**1. Introduction**

Marine Renewable Energy (MRE) devices such as offshore wind turbines or ocean current turbines involve carbon steel structures with complex geometries and/or inner channels of small dimension where seawater may flow. In most cases, cathodic protection (CP) is envisioned to ensure the durability of the immersed part of the MRE devices. Modelling, predicting and quantifying the efficiency of CP for such complex structures may be challenging. Consequently, specific experimental data may be necessary for a reliable modelling of the CP system. An experimental device was designed to simulate a complex steel structure where seawater would flow inside a channel of small dimension. The steel structure inside the channel was mimicked by a series of 12 steel coupons connected to each other. Four of them could be disconnected individually from the system to be studied separately. In the channel, due to the small cross-sectional area, an important ohmic drop should increase the potential of the metal so that deep inside the channel an insufficient cathodic potential could prevail.

In seawater, the increase of the interfacial pH due to CP induces the formation of a mineral layer on the steel surface. At the potentials usually used for CP of carbon steel, this layer is mainly composed of aragonite CaCO3 [1–4], and thus called calcareous deposit. The increase of pH at the steel/seawater interface is due to the increase of the cathodic reaction rate, a direct consequence of the cathodic polarization. In most cases, the main reaction involved is the reduction of dissolved O2:

$$\rm O\_2 + 2H\_2O + 4e^- \rightarrow 4OH^- \tag{1}$$

The production of OH− ions increases the pH, which modifies the inorganic carbonic equilibrium at the steel/seawater interface:

$$\text{OH}^- + \text{HCO}\_3^- \rightarrow \text{H}\_2\text{O} + \text{CO}\_3^{2-} \tag{2}$$

This process finally leads to the precipitation of CaCO3 on the steel surface:

$$\text{Ca}^{2+} + \text{CO}\_3^{2-} \rightarrow \text{CaCO}\_{3(s)}\tag{3}$$

The growth of the calcareous deposit promotes a physical barrier against O2 diffusion so that the current density required for CP decreases with time. This phenomenon could strongly influence the ohmic drop inside a channel of small dimension, such as the one considered in the present study, and thus modify drastically the potential of the metal deep inside the channel.

To our best knowledge, only one study devoted to a similar topic was reported [5]. It aimed to compare experimental results with a computational analysis based on boundary element modelling. Our purpose was to provide more detailed experimental results. In particular, the aim was to monitor over time the potential of the various steel coupons present inside the channel and to assess the efficiency of CP for each coupon. A 3.5-month experiment was then carried out. At the end of the experiment, a surface analysis by X-ray diffraction was achieved to characterize the calcareous deposit formed on the steel coupons. Finally, to estimate CP efficiency, polarization curves were acquired for various coupons along the channel. As performed in previous studies dealing with cathodic protection of steel in soil [6–9], the residual corrosion rate of the metal at the protection potential *E*CP was estimated by extrapolation of the anodic Tafel straight line down to *E*CP.

#### **2. Materials and Method**

#### *2.1. Description of the Experimental Device Used for the Study*

The experimental device was designed so that seawater flows in a channel 50 cm long, 5 mm deep and 5 cm wide, except in a 12 cm long central region, where its width was reduced to 1.5 cm. The channel was machined in one of the two Plexiglas plates (2 cm thick) that constituted the body of the device, as illustrated in Figure 1.

**Figure 1.** Schematic view (front) of the experimental device showing the respective positions of the steel coupon (grey rectangle) embedded in resin (in yellow), the Ag/AgCl microelectrode (in blue) and the Ti disk used as counter-electrode (hatched rectangle). C.-E., Ref. and W denote the three connections to the potentiostat.

