This paper is the first of two that examine the relationship between relative humidity, the hygroscopic behavior of sea salt aerosol proxies, and the atmospheric corrosion of mild steel contaminated with them. Here, the association between the deliquescence phase transitions (solid-aqueous) of a single salt, NaCl, and the corrosion response of steel contaminated with this salt is examined. Specific focus is given to mechanisms that allow corrosion below the deliquescence relative humidity of NaCl (76% RH). Mild steel loaded with aqueous NaCl drops or crystals (9 mu g.cm(-2)) was subjected to isohurnidity exposures for up to 300 days. The resulting damage was quantified by optical profilometry. The corrosion chemistry that developed was identified using EDS and Raman spectroscopy. The wetting and drying of simulated corrosion chemistries were characterized by impedance measurements across an interdigitated electrode sensor. Taken together, these results show that corrosion can initiate by adsorbed water on NaCl crystals at as low as 33% RH. At 53% RH and above, attack proceeded at rates comparable to that observed above the deliquescence RH due to the development of hygroscopic corrosion chemistry. The governing role of the hygroscopic properties of this chemistry on attack rate and morphology is discussed. (C) The Author(s) 2014. Published by ECS. All rights reserved.