Towards Understanding Surface Wetness and Corrosion Response of Mild Steel in Marine Atmospheres

Atmospheric corrosion is often approximated as a discontinuous process reliant on the availability of electrolyte to provide ionic conduction between the anode and cathode sites. A critical question is, when can a surface be considered wet enough for corrosion to proceed at a considerable rate? In marine environments, where sea salt aerosol is the dominant surface contaminant, the deliquescence (solid-liquid) phase transitions of their inorganic constituents are often utilized as guidepost thresholds between wet and dry. There is little direct information, however, that supports the universality of this contention, nor is the relationship between relative humidity (RH), the wetting and drying behavior of sea salt aerosol, and the corrosion response of metallic surfaces well understood. It was the aim of this work to explore this issue as it relates to mild steel at the initial stages of corrosion. To do so, the hygroscopic behavior of sea salt proxies consisting of both singular and mixed salts along with expected resultant corrosion chemistries were characterized using a novel interdigitated electrode sensing method. Complementary experiments were carried out to quantify corrosion response and characterize the evolution of surface chemistry of mild steel subjected to a range of isohumidity conditions under these salts. The results dramatically impact the view of the fraction of time soluble salt contaminated surfaces can be considered wet to support corrosion. In particular, use of the deliquescence phase transitions of salts as delineation of the RH above which sustained corrosion is possible is shown to be inappropriate and, in some cases, highly inaccurate. Mechanisms and physical phenomena that can initiate and sustain corrosion below the deliquescence point are discussed. Furthermore, evidence is presented that brings into question whether surfaces ever dry in ambient marine conditions. The implications of these results are addressed in the context of time of wetness determination methods, accelerated testing and service life prediction models.