Sequence of Steps of Nitinol Pitting in Phosphate Buffered Saline Solution

The effect of non-metallic inclusions on the pitting sequence of nitinol was investigated in the present study.  The pitting was explored directly using polished metallographic samples in phosphate buffered saline solution at 37°C following ASTM F2129.  The results were compared to those of standard electropolished samples from the same material lot.  One of the benefits of this method is that the non-metallic inclusions can be characterized directly from the tested samples.  It was found that the crevice corrosion is the dominant failure mode, i.e., pitting always initiates from the edge of polished samples since a crevice cannot be totally avoided between the sample and mounting epoxy.  After the edges were masked during test, the corrosion resistance of polished samples was similar to that of electropolished counterparts.  Once the pit encounters an inclusion during propagation, the inclusion is attacked first and a crevice corrosion condition is generated, which further corrodes the samples.  Additionally, both TiC and Ti₄Ni₂O_x particles were found in the pits.  This further confirms that both types of inclusions have similar effect on the pitting sequence of nitinol.