A transmission electron microscopy study of constituent-particle-induced corrosion in 7075-T6 and 2024-T3 aluminum alloy
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I.
INTRODUCTION
LOCALIZED (pitting) corrosion has been observed in high-strength aluminum alloys in aqueous environments (electrolytes) and has been identified as a potential origin for fatigue crack nucleation.[1–4] The combination of pitting corrosion and subsequent fatigue crack growth can significantly reduce the lives of structural components made of these alloys in service. Mechanistic understanding of pitting in aluminum alloys and quantification of its kinetics, therefore, are of scientific interest and technological importance. In previous studies, pitting in 7075-T6 and 2024-T3 aluminum alloys has been identified by optical microscopy (OM) and scanning electron microscopy (SEM) with galvanic coupling between the matrix and the constituent particles in these alloys.*[1,2] In particular, severe pit*The term ‘‘constituent particles’’ is used to designate the insoluble, undissolved, or precipitated coarse particles that are formed and distributed heterogeneously in aluminum alloys from impurity elements, excess alloying elements, or improper heat treatment.
ting has been attributed to the successive interactions between the alloy matrix and constituent particles in clusters.[5,6] In this study, a more direct observation of the nature and extent of particle-matrix interactions in an electrolyte was made with the aid of transmission electron microscopy (TEM). Traditionally, the fundamental cause of pitting corrosion is attributed to the local breakdown of passivity of the surface film that forms over a metal surface, which permits subsequent local dissolution of the metallic substrate.[7,8,9] The resistance to pitting is believed to be determined by the electrochemical stability of the ‘‘protective’’ passive
ROBERT P. WEI, Professor, is with the Department of Mechanical Engineering and Mechanics, Lehigh University, Bethlehem, PA 18015. CHI-MIN LIAO, Associate Scientist, is with China Steel Corporation, P.O. Box 47-29, Hsiao Kang, Kaohsiung 81233, Taiwan, R.O.C. MING GAO, Engineering Advisor, is with Mobil Exploration & Producing Center (MEPTEC), Material, Corrosion and Inspection Group, Farmers Branch, TX 75381-9047. Manuscript submitted July 1, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
film and the ability of the film to repair itself in the electrolyte.[9] The propensity for pitting for a given metal-electrolyte system is characterized by its critical pitting potential (Ep), an electrochemical measure for the instability of the passive film.[8,10,11] This approach is macroscopic and implicitly assumes homogeneity for the underlying metals. Nevertheless, extensive efforts have been made to study the effects of alloying elements, temperature, and electrolyte chemistry on Ep for aluminum and its alloys.[12] For engineering aluminum alloys (such as 7075-T6 and 2024-T3), however, the critical pitting potential per se would not serve as an adequate measure of pitting resistance because of microstructural heterogeneity at the alloy surface. These alloys generally contain significant amounts of constituen
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