AbstractBare and varnished steel contaminated by nuclei produce, in humid atmospheres, the type of corrosion characterised by filamental configuration and known as filiform. With some nuclei, corrosion takes place at relative humidities below 60%.Filiform corrosion is characteristic of, and the primary phase in, electrolytic corrosion in atmospheric conditions, but the rate of attack varies from one electrolyte to another and with the degree of humidity. In several cases, corrosion attack has been found to be greater at 70% relative humidity than at 99% relative humidity.The presence of highly hygroscopic salts in the atmosphere can initiate corrosion, even at low relative humidities.Nuclei on the outside of a varnish film will generally start corrosion earlier than those beneath the film, especially if water is present on the surface.