In one of the Plexiglas plates, 12 large holes were drilled to insert 12 carbon steel coupons embedded in a resin matrix, as displayed in Figure 2. These coupons are denoted C1 to C12, with C1 being the coupon at the entry of the channel, where seawater comes in. A copper wire was welded on the rear side of each coupon so that the coupon could be connected to potentiostat #1 that ensured CP. Two kinds of coupons were used, with eight 3-cm diameter coupons in the 5-cm wide parts of the channel and four 1.5-cm diameter coupons in the 1.5-cm wide central part of the channel (Figure 2). The device was set in a tank (120 cm long × 25 cm deep × 15 cm wide) composed of three sections, separated by Plexiglas walls ensuring the sealing of each section. The first section, which can be called the "input section", was filled with artificial seawater. A counter-electrode (titanium grid), and an Ag/AgCl-seawater reference electrode (*E*ref = +0.250 V vs. SHE) were set in this section and connected to potentiostat #1 used for CP. Five additional large (7 cm × 7 cm × 0.8 cm) steel plates were also immersed in the "input section" to constitute the outer part of the "simulated steel structure". They were connected to potentiostat #1 like the coupons inside the channel and thus protected similarly. The cathodic protection was applied at a constant potential of −1060 mV vs. Ag/AgCl-seawater (i.e., −1050 mV vs. SCE) with respect to the reference electrode set in the "input section". This potential value is the lowest value that could reach a steel structure connected to an Al-Zn-In galvanic anode.

The central section of the tank, where the Plexiglas device was set, was not filled with seawater, to facilitate the visual observation of the coupons inside the channel. The third section, i.e., the "output section", was filled with artificial seawater like the "input section". An aquarium pump was placed at the bottom of this section and connected to the channel of the device via a 3-mm diameter plastic tube. The pump ensured a continuous seawater flow inside the channel at a rate, controlled weekly during the 3.5-month experiment, measured between 16 L h−<sup>1</sup> and 28 L h−1. The corresponding flow velocity was then between 1.8 cm s<sup>−</sup><sup>1</sup> and 3.1 cm s<sup>−</sup><sup>1</sup> in the main part of the channel and between 5.9 cm s<sup>−</sup><sup>1</sup> and 10.4 cm s<sup>−</sup><sup>1</sup> in the smaller central section.

The pumped seawater was transported up into a second tank, initially filled with a large volume of seawater. The seawater level of this thank remained constant as the excess seawater overflowed into the "input section" of the first tank. The overall volume of seawater was equal to 20 L. Half of this volume, i.e., 10 L, was renewed after 2 weeks, after 1 month and after 2 months of experiment. A picture of the entire experimental system is shown in Figure 3.

Four of the coupons, namely C1, C4, C9 and C12, could be disconnected individually and studied separately using a second potentiostat. Close to each coupon, an Ag wire covered with an AgCl layer was set inside the channel to be used as a local Ag/AgClseawater reference electrode specific to each coupon. Opposite to each of these four steel coupons, a Ti disk was inserted in the Plexiglas plate where the channel was machined to be used as a local counter-electrode (Figure 1).

Finally, four small holes (3.5-mm diameter) were drilled on top of the device so that a micro pH electrode could be inserted inside the channel for local pH measurements close to coupons C4, C6, C7 and C9 (Figure 2). The holes were closed with small rubber lids except when pH measurements were carried out.

The artificial seawater used here was based on the ATSM D1141 standard [10]. Its composition was NaCl (0.42 mol <sup>L</sup>−1), MgCl2·6H2O (0.055 mol <sup>L</sup>−1), Na2SO4·10H2O (0.029 mol <sup>L</sup>−1), CaCl2 ·2H2O (0.011 mol <sup>L</sup>−1), KCl (0.009 mol <sup>L</sup>−1) and NaHCO3 (0.003 mol <sup>L</sup>−1). Its pH was adjusted at 8.1 ± 0.1 by addition of small amounts of a 0.1 mol L−<sup>1</sup> NaOH solution. Its conductivity was measured at 55.4 ± 0.2 mS/cm, i.e., a typical value for seawater.

S235-JR carbon steel rods (3-cm and 1.5-cm diameter) were used to prepare the coupons. The steel composition (wt %) was 98.2% Fe, 0.122% C, 0.206% Si, 0.641% Mn, 0.016% P, 0.031% S, 0.118% Cr, 0.02% Mo, 0.105% Ni and 0.451% Cu. The steel surface was abraded with silicon carbide (grade 180, particle size 80 μm), rinsed with deionized water, and carefully dried just before the coupons were set in the experimental device. The large steel plates immersed in the "input section" of tank #1 (Figures 2 and 3) were made of the same steel and their surfaces were prepared the same way.

