Fatigue behavior in the potentiostatic passive corrosion regime of the iron-base superalloy A-286
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I.
INTRODUCTION
THE occurrence
of corrosion fatigue, in which corrosion and cyclic mechanical loading are involved simultaneously, can seriously degrade material properties. The degradation is often far greater than predicted from a linear summation of corrosion damage and fatigue damage. This has been known for at least 50 years, since the corrosion fatigue experiments in aqueous environments reported by McAdams tL2"31and Moore t4'5] in several steels as well as in nonferrous alloys. Their results clearly showed the difference between "corrosion + fatigue" and "true corrosion fatigue." In general, three broad classes of corrosion processes can occur in the same material under different environmental conditions, and each can lead to very different types of corrosion fatigue crack initiation behavior: (a) active material dissolution (general surface corrosion); (b) localized corrosion (pitting, crevice corrosion); and (c) passive film corrosion (film rupture, repassivation). Of these, the third is usually of most interest, because the formation of a protective passive layer on the metal surface (which can lower the corrosion rate by several orders of magnitude compared to active dissolution) is often desired and encouraged when materials are used in corrosive conditions. There is experimental evidencO 6-~~ that such a film plays a direct and major role in corrosion fatigue of ferrous alloys, and several recent studies have provided baseline data about passivity, composition, fdm growth kinetics, and film thickness for alloys rich in Ni and Cr. [H-tTl Many of these studies involved monitoring of the corrosion current during a corrosion fatigue experiment. They revealed periodic current fluctuations which scale with
M.A. DAEUBLER, formerly Postdoctoral Associate, Carnegie Mellon University, is with MTU-Deutsche Aerospace, Department ELMA, D-8000 Munich 50, Federal Republic of Germany. A.W. THOMPSON, Professor, is with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University, Pittsburgh, PA 15213. I.M. BERNSTEIN, formerly Professor, Carnegie Mellon University, is Chancellor, Illinois Institute of Technology, Chicago, IL 60616. Manuscript submitted May 29, 1987. METALLURGICALTRANSACTIONSA
the applied cyclic load, DsA9,2~ indicating that localized corrosion attack occurs, which can lead to premature crack initiation, compared to tests in air. Such observations have been incorporated into a description of corrosion fatigue crack initiation which is usually designated as the "film-rapture" model, t7,1~ This model proposes that emerging slip steps rupture the passive film under cyclic loading. The now unprotected slip step is exposed to the corrosive environment, and a certain amount of material may be dissolved prior to repassivation. Repetition of this film-rupture repassivation cycle can lead to a small notchlike feature along an emerging slip band. It is the purpose of this study to examine corrosion fatigue, under passive conditions which are electrochemically controlled, as a
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