Study of Single Al Corrosion Pit Growth By Electrochemical Techniques

Abstract

Electrochemical techniques are commonly used for the study of localized corrosion growth [1]. However, pit growth at open circuit is hard to study by electrochemical techniques because of its difficulty of measuring current at open circuit while pit growth under potentiostatic or galvanostatic condition may be totally different from the open circuit. For stable Al pit growth, it is widely accepted that local pit chemistry and environment are critical; or a critical value of the stability product (x.i), where x the pit depth, and i the current density, must be maintained for pit survival [2-6]. Several techniques have been proposed for studying pit growth rates, including (1) Use of artificial pit (“lead-in-pencil”) electrode[7]; (2) Single pit formed by implantation[8]; laser irradiation[9]; (3) Think film electrode [10] ; (4) The foil penetration technique [11]. In this work, we propose to employ a single Al wire (~100um dia.) mounted in epoxy with different initial pit depth d, which is produced by anodic dissolution, for singe Al corrosion pit growth by coupling with a large Al foil ring as its remote cathode through a zero resistance ammeter (ZRA), as schematically shown in Fig.1. The purpose of this study is, in combined with in-situ scanning vibrating electrode technique (SVET, Fig. 2), to determine quantitatively whether the single Al corrosion pit growth follows a depth-dependent growth model as often predicted. Acknowledgements This work was performed under United Technologies Corporate R&D funding. Fig. 1 Schematic geometry of single al corrosion pit formed by an Al wire mounted in epoxy, by coupling with a large Al foil ring as its remote cathode through a zero resistance ammeter (ZRA). Fig. 2 In-situ SVE images of single Al corrosion pit growth, showing transition from (a) open circuit condition (before coupled) to (b) galvanically coupled condition. References G. S. Frankel, Corros. Sci., 30, 1203 (1990). J. R. Galvele, J. Electrochem. Soc., 123,464 (1976). J. R. Galvele, Corros. Set, 21, 551 (1981). Streblow H.H., Ives M.B., Corr.Sci. 16, 317 (1976). T.R. Beck, Electrochimica Acta, 29, 485 (1984). S.T. Pride, J.R. Scully J.R., Hudson J.L., J. Electrochem. Soc. 141, 3028(1994). H. S. Isaacs and R. C. Newman, Corrosion Chemistry within Pits, Crevices and Cracks, A. Turnbull, Editor, HMSO Books, London (1987). K. P. Wong and R. C. Alkire, J. Electrochem. Soc., 137, 3010 (1990). D. W. Buzza and R. C. Alkire, J. Electrochem. Soc., 142, 1104 (1995). G. S. Frankel, Mat. Sci. For., 247, 1 (1997). F. Hunkeler and H. Bohni, Corrosion (Houston), 37, 645 (1981). Figure 1