A scanning electrochemical microscopy characterization of the localized corrosion reactions occurring on nitinol in saline solution after anodic polarization

Scanning electrochemical microscopy (SECM) in combined amperometric/potentiometric operation was employed to characterize the electrochemical activity of nitinol biomaterial, prior and after anodic treatment, in 0.1 M NaCl solution. SECM operation in the feedback mode proved that the nitinol surface was homogeneously passive following surface finishing and storage in ambient condition, whereas heterogeneous surface characteristics occurred after the application of anodic polarization even for a limited time. That is, the development of anodic and cathodic sites owing to the onset of localized corrosion processes was detected on the metal surface. Hydrogen gas evolution from localized sites was monitored using SECM in the substrate generation/tip collection mode (SG/TC), whereas SECM operated in potentiometric mode was used to map the pH distribution in the electrolyte volume adjacent to the nitinol surface. Local acidification around anodic spots related to Ni2+ discharge, as well as alkalization above the cathodic areas were observed.