Effect of Composition on the Electrochemical Behavior of Austenitic Stainless Steel in Ringer’s Solution

Potentiodynamic cyclic polarization tests on Type 316L stainless steel, a common orthopedic implant alloy, in Ringer’s solution show considerable hysteresis and a protection potential more active than the open circuit corrosion potential. This implies that chances of repassivation of actively growing pits in this alloy are limited. Tests in Ringer’s solution containing hydrochloric acid show that the open circuit potential of Type 316L steel in this solution may exceed in the noble direction the critical pitting potential in the same solution. This signifies that spontaneous breakdown of passivity may occur in a bulk environment which grossly simulates the electrochemical environment within a crevice. Alloying elements such as Mo, Ni, Cr, all improve the corrosion resistance of Type 316L stainless steel in that the critical pitting potential shifts in the noble direction in the alloys having any of the three alloying elements in a higher proportion than in Type 316L steel. Polarization tests in Ringer’s solution on a 20% Cr, 25% Ni, 4.5% Mo, 1.5% Cu austenitic stainless steel, having Mo, Cr, and Ni–all in higher proportions than in Type 316L steel, does not show any critical pitting potential or hysteresis at potentials below that for dissociation of water. However, tests in 4% NaCl solution at 60 C, a more aggressive chloride environment than Ringer’s solution, reveals considerable hysteresis and a very active protection potential, indicating that this behavior is a common feature of austenitic stainless steel in sufficiently aggressive, chloride media.