Effect of Surface Roughness on Corrosion Resistance of Mooring Chains for Offshore Floating Photovoltaics

Abstract

Mooring chains are key components of offshore floating photovoltaic systems. Although their service safety is often affected by the harsh service environment, the influence of surface roughness on their corrosion resistance is not clear. This study investigated the corrosion behavior of mooring chain steel using cyclic salt-spray corrosion and electrochemical tests. Scanning electron microscopy, energy-dispersive spectrometry, optical profilometry, and other analytical techniques were used to study the composition and morphology of the corrosion products. The corrosion behavior was studied by electrochemical polarization curves, alternating current impedance spectroscopy, and X-ray photoelectron spectroscopy. The results show that the salt-spray corrosion resistance of mooring chain steel significantly improved with the reduction in specimen surface roughness, and the number and depth of corrosion pits were reduced. Mass loss after 24 h of salt-spray corrosion was exponentially related to initial roughness ($\mathrm{Ra}$). Improved surface roughness significantly increased the pitting potential of the specimens, widened the passivation range, and enhanced the repassivation capability, thus significantly improving the pitting resistance. The pitting potential is linearly related to the initial roughness of the specimen. The oxide contents of Fe, Mo, and Si in the passivation film tended to increase with a smoother surface, which contributes to its densification. This effectively blocks chloride ion attack, thus improving the corrosion resistance of the mooring chain steel.

1. Introduction

The development and utilization of renewable energy sources have become particularly important in response to global carbon-neutral initiatives [1]. Among many renewable energy solutions, offshore floating photovoltaic (FPV) systems have attracted much attention due to their potential to produce green and clean energy on a large scale [2]. However, the harsh marine environment poses a serious challenge to the corrosion resistance of the mooring chains used in FPV systems.

Mooring chains are key components to ensure the stability and precise positioning of offshore PV platforms [3]. The chain is usually made of high-strength alloy steel. As smelting technology has advanced, ultrahigh-strength R5 grade 21Cr2NiMo steel with tensile strength ≥ 1000 MPa has been developed for use in mooring chains to withstand the high loads and corrosive conditions in the marine environment [4]. During the manufacturing process, the steel is subjected to processes such as forging, welding, heat treatment, and surface treatment to enhance its mechanical properties and corrosion resistance [5]. However, in use, the surface of a mooring chain may produce defects due to factors such as wear and knocking between the links [6]. According to the Chinese National Standard GB/T 32969-2016 [7], surface defects of mooring chains should not exceed 1% of the diameter, as such changes in surface roughness may affect the service life.

Studies on mooring chains have covered several areas, including hydrogen embrittlement [8], fatigue damage [9,10], and weldability [11]. Relevant reports on their corrosion behavior have focused on stress corrosion [3], fatigue corrosion [12], friction corrosion [13], and corrosion behavior under interaction with the environment [14]; however, the effect of surface roughness on the corrosion resistance of mooring chains has not been fully investigated. Surface roughness has been shown to have an important effect on the corrosion resistance of metals. For example, Liu et al. [15] significantly improved the corrosion resistance of superhydrophobic Ti6Al4V surfaces with ground roughness prepared on a large scale by one-step laser interference lithography. Oh et al. [16] found that the corrosion rate of UNS S 32,760 super duplex stainless steel increased with the increase in roughness up to 500 nm; above this value, there was no significant change. Kashs et al. [17] studied the effect of surface roughness on the corrosion behavior of additively fabricated 316 L stainless steel, finding no significant difference in corrosion resistance between specimens with roughness values of 2.24 μm and 1.70 μm. High salinity, humidity, and temperature variations in the marine environment significantly increase the corrosion risk of mooring chains, which seriously affects their structural integrity and safety in service [18]. Therefore, it is important to explore the relationship between mooring chain surface roughness and corrosion resistance to optimize their performance and durability in offshore FPV systems.

The aim of this study was to simulate service conditions of mooring chains for FPV at sea. Salt-spray and electrochemical corrosion tests were conducted on 21Cr2NiMo steel to investigate the influence of surface roughness on the corrosion resistance and the mechanism, to provide a reference for the processing and applications of mooring chain steel.

2. Materials and Methods

This study focused on an R5 grade offshore FPV mooring chain (Φ 30 mm) made of 21Cr2NiMo steel (Hesteel Group Co., Ltd, Shijiazhuang, China). Its chemical composition is shown in

Table 1. Chemical composition of mooring chain steel (wt.%).

