J. BIOMED. MATElt. 1tKS.

VOL. 9, PP. 259-264 (1975)

The Concept of Protection Potential Applied to the Corrosion of Metallic Orthopedic Implants J . It. CAHOON, R. B~ANDYOl'ADHYA, and LAURA TENSESE, Metallurgical Science Laboratory, Department of Mechanical E+ ng weer' i n g , lrniuersity of Maniloba, W i m i p e g , Ma?ritoba, Canada

Summary Cyclic polarization curves obtained for a cobalt-base casting alloy (similar to Vitallium Surgical Alloy) and for 316L stainless steel show that the cobalt-base alloy exhibits a high (noble) protection potential whereas the stainless steel exhibits a low (active) protection potential. It is suggested that the resistance to crevice corrosion of Vitallium Surgical Alloy results from its high valtie of protection potential whereas the susceptibility of 316L stainless to severe crevice corrosion results from its low value of protection potential.

INTRODUCTION At present, general corrosion of mc%allic orthopedic implants is not a problem except in cases where the implant has received improper heat treatment.' However, localized corrosion, particularly crevice corrosion (corrosion occurring between screws and plates or other contacting surfaccs) occurs in almost all multicomponent 316 stainless steel orthopedic devices,2 and this type of corrosion can cause sufficient clinical reaction t o necessitate removal of the On the other hand, cobalt-chromium implants (e.g. Vitallium Surgical Alloy*) do not appear t o suffer from crevice ~ o r r o s i o n . ~The reason for this difference in behavior between 316 stainless steel and cobaltchromium alloys is not evident from published elrctrochemical studies in that the polarization behavior of Vitallium Surgical Alloy and 316 stainless steel is very similar (i.e. rest potential, breakdown potential, and passive current density) for both alloys during in vitro tests in Ringers physiological solution.6 *Vitalhum i s a registered trademark of Howemedica, Inc. 259

@ 1975 by John Wiley & Sons, Inc.

CAHOON, BANI)YOPAl)HYA, AN11 TICNNKSE

260

PITTING INITIATES AND PROWGATES

BREAKDOWN OR CRITICAL POTENTIAL, E,

-

PROTECTION POTENTIAL E

FLADE OR ACTI’ATION-

ACTIVE

P

EXISTING PITS ARE PASSIVATED

t

L CURRENT DENSITY

Fig. 1. Schematic cyclic polarization CIII.VR.~

However, recent studies',* have illustrated the importance of the “protection potential” in characterizing the susceptibility of an alloy to pitting or crevice corrosion.* The protection potential is defined as the potential a t which the current density becomes equal to th(1 passive current density during a cyclic polarization test.? A schrmatic cyclic polarization curve illustrating the protection potential and the suggested corrosion behavior a t various potentials is givcn in Figure 1.’ Wild@ has shown that the stverity of crevice corrosion increases with increasing hysteresis of the cyclic polarization curv(1 and that the weight loss due t o crevice corrosion is approximately linear to the difference between the brcaakdown potential and the protection potential ( E , - EP). In the present study, cyclic polarization curves were pcrformc3d for 316 L stainless steel and a cast cobalt-chromium alloy in order to determine the respective protection potentials and to determine if this type of study can assist in providing an explanation for the difference in crevice corrosion behavior of the two types of alloy. * I t is now generally accepted that pitting corrosion and crevice corrosion occur via the same rnechanism.’~9

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The concept of protection potential applied to the corrosion of metallic orthopedic implants.

Cyclic polarization curves obtained for a cobalt-base casting alloy (similar to Vitallium Surgical Alloy) and for 316L stainless steel show that the c...
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