Cathodic Kinetics on Stainless Steels in Conditions Relevant to Atmospheric Corrosion
Authors: Alexander Christopher L. , Liu Chao , Katona Ryan , Alshanoon Ali , Carpenter Jacob , Kelly Robert G. , Bryan Charles , Schindelholz Eric John 
Published: February 2020

The objective of this work is to develop an understanding of oxygen reduction kinetics on 304 and 316 stainless steels in concentrated electrolytes relevant to saline atmospheric conditions. Atmospheric corrosion in marine environments typically occurs under concentrated electrolyte films of micron-scaled thickness formed by deposited sea salt aerosols. Under actively corroding conditions, the corrosion rate is ultimately controlled by the available current provided by the cathode, which is predominately the result of oxygen reduction on stainless steel.1 The rate of oxygen reduction under thin electrolyte films is often diffusion controlled, and therefore, is dependent on the physicochemical state of the electrolyte, such as film thickness and oxygen solubility. Relative humidity, temperature and salt load are all environmental parameters that control the state of the electrolyte. Therefore, understanding the impact that environmental parameters have in controlling the electrolyte layer chemistry and, in turn, the rate of oxygen reduction is paramount.

Relevant atmospheric electrolytes may comprise up to 6M NaCl, 10M NaOH, and 5M MgCl2 and be as thin as a few monolayers of water. Existing work studying oxygen reduction on stainless steels is, for the most part, confined to NaCl or NaOH solutions of low concentrations (<1M) and low to moderate rotation speeds (<3600rpm), corresponding to diffusion layer thicknesses greater than 10μm.2,3,4 It has been shown, within this range of thicknesses, that the oxide film can cause a reduction in the limiting current predicted by Levich.5 It is expected that this effect will be even more prevalent in thinner diffusion layers. In solutions of high concentrations, the precipitation of salts may also hinder cathodic kinetics.

In this work, an understanding of the influence temperature, relative humidity and salt load have on the cathodic kinetics will be developed. This will be determined by relating experimental parameters to environmental conditions through knowledge of electrolyte layer chemistry and thermodynamics. Many researchers have studied corrosion kinetics under thin electrolyte films by manufacturing thin layer electrochemical cells in which the electrolyte level over the substrate was controlled within the micron to millimeter range.6 In this work, a rotating disk electrode is used as an alternative. By controlling the rotation speed of the disk, the thickness of the diffusion layer, which is analogous to thin electrolyte layers, is also controlled. The advantages and limitations of the rotating disk system to replicate thin layer atmospheric conditions will be discussed.

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