Pit Initiation On Electropolished Nitinol

Various alloys such as stainless steels contain nonmetallic inclusions, and these inclusions can act as sites for pit initiation.  There is some evidence that pitting of electropolished (EP) nitinol is likewise associated with inclusions.  The objective of this study was to investigate pit initiation on EP nitinol, particularly with regard to the type of inclusions present at initiation sites.  Potentiodynamic polarization tests were performed on EP nitinol in phosphate-buffered saline.  The maximum potential was limited to 0.4 V (SCE) to restrict the extent of pitting and possible loss of any inclusions at pit initiation sites.  Even so, the pits typically grew to a relatively large size, and not surprisingly, any inclusions were no longer present.  However, in a few instances, the pits were found to have retained inclusions, and analysis using energy dispersive spectroscopy indicated that the inclusions were likely carbides and oxidized carbides.