Flaky carbonyl iron (FCI) powder is a typical absorbing material with excellent magnetic loss performance. However, its single absorption mechanism, narrow effective absorption bandwidth (RL <
-10 dB), poor corrosion resistance, and high density restrict the application of FCI in marine environments. In this study, carbonized urea-formaldehyde resin (UFC)-coated flaky carbonyl iron (FCI@UFC) composites were prepared by in situ polymerization and pyrolysis. Various characterization techniques were employed to investigate the phase structure, microstructure, absorption performance, and corrosion resistance of FCI@UFC. The results showed that FCI@UFC effectively combined the magnetic loss of FCI and the dielectric loss of the porous carbon layer, achieving a wider effective absorption band (EAB) with a smaller thickness. When the simulated absorption layer thickness was 1.2 mm, the EAB ranged from 10.32 to 18 GHz, which demonstrated excellent microwave absorption performance. Additionally, the porous carbon coating slowed down the direct reaction between the corrosive medium and FCI, and the porous structure could also accommodate electrolytes, maintaining a stable electrochemical environment. As a result, FCI@UFC exhibited a higher corrosion potential (-0.472 V) and a lower corrosion current (1.45 × 10