C	Si	Mn	Cr	Mo	Ni	P	S	Fe
0.21	0.27	1.20	1.60	0.40	0.95	≤0.025	≤0.025	Bal.

. After quenching at 920 °C and tempering at 600 °C, the experimental steel forms a tempered martensite structure with fine carbides evenly distributed on the matrix, as shown in Figure 1. Mechanical performance test results showed that the tensile strength of the test steel was ≥1000 MPa, the yield strength was ≥760 MPa, the elongation at break was ≥12%, and the impact energy at −20 °C was ≥80 J. The original austenite grain size of the quenched and tempered specimen was ≤grade 6, which meets the requirements of Chinese Standard GB/T 32969-2016 [7]. Specimens with dimensions of 15 mm × 10 mm × 3 mm were cut from the chain using electric spark cutting, and the surfaces were polished with 400 mesh, 800 mesh, 2000 mesh, and 4000 mesh sandpapers to obtain surfaces of different roughness, labeled as 4#, 8#, 20#, and 40#, respectively. The polished specimen surfaces were degreased and cleaned using distilled water and anhydrous ethanol.

The effect of surface roughness on the corrosion resistance of the steel was investigated by carrying out salt-spray corrosion testing on the four groups of specimens according to Method B of the Chinese Standard GB/T24195-2009 [19]. Each specimen was weighed using an analytical balance with an accuracy of 0.01 mg. The test cycle parameters were as follows: salt spray for 1 h, carried out at 35 °C, pH 2.5, and a salt solution sedimentation rate of 1.5 ± 0.2 mL/(80 cm²·h); drying for 4 h, at 60 °C and a relative humidity (RH) < 30%; and humidification for 3 h at 40 °C and 85 ± 5% RH. Several specimens were simultaneously placed into the test chamber and removed after 24 h.

An electrochemical workstation (CHI660E, CH Instruments, Austin, TX, USA) was used to measure the action potential polarization curves and electrochemical impedance spectroscopy (EIS). The test environment was 3.5% NaCl solution at 30 °C. A conventional three-electrode system was used, with a platinum plate as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and the specimen as the working electrode. Potentiodynamic polarization tests were performed at a scanning rate of 20 mV/min from −1.4 to 0.4 V relative to the SCE. According to ASTM G59 [20] and ASTM G61 [21] standards, the corrosion current and corrosion potential are determined using the Tafel extrapolation method. The passive current density is defined as the current value when the electrode potential increases gradually without significant change. The pitting potential is obtained using the tangent method when the current density increases rapidly. The properties of the passive film were analyzed using electrical impedance spectroscopy prior to the polarization testing. The specimen was held in 3.5% NaCl solution for 1 h, and its open-circuit potential was recorded until it reached a stable value. A sinusoidal AC potential with an amplitude of 10 mV relative to the open-circuit potential was then applied to the specimen at a frequency range of 0.01 Hz to 100 kHz. The EIS results were fitted and analyzed using ZSimpWin software (Version: 3.2. Ann Arbor, MI, USA) to obtain the Nyquist and Bode diagrams.

The corrosion products produced for different periods of salt spraying were observed by scanning electron microscopy (SEM; TESCAN MIRA LMS, Seoul, Republic of Korea) and analyzed by an energy-dispersive spectrometer (EDS).

X-ray photoelectron spectroscopy (XPS; Thermo ESCALAB 250XI, Waltham, MA, USA) was used to analyze the corrosion products of specimens with different surface roughness. The photoelectrons were excited by an Al Kα X-ray source, and high-resolution spectra of the Fe2p, Cr2p, N1s, O1s, and Cl2p regions were recorded. Thermo Avantage software ( Version: 5.9931. Waltham, MA, USA)was employed to process the spectra, with peak position correction according to the binding energy of the C1s peak (284.8 eV). All spectra were deconvoluted.

The corrosion products were removed according to the method prescribed by Chinese Standard GB/T16545-2015 [22]: 50 mL HCl and 0.35 g hexamethylenetetramine were added to distilled water to prepare a 100 mL solution, which was shaken with the sample in a water bath at 25 °C for 10 min. The mass loss of the corrosion specimen was calculated after de-rusting, cleaning, drying, and weighing.

Three-dimensional confocal surface topography (ADE Phase Shift MicroXAM 3D, Conshohocken, PA, USA) was used to characterize the surface topographies of the initial specimens and the specimens after the removal of salt-spray corrosion products. The scanning field of view was 1 mm × 1 mm.

