On the initiation and growth dynamics of filiform corrosion on carbon steel: New microscopic insights

Filiform corrosion (FFC) represents a specific form of coating failure and localized corrosion that predominantly occurs under elevated humidity. Nevertheless, the mechanisms governing the distribution of electrochemical potential and the initiation and growth of filaments in FFC remain insufficiently understood. Herein, new microscopic mechanistic insights describing the propagation of corrosion filaments are presented, informed by experimental investigations conducted at varying relative humidity levels on coated steel, as well as numerical simulations. The experimental results demonstrate that FFC manifests exclusively at comparatively high humidity, with no observable FFC at lower humidity thresholds. Raman spectroscopy, scanning Kelvin probe, and scanning electron microscopy/energy dispersive X-Ray spectroscopy (SEM/EDX) are employed to systematically investigate the composition of corrosion products, the distribution of electrochemical potential, and the micromorphology at the filament head. The findings suggest that the reduced O₂ concentration at the filament front is a critical determinant of filament activity. Also, the presence of micro-pits at the leading edge of the filament, which is the cause of carbon steel dissolution and coating delamination, is identified as the principal factor driving filament propagation. Furthermore, the spatial distribution of elemental species within the filament structure is analyzed by SEM/EDX and phase field modeling, indicating that the uneven oxygen flux from the head to tail, which causes the unique “V shape” boundary between the head and tail, is attributed to the denser corrosion products and higher Fe^{3 +} concentration at the center of the tail. The microscopic insights presented offer new perspectives on the prevention and control of FFC.