The influence of pore morphology on corrosion

Porous titanium parts fabricated powder-metallurgically are known to have pore morphologies that range from large, open, interconnected pores to small, isolated pores. Although the latter geometry is synonymous with highly dense and mechanically strong materials, it conceivably encourages stagnation of electrolyte and prevents its free flow crucial to the ion incorporation/titanium release process of passivation. In this case, the depletion of oxygen leads to crevice corrosion which further increases the corrosion rate of these large-surface-area systems. To investigate the localised corrosion resistance of porous titanium (porosity 10-30%, pore size 50-100 mu m), cyclic polarisation tests were performed in saline solution at 37 degrees C, on porous purl titanium compacts with varying porosities and pore sizes, solid titanium and 316L stainless steel. As expected, 316L stainless steel exhibited poor corrosion resistance compared to titanium. A distinct hysteresis was observed in the reverse scan of the former which indicates the weak resistance to crevice and pitting corrosion of this well-established metallic biomaterial. The main reason for the excellent corrosion resistance of the solid and porous titanium compacts is the strong affinity of the metal to even a slight trace of oxygen in an aqueous environment, resulting in a highly resistant and self-regenerating passive film. However, the cyclic polarisation studies also showed that the final E-corr value reduces with decreasing porosity. This is due to the small, isolated pore morphology that traps ionic species and exhausts the supply of oxygen. In highly porous compacts with an open, interconnected pore morphology, the free flow of species resulted in a material with much higher resistance to pitting. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.