Unveiling the Role of Seawater Microdroplets in Accelerating Steel Corrosion

In marine environments, splash-induced microdroplets are highly aggressive corrosion media, producing metal corrosion rates several times higher than in fully immersed regions. While macroscopic factors contributing to splash-zone corrosion are well established, the chemical reactivity of seawater microdroplets provides a previously underappreciated complementary mechanism that further enhances corrosion. Here, we demonstrate that the spontaneous generation of reactive radical species within microdroplets significantly accelerates metal oxidation. Notably, the corrosion rate of carbon steel in NaCl microdroplets is over an order of magnitude higher than in NaCl bulk water. Electron paramagnetic resonance and fluorometric assays reveal sustained generation of hydroxyl radicals (·OH) and hydrogen peroxide (H₂O₂) in microdroplets, negligible in bulk. These species drive a two-stage acceleration: (i) surface ·OH initiates rapid Fe oxidation and (ii) H₂O₂ reacts with Fe²⁺ via a cyclic Fenton process, regenerating ·OH and forming an FeOOH layer. Time-of-flight secondary-ion mass spectrometry shows that high-salinity Cl^- subsequently displaces FeOOH to form soluble FeOCl, exposing fresh metal and sustaining localized corrosion. This study highlights spontaneously generated reactive species in microdroplets as a key driver of splash zone corrosion, offering insights for radical-targeted protective coatings.