**4. Conclusions**

From the results of this work, the following conclusions can be proposed about the efficiency of the different ageing tests to differentiate the behaviors of thick marine coatings applied onto steel (ROX system), hot-galvanized steel (RG), and Zn-Al15 thermal spraying coated steel (RM):

When applied to defect-free samples, immersion tests in NaCl 35 <sup>g</sup>·L−<sup>1</sup> solution at a constant temperature (35 ◦C) were only able to show the decrease in the paint resistivity for all systems. No significant difference was observed between the three systems after 1085 days of immersion. Much more time or lower thicknesses may be needed in such tests to expect degradation and eventual differentiation between different systems. When X-cut samples were immersed in NaCl, only the contribution of the sacrificial Zn layers has been shown.

The immersion in natural seawater for 1200 days showed no significant difference for the three systems. This test with true natural conditions is therefore not adapted for the rapid evaluation of thick coating performances.

No significant difference was observed between the defect-free systems during the cyclic NSS test for 1440 h. Only adhesion issues could have been suspected from these ageing conditions, but the phenomena were not clear enough. For panels with defects, the NSS cyclic test was clearly able to show the effect of the sacrificial Zn layers in the same way as the immersion tests.

When applied to defect-free panels, the alternate cycling test was able to clearly emphasize adhesion issues between the paint system and the substrate, as it has been seen with the blistering of RG samples. Moreover, when applied to X-cut panels, corrosion propagation was also involved, and this test allowed us to highlight the contribution of sacrificial Zn layers, as observed in the RM systems.

Thermal cycling is the only test which has been able to clearly differentiate defectfree systems by stressing the adhesion of the coatings. However, care must be taken when defining the thermal cycles so that the maximum temperature remains lower than the wet glass transition temperature. Finally, in the framework of this study, for thick, highly pigmented coatings, X-cut panels and alternate cycling have been the only ageing conditions to fully discriminate different paint systems by involving both corrosion and adhesion loss mechanisms. It can be noted that a long testing time is still needed to rank different thick organic coating systems, therefore more studies have to be performed in order to find rapid realistic ageing conditions. For example, steps of exposition to UV-light could also be introduced in the ageing cycles in order to induce the chemical degradation of the organic compounds and bring the testing to a more realistic level. Mechanical stresses should also be considered, depending on the targeted applications.

Further investigations should also be carried out to study paints applied on other metals, and to understand the degradation of more complex systems such as paints containing corrosion inhibitors or self-healing coatings.

**Author Contributions:** Investigation, A.R., V.P., J.G. and T.S.; resources, C.P.-L.; writing—original draft preparation, T.S.; writing—review and editing, A.R. and T.S.; supervision, S.F., M.K., A.-M.G.; project administration, L.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by IRT Jules Verne (French Institute in Research and Technology in Advanced Manufacturing Technologies for Composite, Metallic and Hybrid Structures).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** This study was part of the ADUSCOR project managed by IRT Jules Verne (French Institute in Research and Technology in Advanced Manufacturing Technologies for Composite, Metallic and Hybrid Structures). The authors wish to associate the partners of this project, respectively NAVAL GROUP, CHANTIERS DE L'ATLANTIQUE, GENERAL ELECTRIC, CETIM and IRT JULES VERNE.

**Conflicts of Interest:** The authors declare no conflict of interest.
