Elucidating the Effects of Oxide Film on Cathodic Limiting Current Density of Aluminum Alloys with Applications to Atmospheric Localized Corrosion
Authors: Liu Chao , Parker Mary , Repasky Russell , Alshanoon Ali , Srinivasan Jayendran , Kelly Robert G. 
Published: February 2020

Aluminum alloys have highly heterogeneous microstructures that include constituent particles and secondary phase precipitates.1,2 Some of these particles and precipitates are anodic to the Al-matrix, which can lead to localized corrosion in the presence of a conductive solution. In atmospheric environments where a thin layer electrolyte or droplet exists on the alloy surface, localized corrosion can develop and propagate only as long as its anodic current can be met by the the cathodic current outside the corrosion site3. In other words, for susceptible materials the degree of localized corrosion can be controlled by the amount of cathodic current available. There is only a limited amount of literature focused on the cathodic kinetics of aluminum alloys under atmospheric environments.4,5 The current study aims to fill this gap by exploring the key factors that control the cathodic kinetics of aluminum alloys in a simulated atmospheric environment.
Aluminum alloys are always covered by an oxide film due to the high affinity of an aluminum surface for oxygen. However, the impact of this oxide film on the oxygen reduction reaction is unclear. Studies on other alloys6,7 suggest that the oxide film works as a diffusion barrier for oxygen, but there is no quantitative conclusion on the dependence of limiting current density on oxide film in these works. If the oxide film is in fact a diffusion barrier for reacting species, different oxidizing species (oxygen, potassium persulfate, hydrogen peroxide etc.) may interact differently with the oxide film. Oxidizing agents stronger than oxygen are routinely used for accelerated corrosion testing of aluminum alloys, and understanding the interaction between oxidizing species and the oxide film on aluminum would allow better accelerated corrosion test design.
The objective of this work is to elucidate the dependence of oxide film thickness on cathodic limiting current density. The effect of aluminum alloy type on the limiting current density is addressed, and AA7050, 7075, 2024 and 2060 are the alloys of interest in this study. In addition to oxygen, other oxidizers including hydrogen peroxide and potassium persulfate were considered. The first step of this work was to grow different oxide film thicknesses on the aluminum surface through anodization. The details of anodization are described elsewhere8. The film thicknesses were measured using both electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The anodized alloys were then machined to fit the rotating disk electrode configuration (RDE) and used to measurement cathodic kinetics. The RDE technique works by creating a fixed boundary layer to simulate thin layer electrolyte conditions. Three sets of RDE testing were conducted: 1) testing with different rotation speeds for a fixed film thickness on a specific alloy to investigate the effect of water layer thickness on the cathodic limiting current density; 2) testing with a fixed rotation speed for a series of film thicknesses on a specific alloy to illustrate the effect of oxide film thickness; 3) testing in solutions made with different oxidizing agents (oxygen, persulfate, hydrogen peroxide) to explore the impact of oxidizing species on the cathodic limiting current density.

Acknowledgement

This work has been supported by the Office of Naval Research (ONR) Grant N00014-14-1-0012. Mr. William Nickerson, Technical Officer at Office of Naval Research is gratefully acknowledged.

References

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