*3.3. Voltammetry*

The polarization curves obtained for coupons C1, C4, C6 and C9 are displayed in Figure 7. The curve obtained for C12, very similar to that of C9, was omitted for clarity.

**Figure 7.** Polarization curves (log|*j*| vs. *E*) acquired at the end of the experiment for coupons C1 (black line), C4 (red line), C6 (brown dotted line) and C9 (green line).

First, it is observed that the corrosion potential varies between −0.82 V vs. Ag/AgClseawater and −0.76 V vs. Ag/AgCl-seawater, without any apparent link with the position of the coupons inside the channel. In contrast, both anodic and cathodic parts of the curves are shifted to higher current densities from C1 to C9. This evolution can be attributed to the calcareous deposit, which may be more protective for the coupons that were polarized at a lower potential during a longer time. Numerous studies devoted to calcareous deposition on steel in seawater demonstrated that a decrease in potential, in the range between −0.85 to −1.05 V vs. Ag/AgCl-seawater, led to denser and thicker layers [3,4,12]. A longer polarization time, at a given potential, also increases the thickness and decreases the porosity of the calcareous layer [3,4,13].

The anodic part of the log|*j*| vs. *E*IR free curve proved linear in any case, with similar anodic Tafel slopes, for C4, C6 and C9, (i.e., 281, 287 and 272 mV/decade) and a significantly different slope for C1 (220 mV/decade). As for the cathodic part, two behaviors can be observed. In the case of C1 and C6, the polarization curve bends progressively, from *E*cor to more cathodic potentials, so that the slope of the curve becomes very small below −0.9 V vs. Ag/AgCl-seawater. This shows that the kinetic of the cathodic reaction, i.e., mainly O2 reduction, is strongly influenced by mass transport. Thus, the polarization curve tends towards a diffusion plateau at the lowest potentials. In the case of C4 and C9, the influence of mass transport is less important, and the slope of the polarization curve remains high at the lowest potentials. This difference is thoroughly discussed in Section 4.

The cathodic reaction being partially controlled by diffusion, the polarization curves did not obey Tafel law in the cathodic domain. Consequently, only the anodic part of the curve was considered for an interpretation of the voltammetry results based on the Tafel method. The anodic Tafel lines were then drawn and extrapolated down to *E*CP, i.e., the final potential reached by the coupon at the end of the CP experiment. This extrapolation led to an estimation of the corrosion current density *j*cor, i.e., the value *j*A(*E*cor) of the anodic current density at *E*IR free = *E*cor. Similarly, the value *j*A(*E*CP) of the anodic current density at the potential applied during CP gave an estimation of the residual anodic current density, i.e., the residual corrosion rate, expected low, reached under CP (see references [6–9] for more details). The obtained anodic Tafel lines, drawn for each coupon between *E*cor and *E*CP, are displayed in Figure 8. Using Faraday's law, the obtained *j*cor and *j*A(*E*CP) values could finally be converted to corrosion rates. All the obtained values (potential, current density, and corrosion rate) are listed in Table 3.

**Figure 8.** Anodic Tafel lines deduced graphically from the anodic branch of the polarization curves, extrapolated down to the last measured *E*CP value, and drawn between *E*CP and *E*cor for coupons C1 (black line), C4 (red line), C6 (brown dotted line) and C9 (green line).


**Table 3.** Voltammetry measurements and data obtained via the extrapolation of the anodic Tafel line down to *E*CP. *E*cor is given in V vs. Ag/AgCl-seawater, *j*cor and *j*a(*E*CP) in mA cm<sup>−</sup>2, and the corrosion rate and residual corrosion rate in μm yr<sup>−</sup>1.

First, as can be seen in Figure 8 and read in Table 3, the corrosion rates of C1 and C4, almost identical, are significantly lower than those of C6 and C9, which are quite similar. This effect can be attributed to the calcareous deposit [4], which constitutes a more protective barrier against corrosion for coupons C1 and C4 polarized at lower potentials than C6 and C9 all through the experiment (see for instance Figure 5 to compare C1 and C4 with C9). The corrosion rate estimated for C1 and C4, i.e., 7 μm yr<sup>−</sup>1, is actually very low, which indicates that the calcareous deposit provides an efficient protection against corrosion, at least a short time after the interruption of CP. Besides, it cannot be excluded that the increase of the interfacial pH promoted the formation of a nanometric passive or pseudo-passive layer on the steel surface.

The estimated residual corrosion rate increases from C1 to C9. As illustrated by Figure 8, this rate is not only linked to the *E*CP values, but also to the respective positions of the anodic Tafel lines. Let us consider C6 and C9. In this case, the anodic Tafel lines are close but the *E*CP value of C6 is significantly lower than that of C9. Consequently, CP is more efficient for C6 because the potential of this coupon is more cathodic. If we now compare C6 with C4, we can see that the *E*CP values are not so different, but the anodic Tafel line of C4 is located at much lower current density values. In this case, CP is more efficient for C4 mainly because of the positioning of its anodic Tafel line. As explained earlier, the decrease of both anodic and cathodic current densities is due to the calcareous deposit. In other words, at the end of the experiment, CP was more efficient for C4, if compared to C6, because C4 had been previously polarized at a lower average cathodic potential, which had led to the formation of a more protective calcareous deposit (and maybe a more protective nanometric pseudo-passive layer). In the case of coupon C1, the anodic Tafel slope, different from that characteristic of coupons C4, C6 and C9 (Figure 8), is another factor that explains the very low residual corrosion rate (Table 3).

## *3.4. XRD Analysis*

The XRD patterns obtained for coupons C7 and C9 are displayed in Figure 9.

In any case, the surface of the coupons, once extracted from the experimental device, was gently rinsed with deionized water. For the coupons covered with a fluffy layer of orange corrosion products (C11 and C12 for instance, see Figure 6), this rinsing removed most of the corrosion products. These products were analyzed separately and consisted mainly of lepidocrocite γ-FeOOH (data not shown).

Therefore, the XRD analysis reveals only the nature of the mineral layer formed on the steel surface below the orange corrosion products (if present). For all coupons, this mineral layer proved to be composed only of aragonite CaCO3, as illustrated for C7, C9 and C10 in Figure 9. This is consistent with previous works dealing with calcareous deposition on steel immersed in seawater under cathodic protection [1–4].

**Figure 9.** XRD analysis of coupons C10, C9 and C7. The diffraction lines of aragonite are denoted with the corresponding Miller index. H = main diffraction line of halite (NaCl).

When compared to the corresponding ICDD-JCPDS file, it appeared that the aragonite crystals exhibited a preferential orientation, revealed by an increased intensity of the 012 diffraction line. This phenomenon proved much more pronounced for coupons C5–C8, i.e., for the coupons set in the smaller part of the channel. The XRD pattern of coupon C7, compared to those of C9 and C10 in Figure 9, clearly illustrates this result.
