Corrosion behaviour of SLM CoCrMo alloys in solutions reproducing biological fluids

摘要

CoCrMo base alloys are widely used for the production of orthopaedic implants because of their excellent mechanical properties and the high corrosion resistance in biological fluids. Despite the advances in this sector, still untimely failures and dissatisfaction are observed in patients, due to many reasons, such as mismatch between the joint prosthesis and the natural joint conformation, non-physiological load transfer and scarce osseointegration. The fabrication of customized implants by Selective Laser Melting should permit to overcome some of these problems. In fact, through appropriate production strategies, with this technique it is possible to realize complex shapes with modulated structure and controlled porosity. However, some parameters related to the process optimization in terms of final density and mechanical properties are critical and the evaluation of their influence on corrosion resistance of the components is reputed fundamental. For these reasons, SLM technique was used to produce two types of samples based on ASTM F1527 / F75 CrCoMo alloy and obtained by using different process parameters. The corrosion behaviour of the two materials was investigated by recording the EIS spectra and the polarization curves during 15 days of immersion in Phosphate-Buffered Saline (PBS) solutions at pH 7.4 and 4.0, in the absence and in the presence of H_2O_2, in order to simulate the compositions of body fluids under normal or inflammatory conditions. The microstructure of the samples and the surface composition before and after exposure to the aggressive solution were studied by means of SEM / EDS, X-ray diffraction and FTIR spectroscopy. The samples manufactured with SLM technique showed a very fine and elongated cellular microstructure, characterized by enrichment in Si and Mo at the cell boundaries. Furthermore, the high cooling rate and the high intrinsic temperature gradient, typical of the SLM process, produced non-equilibrium structural conditions, with suppressed carbide precipitation. Both types of samples showed low corrosion rate in the studied environments because of the formation of a very protective oxide film on the sample surfaces with high resistance to localized corrosion.