Effect of the sigma (σ) phase in Co-29Cr-6Mo alloy on corrosion behavior in saline solution has been investigated. The area fraction of the σ phase contained in Co-29Cr-6Mo alloy varies depending on the aging time at 1023 K. The σ phase is mainly observed at grain boundaries. The area fraction of the σ phase increases with increasing aging time and reaches 0.6% after aging at 1023 K for 21.6 ks. The Co-29Cr-6Mo alloys aged at 1023 K for various aging times were immersed in saline solution at 310 K for 1 week and metal ions released from the alloys were examined by inductively coupled plasma atomic emission spectrometer (ICP-AES). The quantity of a released Co ion does not depend on the aging time and shows almost the same value regardless of the different area fraction of the σ phase, while that of released Cr, Mo and Ni ions increases with increasing area fraction of the σ phase. Results of polarization test in saline solution at 310 K revealed that passive current density and breakdown potential of Co-29Cr-6Mo alloys aged at 1023 K for various aging times exhibit almost the same values, although the alloys have different area fraction of the σ phase. These results suggest that a small amount of the σ phase (<0.6%) hardly affect the formation and breakdown of passive film in the Co-29Cr-6Mo alloy aged at 1023 K.
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