Corrosion-resistant alloys in chloride solutions: materials for surgical implants

The behaviour of a number of corrosion-resistant alloys in chloride solutions, in Hanks’s physiological solution (simulating the extracellular body fluids) and as surgical implants has been investigated by electrochemical means, namely potential-time curves for isolated specimens, potential-current density curves for anodes, and current density-time curves for anodes maintained electronically at constant potential. Microscopical observation of pitting attack has also been made. Alloys based on iron (e. g. stainless steels), nickel (e. g. Inconel, Nimonic 75, etc.), cobalt (Vitallium), titanium and tantalum exposed to chloride solutions all show a range of potential in which they are passive, and, at sufficiently high chloride concentration and sufficiently positive potential, breakdown giving rise to pits that are electrobrightened. This general phenomenon occurs, in 0·17M sodium chloride solution, at 0·2 to 0·5 V (normal hydrogen scale) for stainless steels, ca. 0·9 V for the cobalt·based alloys, and ca. 20 to 30 V for certain titanium alloys and tantalum. In the passive range, all the alloys show anode current densities in the range 10-6 to below 10-9 A/cm2, the smaller current densities given by the most passive alloys (e. g. titanium-5 % niobium) often tending to decrease yet further with passage of time. We conclude that stainless steels (even of the higher chromium-nickel quality) and nickel alloys are unlikely to resist all breakdown by pitting when exposed to the body fluids (or other media containing chloride) indefinitely; that the cobalt-based alloys may well withstand such exposure for very long times; and that titanium and (especially) some of its alloys should withstand such exposure for an indefinite period. The (extremely slow) passage of cobalt and titanium into the environment is caused by passage of cations through their passivating oxide films, without breakdown.