Enhancement of the corrosion properties of selective laser melted Co-Cr-Mo alloy by selective oxidation annealing

Abstract

This study investigated the surface modification effect of the selective oxidation annealing (SOA) on the corrosion properties of cobalt-chromium-molybdenum (Co-Cr-Mo) alloy fabricated by selective laser melting (SLM). For the SOA, each element's oxidation driving force was calculated, and the condition in which only Cr was selectively oxidized was thermodynamically analyzed. The oxidation potential was controlled by the partial pressure ratio of hydrogen gas and water vapor, and the alloy samples were annealed following the condition of the selective oxidation of Cr. The changes in the oxide layer and corrosion properties according to the SOA time were examined. On the as-built sample's surface, a very thin native oxide film was formed. After the SOA, a dense chromium oxide (Cr2O3) layer was formed on the surface, and its thickness increased along with the SOA time. These changes in the oxide layer had an impact on corrosion properties. Even though all samples before and after the SOA were under the passivation state, the corrosion performances in terms of corrosion potential and corrosion current density were improved by the SOA. The electrochemical impedance spectroscopy (EIS) results further confirmed that the improved corrosion resistance mainly came from the increased oxide layer resistance (Ro) because of the thickened oxide layer.