The mechanism of group (I) chloride initiated filiform corrosion on iron

The initiation and propagation of filiform corrosion (FFC) on polymer-coated iron following contact with aqueous group (I) chloride salts is investigated using in situ scanning Kelvinprobe (SKP) and ex situ SIMS measurements. Prior to the onset of FFC a circular area of cathodic delamination is observed to grow radially from a penetrative coating defect. The delaminated zone is abundant in group (I) cations, whereas Cl⁻ anions are substantially excluded. As cathodic delamination slows to a halt corrosion filaments initiate at the coating defect and propagate across the delaminated zone at ca. 0.33 μm min⁻¹. Filaments travelling beyond the delaminated zone show only a small (<20%) decrease in propagation rate. It is shown that Cl⁻ is substantially conserved in the filament-head electrolyte, whereas group (I) cations are effectively excluded. When FeCl₂ is used as the initiating salt, a cathodic delamination phase is not observed but FFC occurs as before. The implications of these findings are discussed with respect to the role of cathodic delamination processes in FFC on iron.