undergo reduction reactions without reaching the metal surface, thus accepting the electrons generated by the metal dissolution reaction. Consequently, the cathodic current generated by oxygen reduction, in conjunction with the slowly increasing calcium deposition layer on the surface, causes the R_ct value of the rust-bearing group to exhibit a slight increase in stability rather than maintaining a significantly larger growth like the rust-free group with 0 days of corrosion.

In the fitted data, the constant phase element CPE₁ helps explain why there is a significant difference in cathodic protection current density during the initial stages of cathodic protection. A comparison between Table 3 and Table 5 reveals that the rust layer with a larger specific surface area results in a ranking of CPE₁ values as follows: 30-day rust group > 7-day rust group > 0-day rust group. The larger the constant phase element value, the greater the current required to charge it until it reaches full capacity. Consequently, upon completion of double-layer charging, the experimental group with the highest constant phase element value exhibits the most significant decrease in current.

4. Conclusions

During the cathodic protection process, the rust layer has both advantages and disadvantages for cathodic protection and the deposition growth of calcium layers. In the initial stages of cathodic protection, the rust layer with a larger specific surface area increases the constant phase element value, necessitating a higher current to charge it. Consequently, a sacrificial anode with a higher voltage is required for cathodic protection, placing greater demand on the sacrificial anode.

From the perspective of the early-to-middle stages of cathodic protection, the rust layer also provides certain protective effects. It facilitates the rapid deposition of calcium layers around its core, thereby facilitating easier and faster pore filling in the rust layer. This results in a prompt enhancement of metal substrate protection.

In the later stages of the experiment, the rust layer is susceptible to detachment, leading to more severe reactions on the exposed steel surface. Compared to calcium–magnesium deposition layers, the porous nature of the rust layer results in inferior protection for the deposition layer mixture. Under long-term cathodic protection, pure calcium magnesium deposition layers offer optimal protection for the metal substrate.