3. Results and Discussion

3.1. Salt-Spray Corrosion

Figure 2 shows the three-dimensional morphologies of the surfaces of the mooring chain specimens after roughening using the different sandpapers, where a1, b1, c1, and d1 correspond to the 4#, 8#, 20#, and 40# specimens in random fields of view, respectively.

Figure 2(a2,b2,c2,d2) shows contour curves along the paths indicated by the arrows. The number and depth of pits on the specimen surface significantly decreased with increase in the sandpaper mesh size: the maximum pit depth decreased from 14.4 μm for specimen 4# to 0.2 μm for specimen 40#. The roughness (Ra1) of specimens 4#, 8#, 20#, and 40# are 1.98 ± 0.07 μm, 1.65 ± 0.06 μm, 0.62 ± 0.04 μm, and 0.08 ± 0.01 μm, respectively. The Ra1 of the specimen was significantly reduced with increase in fineness of the grinding paper.

Figure 3 shows the statistics for Ra1 and maximum pit depth (D1) for the data in Figure 2; these are fitted with the sandpaper mesh numbers as (1) and (2):

$$\mathrm{Ra}1=1.6{\mathrm{N}}^2-0.001\mathrm{N}+2.49\left(\mathsf{\mu }\mathrm{m}\right)$$

(1)

$$\mathrm{D}1=7.02{\mathrm{N}}^2-0.006\mathrm{N}+16.38\left(\mathsf{\mu }\mathrm{m}\right)$$

(2)

Figure 4 shows SEM images of the chain steels of different surface qualities after 24 h of salt-spray corrosion. With the increase in the initial surface roughness, the surface roughness of the specimen after corrosion was significantly reduced and the morphology of the corrosion products changed. The surface of specimen 4# (Figure 4(a1,a2)) was rough and the corrosion products mainly occurred as flakes and needle clusters, indicating that a poor surface roughness easily gives rise to loose and porous corrosion products, which may aggravate corrosion. Specimen 8# (Figure 4(b1,b2)) showed an improvement in roughness: the corrosion products were still clustered, but their distribution was more uniform and the morphology was denser than that of specimen 4#. With further refinement of surface treatment, specimens 20# (Figure 4(c1,c2)) and 40# (Figure 4(d1,d2)) showed flatter and more uniform corrosion product morphology. Particularly for specimen 40#, the corrosion products tended to be spherical or granular, and the structure was more compact, indicating that reduction in surface roughness effectively inhibited expansion by corrosion.

In summary, initial roughness had a significant impact on the surface morphology of the chain steel after salt-spray corrosion. Surfaces with higher roughness tended to form non-uniform and loose corrosion products, while finely treated surfaces tended to form dense and uniform corrosion layers, thereby improving their corrosion resistance.

Figure 5 shows the EDS element distribution on the corroded surfaces of specimen 20#. Figure 5b,c shows widespread distributions of oxygen and chlorine, respectively, in the corrosion products, indicating that they mainly comprised oxides and chlorides. Figure 5d shows that Fe was relatively uniformly distributed in the corrosion products. Combined with the distributions of O and Cl, it can be inferred that these regions mainly comprised iron oxides and chlorides. Prior research has shown that iron oxide is one of the main components of the passive film on steel [23]; in a salt-spray environment, the Cl⁻ anion will reduce the local pH and cause dissolution of the passive film, thus intensifying the corrosion [24]. This explains the large amount of chloride on the corrosion surface. Furthermore, Figure 5e,f shows that Mo and Si, respectively, are also involved in the corrosion process. Molybdenum and silicon oxides are relatively stable and dense [25,26], and their formation can effectively inhibit the generation and expansion of pitting corrosion, thus improving corrosion resistance.

Figure 6 shows the surface morphologies of the mooring chain steels with different surface roughness values after 24 h salt-spray corrosion and rust removal. Compared with Figure 2, all specimens, especially 4#, showed relatively serious corrosion. The three-dimensional morphology showed many pits on the surface (Figure 6(a1)), with a maximum pit depth of 24.4 μm (Figure 6(a2)). The number of surface pits of specimen 8#, which had a smoother surface, was significantly reduced (Figure 6(b1)), and the maximum pit depth was reduced to 15.4 μm (Figure 6(b2)). When the surface roughness was further reduced, the number of surface pits of specimen 20# was further reduced (Figure 6(c1)), and the maximum pit depth was reduced to 12.7 μm (Figure 6(c2)). The surface of specimen 40# was very flat (Figure 6(d1)); the local enlarged view shows that the maximum pit depth was only about 9.5 μm (Figure 6(d2)). The roughness (Ra2) of corrosion specimens 4#, 8#, 20#, and 40# are 4.23 ± 0.12 μm, 3.03 ± 0.09 μm, 1.80 ± 0.05 μm, and 1.58 ± 0.02 μm.

