Marine Aerosol Drop Size Effects On The Corrosion Behavior Of Plain Carbon Steel

This research focused on the development and use of a novel methodology to study corrosion under aqueous electrolyte drops ranging in diameter from 20-1000μm, the most common aerosol size range found in coastal marine locations. The objective of this work was to examine the relationship between drop size and corrosion attack of bare low carbon steel under simulated atmospheric marine conditions. It has been recently reported in the literature that corrosion rate appears to be a function of deposited aerosol drop size; however, empirical data are insufficient to establish a strong, quantitative correlation between drop size and corrosion rate. For this work, a drop-on-demand inkjet printer was custom built by coupling a commercial jetting device with a programmable two-axis stage. Singular droplets of aqueous sodium chloride were deposited on polished 1010 steel substrates followed by exposure to 75% relative humidity for 24 hours. It was found that, in general, the volume loss increased with drop diameter according to a power law. Various attack morphologies were observed; attack was primarily filiform for drops 20-100μm in diameter whereas deep, separate pits formed under drops 300-1000μm in diameter. Filiform attack is characterized by shallow pits that occur under and travel outside of the initial drop area. Corrosion always occurred for drops greater than 150μm in diameter, but was not consistently observed under drops in the 20-100μm range. The fraction of drops that showed no corrosion increased with decreasing drop size below 100μm in diameter. However, drops smaller than 100μm that corroded were found to have the greatest volume loss compared to their salt loading density. The results from this study suggest that the manner and geometry in which salt is deposited onto a surface will significantly affect corrosion loss along with the mode of attack. From a technical standpoint, these results suggest that service life prediction approaches including accelerated testing have the potential to more accurately simulate natural atmospheres if electrolyte geometry is taken into account.