The Remarkable Improvement of Pitting Corrosion Resistance of Chemically Modified 316L Stainless Steel In Simulated Saline Solution

316L stainless steel (SS) has been used widely for cardiovascular stents due to its excellent mechanical properties and low cost. However, 316LSS is vulnerable to pitting corrosion in physiological solutions so it may release toxic ions such as chromium and nickel ions, or even fail prematurely. 

We studied the corrosion of the thin wire samples of 316LSS in simulated saline solution using electrochemical techniques as well as several in-situ electrochemical microscopic techniques such as optical microscopy, atomic force microscopy and ellipsometry for surface imaging. We found that untreated 316LSS is susceptible to the pitting corrosion in saline solution and has its pitting potential around 350mV (Ag/AgCl). The pitting corrosion of 316LSS in saline solution originates from the metastable pits which are initiated from surface defects such as inclusions. 

A novel chemical surface modification technique for 316LSS has been developed which dramatically improves the corrosion property of 316LSS in saline solution, increasing the film breakdown potential up to 1100 mV. The enhancement of passivity of 316LSS in saline solution is attributed to the removal of inclusions along with the enrichment of chromium oxide on the surface of 316LSS as supported by XPS and AFM measurement results.