Delamination of thermally sprayed ZnAl15/organic duplex coating systems on structural steel for offshore wind applications

Duplex coating systems incorporating both thermally sprayed metal (TSM) and organic coatings have demonstrated excellent corrosion protection performance in highly corrosive offshore applications. However, the present work showed extensive delamination of organic coatings from the TSM (ZnAl15) layer during ISO 12944-9 cyclic ageing test. A positive correlation was uncovered between cathode-to-anode ratio and the extent of the delamination, which was further verified by delamination measurement on samples with varying artificial scribes. A depletion of aluminum at the tip of propagating cracks in the TSM layer was detected by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), suggesting the occurrence of local acidification driven by hydrolysis of Al3+. X-ray diffraction (XRD) analysis confirmed ZnO, Zn5(OH)8Cl2·H2O, and AlO(OH) as the main corrosion products in the artificial scribe. Careful considerations and supplementary real-field assessment were recommended when employing ISO 12944-9 cyclic ageing test for a robust anticorrosive performance evaluation of this type of duplex coating system.