All specimens exhibited small deep pits. As anodic dissolution progresses in a salt-spray test, the corrosion products hydrolyze and block the transport of material, which increases the concentration difference between the inside and outside of the initial pit, forming an occlusive cell, and further accelerating the corrosion process by self-catalysis. In the early stages of pitting corrosion, anodic dissolution inside the pit produces Fe²⁺, while cathodic reactions outside generate OH⁻. The increased pH at the pit opening and outward migration of metal ions lead to secondary reactions (Equations (3) and (4)). Fe(OH)₃ forms at the pit opening, blocking material exchange, creating oxygen depletion inside and enrichment outside, thus forming a concentration cell. As Fe²⁺ increases, Cl⁻ migrates into the pit, forming chlorides (FeCl₂, NiCl₂, CrCl₃) [27], which hydrolyze with Fe²⁺ (Equations (5) and (6)), lowering the pH and accelerating anodic dissolution, forming a self-catalyzing occluded cell.

$${\mathrm{F}\mathrm{e}}^{2+}+2{\mathrm{O}\mathrm{H}}^{-}\to {\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_2$$

(3)

$$4{\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_2+2{\mathrm{H}}_2\mathrm{O}+{\mathrm{O}}_2\to {4\mathrm{F}\mathrm{e}\left(\mathrm{O}\mathrm{H}\right)}_3$$

(4)

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

(5)

$$\mathrm{MC}\mathrm{l}+2{\mathrm{H}}_2\mathrm{O}\text{→}\mathrm{MOH}+{\mathrm{H}}^{+}+\mathrm{C}\mathrm{l}$$

(6)

Figure 7 shows the statistics for Ra2 and mass loss (Δm) in Figure 6 and can be fitted to the Ra1 of the original specimen as (7) and (8). The data show a nonlinear positive correlation between surface roughness and maximum pit depth of the specimen after salt-spray corrosion and roughness of the original specimen. This indicates that reducing the surface roughness of the specimen can improve its salt-spray corrosion resistance.

$$\mathrm{Ra}2=0.09\text{·}\mathrm{EXP}(\mathrm{Ra}1/0.58)+1.5\left(\mathsf{\mu }\mathrm{m}\right)$$

(7)

$$\mathsf{\Delta }\mathrm{m}=\mathrm{EXP}(\mathrm{Ra}1+0.303)(\mathrm{mg}\text{·}{\mathrm{cm}}^{-2})$$

(8)

3.2. Polarization Behavior

Figure 8 shows the polarization curves for different surface roughness specimens in 3.5% NaCl solution. Figure 8a shows that during anodic polarization, with increasing potential, all specimens show different ranges of passive regions after passing through the activated dissolution region. The potential and current density results of specimens with different roughness are shown in

Table 2. The potential and current density results of different specimens.

	Pitting Potential (V vs. SCE)	Passive Current Density (A/cm2)	Self-Corrosion Potential (V vs. SCE)	Self-Corrosion Current (A/cm2)
4#	−0.36	1.89 × 10−5	−0.705	6.665 × 10−6
8#	−0.33	1.12 × 10−5	−0.693	5.622 × 10−6
20#	−0.24	1.05 × 10−5	−0.688	5.418 × 10−6
40#	−0.18	8.53 × 10−6	−0.672	4.045 × 10−6

Figure 8b shows the statistical results of the pitting potential of the specimen, indicating that the pitting potential of the specimen increases significantly with increasing surface roughness, from −0.36 V vs. SCE for specimen 4# to −0.183 V vs. SCE for specimen 40#. Fitted analysis showed a linear positive correlation between pitting potential P and initial roughness Ra1:

$$\mathrm{P}=-0.09\mathrm{Ra}1-0.18(\mathrm{V}\mathrm{v}\mathrm{s}.\mathrm{S}\mathrm{C}\mathrm{E})$$

(9)

The passivation potential range significantly increased with increasing surface roughness, from 0.03 V for specimen 4# to 0.31 V for specimen 40#, while the passivation current density was maintained at 1.89 × 10⁻⁵ A/cm² to 8.53 × 10⁻⁶ A/cm². These results show that reduction in surface roughness significantly enhanced pitting resistance of the specimen. In addition, specimens 20# and 40# showed significant re-passivation behavior when the polarization potential increased to −0.55 V, further confirming that reduced surface roughness enhanced the re-passivation capability of the specimens.

