(Corrosion Division Morris Cohen Student Award) Impact of Salt Deliquescence on the Humidity-Dependence of Atmospheric Corrosion

In atmospheric environments, corrosion is often approximated as a discontinuous process reliant on the availability of electrolyte to provide ionic conduction between cathodic and anodic sites. A critical question in atmospheric corrosion is when can a surface be considered wet enough for corrosion to occur at a considerable rate? For surfaces contaminated with hygroscopic salts, such as those which comprise atmospheric sea salt particles, the relative humidity (RH) associated with the deliquescence of these salts is often assumed to serve as the wet/dry threshold in this regard. Deliquescence is the bulk phase transformation whereby solid salt spontaneously absorbs water to form an aqueous electrolyte at and above a specific RH, the deliquescence RH (DRH).

In this work, we examined the relationship between the hygroscopic phase transitions of NaCl, MgCl₂ and ASTM artificial seawater (ASW) and the response of mild steel contaminated with these sea salt simulants at the initial stage of corrosion. The hygroscopic behavior of salt microparticles of each composition, similar in size to natural atmospheric aerosol, was characterized by impedance measurements across an interdigitated electrode sensor. The corrosion behavior of mild steel loaded these microparticles was investigated as a function of RH under isohumidity, room temperature conditions.  Both the attack morphology and volume loss rates were quantified using optical profilometry. Ex-situ characterization of the dried salts on the corroded steel and sensor were carried out using SEM/EDS and Raman spectroscopy.

Trends in corrosion loss versus RH were not directly reflective of the deliquescence phase transitions observed or predicted for the salts examined.  Considerable corrosion was observed below the DRH of the salts and attributed to (1) the presence of non-equilibrium electrolyte due to kinetic limitations during drying, (2) alteration of the deliquescence behavior by the corrosion chemistry and (3) adsorbed electrolyte at the solid salt/metal interface.

For NaCl, sustained corrosion was observed down to 33% RH for up to 300 days. At 53% RH and above, attack proceeded at rates comparable to that observed above the DRH (76%) due to the development of hygroscopic corrosion chemistry. There was no significant difference in attack depth and volume loss trends between samples that were originally loaded with droplets and those loaded with NaCl crystals prior to isohumidity exposure. The initiation of corrosion under NaCl deposits well below the DRH can be explained in terms of the presence of adsorbed water associated with the salt crystals in contact with the steel.

For MgCl₂ and ASW, sustained corrosion was detectable down to 11% and 23% RH, respectively, for up to 30 days. This is well below the DRH of MgCl₂ (33%), which is often considered the active component of sea salt mixtures at low humidity levels. Significant admittance was measured across ASW and MgCl₂ deposits on the electrode sensors at < 2% RH after 24 hours, likely due to trapping of supersaturated MgCl₂ fluid under solid salt crusts. Trends in attack morphology and volume loss above 33% RH of ASW-loaded samples were similar to those loaded with NaCl alone. This may be explained by the similar corrosion chemistry that developed on the NaCl and ASW samples.

Together, these results show that DRH of an individual salt contaminant for the systems under study did not serve as the delineation between a wet and a dry surface, nor as the boundary between significant and insignificant corrosion attack. The impact of these findings on humidity-dependent atmospheric corrosion models along with translation to understanding atmospheric corrosion of other alloy systems will be discussed.