Synergistic acceleration effects of ultraviolet and salt spray on the degradation and failure of electromagnetic wave-absorbing coatings

There is an urgent need to reliably and rapidly evaluate the risk of failure of electromagnetic wave absorbing (EWA) coatings under the complex oceanic environment, which involves multiple weathering factors such as ultraviolet light, salt spray, etc. Unfortunately, conventional accelerated aging methods, which only consider single factor or apply multiple factors alternately, cannot adequately reproduce the failure modes and mechanisms of EWA coatings during real service. As a result, even though EWA coatings have passed existing qualification tests, their performance still could deteriorate much faster than expected. To this end, we investigated the synergistic acceleration effects of UV and salt spray on the degradation and failure of electromagnetic wave-absorbing coatings. Epoxy/flaky carbonyl iron (FCI) coating was prepared as the model sample. Effects of alternating- and combined-UV-salt spray (A-UV-SS and C-UV-SS) accelerated aging on the failure behaviors and underlying mechanisms of EWA coatings were investigated and compared thoroughly. It was found that EWA ability of coatings aged under C-UV-SS deteriorated much faster than that under A-UV-SS (more than two times). This additional acceleration aging effect was attributed to synergistic mechanisms with the coexistence of UV and salt spray: 1) acetic acid from the photo-oxidation of epoxy act synergistically with salt spray to accelerate the corrosion of Fe; 2) photovoltaic effect and salt spray synergistically promote the formation of iron oxides. This novel method has the potential to identify new failure mechanisms of EWA coatings and significantly reduce the test time, which is critical for their reliability evaluation and performance optimization.