The effect of Mn-depleted surface layer on the corrosion resistance of shape memory Fe-Mn-Si-Cr alloys
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I.
INTRODUCTION
THE Fe-based shape memory alloys (SMAs) have undergone a significant development in recent years.[1–6] These low cost materials have large significant applications in different fields, i.e., as thermal alarm actuators, as coupling systems for underground pipes, and in general for many applications requiring materials having a high damping capacity. One of the major problems that limits the use of these alloys is the low corrosion resistance due to the presence of a high percentage of Mn. This element is essential for obtaining a high performance SMA. For the alloy of interest, Cr has been added in order to stabilize the surface. But we have experimentally verified that there is no improvement with respect to these alloys deficient in chromium. In this work, we present an alternative method for giving corrosion resistance without modifying the basic composition of the alloy. It consists of reducing the Mn content in the surface layer by up to less than 50 pct of its nominal value by means of an air oxidation process at high temperature. Oh et al.[7,8,9] have found preferential oxidation of Mn in Fe-Mn-Si and Fe-Mn-Ni-Si alloys having a Si content of 2 pct. They indicate that the selective oxidation results in the growth of a Mn-depleted layer under the oxide scale. We have verified a similar behavior in an alloy with a higher content of silicon and containing chromium. The concentration of Mn drops gradually across the depleted layer and reaches a minimum at the scale-metal interface. The selective removal of Mn from the alloy results in the transformation from fcc (austenite) to bcc (ferrite) structure. The mechanism for preferential oxidation of manganese can be summarized as follows. The Mn and Si oxides are more stable than the oxides of the other elements present in the alloy. In the oxide scale forming on the alloy, the Mn cations are more mobile of the Si4+ and O22 ions.[7] G. COCCIA LECIS, Professor, C. LENARDI, Researcher, and A. SABATINI, Postdoctoral Student, are with the Department of Applied Physical Chemistry, Politecnico I Milano, I-20133 Milano, Italy. Manuscript submitted July 16, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
Thus, a layer of Mn oxides growths over a layer of mixed oxides of Si, Mn, Fe, and Cr at the time of oxidation process. The thin silicate layer is almost entirely amorphous. It forms at the interface scale alloy. This layer limits the migration of all the other elements except for Mn, which continues to diffuse toward the outer surface. Thus, the spalling scale is formed by a crystalline part of Mn oxides and by an amorphous part of silicates. At a constant temperature T, steady-state conditions are reached at the interface between the crystalline zone and the amorphous zone, between the amorphous zone and the depleted layer, and between the depleted layer and the bulk. As the scale is removed, the Mn-depleted layer reaches conditions of passivity in weakly aggressive environments. We have also verified that this depleted layer does not substantially affec
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