**4. Discussion**

From the previous sections it was possible to observe that MWCNT incorporation decreased the anodic dissolution rate of the Ni–P layer, leading to a significant reduction in the electrochemical activity, as shown in the SECM maps displayed in Figure 7. As mentioned in Section 3.4, literature reports ascribe the beneficial effect of carbon nanotubes on reducing the corrosion rate of Ni–P-coated substrates to its blocking effect with respect to intrinsic coating defects [47]. Alishahi et al. [28] reported a reduction of approximately 55% in the corrosion current density of a Ni–P/MWCNT composite film, with respect to a conventional Ni–P layer in 3.5 wt.% NaCl solution. Our results point to an even higher protection efficiency of the Ni–P/MWCNT layer. Based on these results, the proposed corrosion mechanism of the Ni–P/MWCNT coated samples is schematically illustrated in Figure 8.

Figure 8A shows the corrosion mechanism of the conventional Ni–P binary coating where anodic dissolution of the steel substrate occurs through intrinsic coating defects such as pores as cracks, releasing Fe2+ ions to the electrolyte. In fact, the high electrochemical activity of this sample was observed in the SECM map displayed in Figure 7B, supporting the occurrence of this mechanism. A different scenario was established after MWCNT incorporation in the Ni–P matrix. The electrochemical activity was gradually reduced, as described in Section 3.5, slowing down the anodic dissolution rate of the steel substrate. As a consequence of the blocking effect of the MWCNT particles, the release of Fe2+ ions was gradually reduced. This situation is illustrated in Figure 8B for the CNT-0.25 sample. As the MWCNT loading in the composite film increased, the amount of Fe2+ ions released to the solution was further reduced. The progressively lower electrochemical activity of the CNT-0.50 (Figure 7D) and CNT-1.0 (Figure 7E) films would be a consequence of this effect, as illustrated in Figure 8C,D, respectively.

**Figure 8.** Schematic illustration of the corrosion mechanism of the Ni–P and Ni–P/MWCNT coatings: (**A**) Ni–P layer; (**B**) CNT-0.25; (**C**) CNT-0.50; (**D**) CNT-1.0.
