Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies: II. MgCl2, Artificial Seawater

Abstract

This paper is the second of two that examines between the hygroscopic behavior of sea salt aerosol proxies and the atmospheric corrosion of mild steel contaminated with them. In this work, artificial seawater (ASW) and MgCl2 were examined. The wetting and drying behavior of ASW was characterized by impedance measurements across an interdigitated electrode sensor. Steel coupons loaded with ASW and MgCl2 microparticles were subjected to isohumidity exposures for up to 30 days. The resulting damage was quantified by optical profilometry. The corrosion chemistry that developed was identified using EDS and Raman spectroscopy. Together, the results bring into question whether sea salt-contaminated surfaces ever dry in ambient outdoor environments. Sustained corrosion was detectable down to 11% RH for MgCl2 and 23% RH for ASW, with significant admittance of ASW deposits at <2% RH after 24 h, likely due to fluid trapping under a solid salt crust. Trends in corrosion loss versus RH were not directly reflective of the major liquid-solid phase transitions observed or predicted for ASW or MgCl2 alone. In light of this, common time of wetness determination methods are contended to be fundamentally flawed as quantitative indicators of electrolyte presence and the potential for significant corrosion.