A Study of Localized Corrosion in Al Resulting From The Controlled Introduction of Cl
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A Study of Localized Corrosion in Al Resulting From The Controlled Introduction of Cl C. M. Johnson, F. D. Wall, J. C. Barbour, M. A. Martinez Sandia National Laboratories P.O. Box 5800 Albuquerque, NM 87185-1056
ABSTRACT High purity Al samples were implanted with 35 keV Cl+ then polarized in Cl--free electrolyte in order to ascertain corrosion behavior as a function of Cl content. Electrochemical data indicated implant fluences between 5x1015 and 2x1016 Cl+cm-2 resulted in little or no localized attack. Implant fluences of 3x1016 and 5x1016 Cl+cm-2 resulted in significant attack with the severity scaling as a function of implant fluence. The low variability in the behavior of localized corrosion for the 5x1016 Cl+cm-2 sample suggests that this implant dosage may result in a critical Cl- in the sample for pit nucleation. Optical and Electron microscopy images indicated some reactivity associated with lower fluences (1x1016Cl+cm-2), possibly indicating enhanced general dissolution. At high magnification the corrosion was observed to occur outside the implanted boundary through mechanisms of tunneling or altered local chemistry at those sites. The SEM images reveal a form of localized corrosion with a crystalline character.
INTRODUCTION Breakdown of the passive oxide layer on an Al surface leads to corrosion of the underlying metal and may lead to failure of electronic and structural components. To gain a thorough understanding of the breakdown process, it is desirable to gain a greater understanding of the fundamental mechanisms influencing the initiation of pits upon the surface. Different factors impacting the initiation process have been hypothesized including Cl ions, which are known to enhance corrosion [1,2]. The presence of other impurities and/or defects in the oxide structure may also influence the initiation. Previous approaches have focused on the relationship between Cl- concentration in solution, and pit nucleation. In these cases, samples were exposed to Cl-containing solution and the degree of pitting was correlated to the Cl- concentration in solution [3-11]. In the present work, a different approach is taken. Cl is intentionally introduced into the samples which are exposed to Cl-free solutions and the degree of pitting is correlated to the amount of Cl actually in the sample: i.e., the oxide is artificially modified and the influence on pit nucleation is determined. Specifically, aluminum oxides were implanted with varying levels of Cl- and the resulting localized corrosion was characterized herein using potentiodynamic experiments and microscopy observations. These experiments were intended to address the following questions: (1) Is Cl- incorporation in the oxide sufficient to cause pit nucleation? (2) Is there a critical Cl- level for localized corrosion? (3) Does ion implantation influence corrosion behavior? Pitting corrosion is a localized form of attack upon the passive surface. To perform tests that initiate corrosion and breakdown, samples are commonly immersed in an electrolyte co
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