Effect of Relative Humidity on Corrosion of Steel under Sea Salt Aerosol Proxies I. NaCl

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. © The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any This paper is the second of two that seek to further the understanding of the relationship between relative humidity, the hygroscopic behavior of soluble salts, and the corrosion response of mild steel surfaces contaminated with them. Part 1 considered NaCl as a proxy for sea salt aerosol (SSA) in order develop an understanding of a single salt system as the basis for examining more challenging and relevant environments. 1 This paper extends the single salt work to MgCl2, often considered the dominating component of SSA with respecttolowhumidity(<50%RH)corrosion 2‐4 andfurtherextendsthe work to artificial seawater (ASW), which is representative of the inorganic components of natural seawater. The aims of this study were to: (1) quantitatively characterize the relationship between solid-liquid phase transitions of these salts and the corrosion response of steel contaminated with them in terms of RH, and (2) establish the RH and time frame under which there is sufficient electrolyte to cause corrosion. The understanding developed is used to assess the accuracy and appropriateness of the time of wetness parameter for describing corrosion of SSA-contaminated surfaces. The impact of these results on the design of accelerated test regimes and the interpretation offield exposure data are also discussed. Toachievetheseobjectives,boththehygroscopicbehaviorofthese sea salt proxies and the corrosion behavior associated with them were studied. The hygroscopic behavior of ASW microparticles was characterized using an interdigitated electrode (IDE) sensing method. Major solid-liquid transitions were characterized. The phases associated with these transitions were identified by ex-situ analyses of the dried seawater deposits. The corrosion behavior of mild steel contaminated with microparticles of ASW and MgCl2 (separately) was investigated asafunctionofRH.Boththeattackmorphologyandvolumelossrates were quantified. As with the sensor experiments, the dried salts on the corroded steel were characterized; the type and spatial distribution of the minerals were used to better understand the location and nature of theelectrochemicalreactions.Takentogether,theresultsfromthesensor experiments and the steel coupons allow relationships to be drawn between the hygroscopic behavior of these surface contaminants and the corrosion response.