The influence of small amounts (similar to2 mug/cm(2)) of deposited NaCl on the initial atmospheric corrosion of iron has been investigated in situ at varying relative humidities. The deliquescence of the NaCl crystallites and corrosion product formation was followed at relative humidities >75% with quartz crystal microbalance combined with either infrared reflection absorption spectroscopy or optical microscopy. The resulting corrosion attacks initiate at droplets of NaCl solution and form filaments characteristic of filiform corrosion. During the progress of filiform corrosion specific features observed include a constant mass increase rate with time at a given relative humidity, a NaCl-depleted radial zone in front of the active filament head, a chlorine-enriched filament head, and a step-wise growth of the filament. The chloride transport towards the filament head is believed to be mass-transport limited and the filament growth is driven by a differential aeration cell within the filament. (C) 2004 The Electrochemical Society.