Characterization of Corrosion Pit Initiation in Aluminum Using Advanced Electron Microscopy Techniques

Abstract

The resistance to pitting corrosion in aluminum is due to the presence of a compact thin, approximately 5 nm, oxide. Certain conditions locally attack this protective oxide layer leading to its breakdown and resulting in the formation of corrosion pits. Numerous studies have investigated the growth and propagation stages of pitting corrosion yet the initiation stage remains not clearly defined nor well understood. The presence of aggressive chemical species, such as chloride, plays a critical role in the pitting phenomenon and is explored in this investigation.

This dissertation focuses on the localization of pitting corrosion in high purity aluminum in order to accurately predict where and when the pit initiation process will occur so that microstructural changes associated with pit initiation can be easily identified and characterized using electron microscopy. A comprehensive investigation into the corrosion initiation process was attempted utilizing advanced characterization techniques in the transmission electron microscope (TEM) coupled with high-resolution microanalysis. Localization of pitting was successful through use of different sample geometries that reduced the length scale for which pitting events occurred. Three geometries were investigated, each with unique features for pitting corrosion. Electropolished Al needles localized pitting to a sharp tip due to a geometric field enhancement effect, while other experiments employed an Al wire micro-electrode geometry. Both geometries minimized the area where corrosion pits initiated and were electrochemically tested using a solution that contained the chloride species. A third geometry included electron beam evaporated Al films implanted with chloride, which induced pitting corrosion in an otherwise chloride-free environment.

Localization of pitting was successfully achieved using novel sample geometries that isolated the desired stages of pitting corrosion, i.e. metastable pitting, through controlled electrochemical tests. Potentiodynamic pitting experiments were performed on the different sample geometries and advanced TEM was utilized for characterization and microanalysis of the samples both prior to and following polarization. Automated eXpert Spectral Image Analysis (AXSIA) was one technique employed that allowed for spatial resolution of chloride in our material.

Preliminary experiments using the Al needle and micro-electrode geometries aided in defining electrochemical parameters and sample properties. Results from these geometries will be presented. A more in-depth study was performed using the Al thin films. TEM samples were made from the Al films in both cross-sectional and plan-view, which provided more information into the size and distribution of the chloride species. Oxide thickness increased locally prior to pitting when local areas high in chloride concentration were present. Results obtained from advanced TEM characterization and sophisticated microanalysis are presented in this dissertation and provide striking information into sample morphology and structural changes that resulted from electrochemically induced pitting corrosion.