Marine Aerosol Drop Size Effects on the Corrosion Behavior of Low Carbon Steel and High Purity Iron

Abstract

This research focused on the development and use of a novel methodology to study corrosion under aqueous electrolyte drops ranging in diameter from 20-1000 μm, the most common deposited aerosol size range found in coastal marine environments. A drop-on-demand inkjet printer was custom built by coupling a commercial jetting device with a programmable two-axis stage. Singular droplets of sodium chloride solution were deposited onto steel and pure iron substrates with various surface finishes, followed by an isohumidity exposure for 24 hours. The volume loss increased with drop diameter according to a power law with an exponent of approximately 2 for highly polished steel and iron, increasing to 3 for iron with higher surface roughness. Attack was primarily filiform-like for the smallest drops, whereas deep, separate pits formed under the largest drops. Corrosion was not consistently observed under all drop sizes. The fraction of drops that showed no corrosion increased with decreasing drop size for all substrates. This behavior appears to have a strong dependency on the microstructure and surface finish. Corrosion initiation on 1010 steel was dominated by manganese sulphide inclusions when a mirror surface finish was maintained. For high purity iron, initiation was dominated by surface roughness. In marine environments, corrosion is generally driven by the deposition of hygroscopic sea salt aerosols. These aerosols range from a few angstroms in diameter to several hundred microns. In coastal marine locations (<2 km from the beach), the most common aerosols deposited are in the 'coarse-mode' size range: 1-100 μm in diameter. 1Laboratory atmospheric corrosion studies and a number of accelerated tests, however, have traditionally focused on corrosion under large drops (several millimeters in diameter) or thin films.From studies on large droplets, a pitting mechanism has been proposed. An aqueous droplet in contact with a metal surface creates a localized electrochemical cell -often called an "Evans drop". In the classic Evans drop model, the cathode forms at the drop edge, where oxygen is readily available due to the short diffusion path length. 2The anode is then established in the center of the drop where oxygen is deficient and iron dissolution is sustained. This cell results in the formation of a pit at the center of the drop. This type of behavior has been commonly observed for a variety of alloy systems that are susceptible to corrosion (e.g., low carbon steel) for drop sizes in the microlitre size range. [3][4][5][6] Although corrosion behavior under 'coarse' droplets has received less attention, there is a growing body of evidence that suggests that it can deviate from Evans' drop behavior, and that corrosion may not appreciably occur under drops on the lower end of the size range for plain carbon steel (<50 μm).It has been recently reported that the corrosion rate appears to be a function of deposited aerosol drop size under field and laboratorysimulated marine conditions. 7-9 Furthermore, several studies ha...

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