Solidification Analysis, Microhardness, and Corrosion Resistance of a Cast Hip Prosthesis in ASTM F745 Austenitic Stainless Steel

The solidification of a hip prosthesis has been studied using cooling curves in two differentiated regions: the area near the acetabulum that presents a greater mass and the distal stem area which is the thinnest area of the prosthesis. The differential thermal analysis has shown that the cooling rate, the undercooling and the thermal gradient in the distal stem have been higher due to a higher caloric extraction recorded in this area. As a consequence, a thinner dendritic structure has been obtained, with smaller primary and secondary dendritic spacings. At the same time, it has been observed that the microsegregation of the Cr and Mo elements has been lower in the distal stem. Furthermore, there is a correlation between secondary dendritic spacing and microhardness, which has shown that the dendritic spacing increases, as the microhardness increases. Additionally, corrosion resistance has been evaluated in those regions by means of the cyclic polarization test and electrochemical impedance spectroscopy (EIS). The analyses have been carried out at pH between 5 and 9, and at 37 C and 42 degrees C. The results have shown that the susceptibility to localized corrosion decreased with increasing pH in the evaluated range. The impedance results indicated that hip prosthesis behaves better at 37 degrees C and pH 7, and if this condition is broken, due to an increase of temperature or change of pH, the area near the acetabulum presents lower resistance corrosion.