3.3. Electrochemical Impedance Spectroscopy

Figure 9 shows the EIS test results for the different specimens in 3.5% NaCl solution. Figure 9a shows the respective Nyquist plots, all of which exhibit a single circular arc, the radius of which significantly increased with increasing surface roughness. The arc radius is positively correlated with interfacial polarization resistance [28]. After fitting the EIS results using ZSimpWin software (Version: 3.2. Ann Arbor, MI, USA), the polarization resistances (R_corr) of specimens 4#, 8#, 20#, and 40# were 2310.4, 2498.3, 3069.3, and 4994 Ω·cm², respectively. Therefore, the reduction in surface roughness significantly increased the polarization resistance of the samples, corresponding to the decrease in current density shown in Figure 8a.

Figure 9b shows the corresponding Bode plots. Impedance in the low-frequency region significantly improved with the reduction in surface roughness, which indicates improvement of the corrosion resistance of the passive film. The value of the fixed frequency of 0.1 Hz corresponds to the interface polarization resistance [29]. In addition, the peak phase angles slightly increased with the increase in surface roughness, which indicates improved integrity and uniformity of the passivation films. A significant shift of the phase-angle curve to the right indicates a shortening of the corrosion process time, i.e., an increase in the re-passivation capability of the specimen.

The equivalent circuit diagram of the EIS is shown in Figure 9b, where Rs represents the solution resistance, which measures the conductivity of the electrolyte solution; Q_f and R_f denote the capacitance and resistance of the oxide film, serving as important parameters for assessing the quality of the passive film; and Q_dl and R_ct represent the double layer capacitance and charge transfer resistance at the metal/electrolyte interface, respectively. The EIS fitting results in

Table 3. Fitting data of the EIS spectra.

	Rs/(Ω·cm2)	Qf/(μΩ−1·cm−2·sn)	Rf/(Ω·cm2)	Qdl/(μΩ−1·cm−2·sn)	Rct/(Ω·cm2)
4#	3.85	21.49	211.4	37.68	2099
8#	3.95	23.21	322.3	29.91	2176
20#	4.33	77.24	492.3	13.17	2577
40#	4.46	17.99	1586	24.48	3408

indicate that the R_s values of different samples fluctuate little, suggesting that the solution used in the experiment is consistent, thereby ensuring the reliability of the experimental results. The higher R_f and R_ct values imply that the oxide film and double layer play significant roles in the passive film, having a notable impact on the charge transfer process. This indicates that the overall quality of the passive film and its corrosion resistance are influenced not only by the oxide film itself but also by the structure of the double layer at the metal/electrolyte interface. As shown in Table 3, the reduction in surface roughness significantly increases the R_f and R_ct values, indicating an increase in resistance. Notably, for specimen 40#, both R_f and R_ct show significant improvements, suggesting that lower surface roughness enhances the density and repassivation capability of the passive film [30], thereby increasing its polarization resistance R_corr. The relationship between R_corr, R_f and R_ct is given by: R_corr = R_f + R_ct [31].

3.4. X-ray Photoelectron Spectroscopy

Figure 10 shows XPS results for the specimens of different surface roughness after the salt-spray tests. High-resolution spectra for O 1s, Fe 2p3/2, Mo 3d, and Si 2p are presented. The intensities of all characteristic oxide peaks decreased with the increasing surface roughness (Figure 10). The iron oxide peak was significant in specimens of lower roughness (Figure 10(a2,b2)), indicating a higher iron oxide content. However, the intensity of this peak significantly decreased as the roughness increased (Figure 10(c2,d2)), indicating a decrease in iron oxide content. Similarly, the intensities of the oxide peaks of molybdenum (Figure 10(a3,b3,c3,d3)), especially the MoO₃ peak, decreased with increasing roughness, indicating that roughness has a significant impact on the formation of molybdenum oxide: a smoother surface is more conducive to the formation of a dense passive film. Similarly, the intensities of the SiO₂ peaks (Figure 10(a4,b4,c4,d4) decreased with the increase in roughness, indicating that the SiO₂ layer was denser on a smoother surface and a rough surface was unfavorable for its formation. Metal oxide is the main component of passive film, the formation of which is significant in improving the corrosion resistance of a metal. However, as the roughness increases, the surface oxide content decreases, which means that the density and integrity of the passive film may be damaged, thus reducing the corrosion resistance of a specimen.

