Study of the electrochemical behaviour of SLM and traditional CoCrMo alloys in simulated body fluids

In this work, the electrochemical behaviour of the selective laser melting (SLM) CoCrMo alloy in simulated body fluid was investigated and compared with that of traditional CoCrMo alloy. The microstructure of the studied samples was characterized by SEM/EDS and X-Ray Diffraction. The electrochemical behaviour was investigated during immersion in phosphate buffer saline (PBS) solution at pH 7.4 and at pH 4 with and without the addition of H2O2 at the concentration of 30 mM, in order to simulate both normal and inflammatory conditions. During the exposure to simulated body fluids, electrochemical impedance spectroscopy (EIS) and cyclic voltammetry tests were conducted to study the electrochemical behaviour of CoCrMo alloys. The alloy manufactured with SLM technique showed a very fine and elongated cellular microstructure, characterized by enrichment in Mo at the cell boundaries and suppressed carbide precipitation. Both the SLM and traditional alloys showed low corrosion rates in the studied environments because of the formation of a very protective oxide film on the sample surfaces with high resistance to localized corrosion, but the traditional alloy exhibited the lowest corrosion rates. A comparison of cyclic voltammetry tests carried out on SLM and traditional CoCrMo alloys revealed that, even if the alloys are passive in all PBS solutions, the passive film in the presence of inflammatory species is not as protective as that formed under normal physiological conditions. The CV results showed that the elemental Co, Cr and Mo react with the electrolyte medium and the passive film is mainly composed of Cr2O3 with some amounts of Co(II) hydroxide (only at pH 7.4), MoO2 and likely Mo complex oxides.