The experiment was performed twice for 3.5 months, after an initial shorter experiment of 21 days, at room temperature (22 ± 2 ◦C) in each case. These experiments gave similar results but the article is focused on the last of the three experiments because, in particular, additional information about coupon C6 was only acquired in this case (see Section 2.2).

## *2.2. Electrochemical Measurements*

When the ohmic drop is important, the "real" potential of the metal differs significantly from the applied potential *<sup>E</sup>*app. This potential is the potential corrected from ohmic drop, *E*IR free, expressed as:

$$E\_{\text{IR free}} = E\_{\text{app}} - RI \tag{4}$$

In Equation (4), *R* is the resistance of the electrolyte that separates the reference electrode from the working electrode (steel coupon), and *I* the current required for CP. For a cathodic current, the value of *I* is considered negative, which implies that *E*IRfree > *<sup>E</sup>*app.

In our experimental conditions, *<sup>E</sup>*app is equal to −1060 mV vs. Ag/AgCl-seawater. It corresponds to the electric voltage between each coupon under CP and the reference electrode used for CP, immersed in the "input section" of tank #1.

The potential *E*IR free of coupons C1, C4, C9 and C12 could be measured using potentiostat #2 via the determination of the electric voltage between the coupon and the paired Ag/AgCl-seawater microelectrode set nearby inside the channel. These measurements were performed daily during the first 20 days, weekly until day 81 and one last time at the end of the experiment (day 109). In this last day, an additional measurement was performed for coupon C6 using a hole designed for pH measurement to set an Ag/AgCl-seawater microelectrode close to the coupon.

On the last day of experiment, polarization curves were acquired for coupons C1, C4, C9 and C12 using the associated counter-electrode and Ag/AgCl-seawater microelectrode. It was also achieved for C6, using an Ag/AgCl-seawater microelectrode as explained above and the closest counter-electrode, i.e., the one facing coupon C4. The curves were obtained from *E*IR free(1) = −1.10 V vs. Ag/AgCl-seawater to *E*IR free(2) = −0.60 V vs. Ag/AgCl-seawater, at a scan rate d*E*/d*t* = 0.1 mV/s. Potentiostats #1 and #2 were both VSP potentiostats (BioLogic, Seyssinet-Pariset, France). The polarization curves were measured with the water still flowing in the channel and CP was not interrupted before the polarization curve was acquired. The lowest potential (−1.1 V vs. Ag/AgCl-seawater) was chosen to be only slightly smaller than the applied potential *<sup>E</sup>*app, so that water reduction remained negligible. The highest potential (−0.6 V vs. Ag/AgCl-seawater) was chosen so that in any case a sufficiently large anodic region could be investigated.

All pH measurements were performed with a Mettler-Toledo micro pH electrode (Mfr # 51343160) and a SevenExcellence pH meter S400 (Mettler-Toledo SAS, Viroflay, France). They were carried out at day 28 while the seawater flow inside the channel had, exceptionally, stopped, and every two weeks with seawater flowing normally inside the channel.

#### *2.3. X-ray Diffraction Analysis*

After the experiment, the surface of the coupons was analysed by X-ray diffraction (XRD). The coupons were directly set in the sample holder and the analysis was carried out with a classical powder diffractometer (Brucker AXS® D8-Advance), using Cu-Kα wavelength (λ = 0.15406 nm) in Bragg-Brentano geometry. The acquisition was performed at 40 kV and 40 mA, from 2*θ* = 10◦ to 2*θ* = 70◦, with an angular interval of 0.04◦ and a counting time of 3 s per angular position.

The various phases were identified using the ICDD-JCPDS-PDF-2 database (ICDD, Newtown Square, PA, USA) via the files 01-075-2230 (aragonite = CaCO3), 00-005-0628 (halite = NaCl), 01-044-1415 (lepidocrocite = γ-FeOOH) and 03-065-4899 (α-Fe).
