Microfabrication of Crevice Corrosion Samples
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Microfabrication of Crevice Corrosion Samples Xiaoyan Wang, Robert G. Kelly1, Jason S. Lee1, and Michael L. Reed Department of Electrical Engineering, University of Virginia, Charlottesville, VA 22904-4743, U.S.A. 1 Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904-4745, U.S.A. ABSTRACT A major challenge in developing computer models for crevice corrosion lies in fabricating appropriate experimental crevice samples. The geometry and dimensions of these samples must be controlled to a high order of precision in order to be amenable for comparison to computational models. In this work we report an effort to construct crevice samples with rigorously defined dimensions by using microfabrication techniques developed for microelectromechanical systems (MEMS). These techniques include microfabrication with SU8, electroplating, and other standard semiconductor device fabrication techniques as well. The crevice substrates contain one-dimensional arrays of metal electrodes to be studied, which are isolated by walls of SU-8. The electrodes have individual electrical connections so that spatial information of the in-situ corrosion process can be obtained. The crevice formers with SU-8 posts were coupled to crevice substrates to maintain a uniform crevice gap. Further, crevice formers with regular rectangular subcrevices were fabricated to study the roles of subcrevices in crevice corrosion. INTRODUCTION Crevice corrosion is one type of localized corrosion that occurs over only a small percentage of the total surface area of metals. Figure 1 defines the geometric aspects of a crevice. The crevice substrate is the metallic material of interest. The crevice former may be any type of material that is close enough to the surface of substrate to form an occluded region (crevice). The gap between the crevice former and substrate is called the crevice gap (g), and the length of the crevice former is called the crevice length (L). Inside the crevice, corrosion is accelerated, leading to severe damage that results in both high repair costs and potential safety problems. Considerable experimental work has been done on crevice corrosion of metals in a variety of environments, and both conceptual and computational models have been built based on these results. However, there are difficulties in the construction of a comprehensive, accurate, and reliable model. The first challenge is experimentation on the relevant size scale. The dimensions L
Crevice former Boldly exposed surface
g
Crevice substrate (metallic) Crevice corrosion Figure 1. Schematic of cross section of a crevice (not to scale)
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of real crevice gaps are on the order of 0.1–100 µm, with lengths of 1–10 mm. It is difficult to reproducibly fabricate crevices on this size scale using conventional techniques. The second obstacle is dimensional variation within crevices. Computer models assume that crevices have ideal geometries, with perfectly vertical walls and uniform crevice gaps, whereas practical crevices do not ha
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