To further analyze the elemental composition of the corroded surface, XPS analysis of Cl 2p was performed on specimens of different surface qualities. The results are shown in Figure 11. Figure 11a–d corresponds to spectra for the low to high surface roughness specimens. The intensity of the Cl⁻ peak gradually increased with the increase in surface roughness, reaching a maximum for the specimen with the highest roughness (Figure 11d), indicating that the amount of chloride adsorbed on the surface significantly increased with the increase in surface roughness. This is inversely related to the reduction in oxide content, i.e., thinning of the oxide layer reduces the protection of the base metal, making chloride more susceptible to deposition and spreading on a rough surface. The area of the M–O–Cl peak increased slightly with increasing roughness, indicating that chloride is not only present on the rough surface as the simple anion but also may combine with metal oxide to form more complex corrosion products.

The results of Figure 5 and Figure 10 suggest that an increase in surface roughness significantly reduces compactness of the passivation film, resulting in a decrease in oxide content, which makes it easier for chloride ions to contact the matrix and form a chloride-containing product. This is consistent with the results of the EIS (Figure 9). Owing to its large specific surface area and high surface energy, it is easier for a rough surface to adsorb and retain corrosive ions, such as Cl⁻. Under these conditions, chloride formation is not only a sign of corrosion, but may also accelerate the corrosion process. Loose porous chloride layers may become active sites of corrosion, further accelerating degradation of the base metal. Thus, an increase in surface roughness reduces the chloride corrosion resistance of the steel to some extent, resulting in more severe corrosion phenomena (Figure 4). In contrast, a smoother surface forms a denser passive film, which more effectively blocks chloride ion penetration and enhances corrosion resistance of the specimen.

The reduction in surface roughness plays a crucial role in enhancing the corrosion resistance of metals by promoting the formation of a denser and more uniform passive film. A smoother surface allows for a continuous oxide layer, which improves the barrier properties against corrosive agents. This denser passive film results in higher polarization resistance, as it limits ion diffusion and minimizes electrochemical reactions at the metal surface, leading to lower current density during corrosion processes.

Additionally, a compact oxide layer decreases electron exchange between the metal and its environment, further reducing corrosion rates. The effective hindrance of aggressive ions, such as chlorides, prevents localized corrosion by restricting their penetration into the passive film. Moreover, the self-repairing capability of the passive film is enhanced with reduced roughness. When localized breakdown occurs, the denser oxide layer can facilitate rapid reformation of the protective film, limiting damage and preventing widespread corrosion.

In summary, smoother surfaces contribute to a more robust passive film, increasing polarization resistance, decreasing electron exchange, and enhancing the self-healing capabilities of the oxide layer, all of which significantly improve corrosion resistance.

4. Conclusions

The effect of surface roughness on corrosion behavior of mooring chains for offshore FPV systems was studied using dry-and-wet cycle salt-spray corrosion and electrochemical tests. The main conclusions are as follows:

(1)

The salt-spray corrosion resistance of mooring chain steel is significantly enhanced by reducing surface roughness: the number and depth of corrosion pits are significantly reduced;

(2)

With an increase in roughness of the original surface, the mass loss after 24 h salt-spray corrosion exponentially increased:

$$\mathsf{\Delta }\mathrm{m}=\mathrm{EXP}(\mathrm{Ra}1+0.303)(\mathrm{mg}\text{·}{\mathrm{cm}}^{-2})$$

(10)

(3)

Reduction in surface roughness significantly increased pitting potential of a specimen and enhanced the re-passivation ability, thereby significantly improving the pitting resistance: pitting potential was linearly related to initial roughness of the specimen:

$$\mathrm{P}=-0.09\mathrm{Ra}1-0.18(\mathrm{V}\mathrm{vs}.\mathrm{SCE})$$

(11)

(4)

A decrease in surface roughness improved the oxide contents of Fe, Mo, and Si in the passive film, improved its density, effectively blocked chloride ion erosion, and improved corrosion resistance of the mooring chain steel.