In vitro corrosion fatigue of 316L cold worked stainless steel Masayuki Taira* and Eugene P. Lautenschlagert Division of Biological Materials, Northwestern University, 311 E. Chicago Ave., Chicago, lllinois 60611 The corrosion resistance of 316L cold worked stainless steel depends upon its thin protective oxide layer; and if this is partially broken down, corrosion resistance depends upon its tendency for repassivation. Since the intended function of stainless-steel implants is to sustain musculoskeletal forces, research toward the stability of the oxide film during dynamic loading i n simulated bodylike fluids is warranted. A pilot corrosion fatigue study was, therefore, performed on uniaxial tension fatigue specimens cycled to various maximum stress levels near their yield point while immersed in 37°C isotonic saline solution, and combined with the electrochemical insult of (a) imparting a n 800 mV vs. SCE anodic potential for 20 s to stimulate local film
breakdown, and then (b) returning to a constant 200 mV vs. SCE anodic potential and maintaining that potential during cyclic loading until the specimens broke in two. During the anodic polarization by continuously monitoring the current it was possible to (a) observe the repassivation and corrosion behavior following stimulation, and (b) detect crack initiation, crack propagation and failure onset. The combined effects of accelerated corrosion and mechanical f a t i g u i n g disturbed the repassivation tendency and reduced the crack initiation times and the fatigue lives as compared to air and saline controls. A s the maximum cyclic load levels were increased, the fatigue lives were further foreshortened. 0 1992 John Wiley & Sons, Inc.
INTRODUCTION
Type 316L cold worked stainless steel has been successfully used for internal fixation devices of various designs for many years.’ Its use in medical application is, however, sometimes limited in duration in vivo because of localized corrosion and corrosion related Resultant corrosion products and loss of function of the implant can cause several clinical problems, including pain and implant loosening.” The objective of the present study was to investigate the combined effects of cyclic tensile stresses plus an imposed accelerated corrosive environment somewhat mimicking that found in ASTM Standard F 746.13While immersed in 37°C saline a large anodic potential is applied for 20 s to break down the *Now on the faculty of the Department of Dental Materials, School of Dentistry, Hiroshima University, Japan. ‘To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 26, 1131-1139 (1992) CCC 0021-9304/92/091131-09$4.00 0 1992 John Wiley & Sons, Inc.
TAIRA A N D LAUTENSCHLAGER
1132
oxide film. Then the 316L stainless steel is returned and maintained during cyclic loading at a much lower anodic potential, just below Ecritical,which is that potential at which the metal will no longer spontaneously passify. This is meant to be a pilot study with only one specimen per condition.
MATERIALS A N D METHODS
AISI 316L cold worked stainless steel was supplied as 1/4-in. rods (Carpenter Technology Corp., PA). This steel is known as ”consumer type 316L orthopedic implant stock” and had been vacuum melted, annealed, and cold worked. The average yield point and ultimate tensile strength of this material was measured to be 700 MPa and 1050 MPa, respectively. The 1/4-in. rod (6.35 mm diameter) was cut into 110-mm lengths and machined under cold air spray to an hourglass configuration over a 25.4-mm central gage length with the narrowest diameter occurring at the center and being 5.1 mm diameter (0.20 in.). After a 1-h 315°C treatment in air to eliminate the effects of machining in the lathe,’l the specimens were surface finished. They were sequentially polished along the long axis with dry 240-, 400-, and 600-grit Sic papers. Electropolishing followed (0.2 A, 2 V for 6 min) in an aqueous mixture of 600 mL of 20% H,PO, and 400 mL of 10% H2SO4.’’ The corrosion environmental chamber employed in conjunction with the dynamic lnstron machine model 1350 (Instron, Canton, Mass) was a rectangular Plexiglas box measuring 83 X 180 X 35 mm, see Figure 1. The top of the chamber was kept open to the atmosphere. The ends of the specimen beyond the hourglass gage length were coated with fingernail polish to confine the chemical attack to the gage length, and to seal off the aidliquid interface which occurs when the upper portion of the specimen leaves the 0.9 wt % NaCl isotonic saline solution to be inserted into the top grip of the fatigue machine. Our very early work showed that during combined fatigue and corrosion regimens, such as to be used in the present work, fatigue fracture 121
s o l u t i o n out
solution in
Figure 1. Schematic of the corrosion fatigue chamber and its accessories.
CORROSION FATIGUE OF 316L SS
1133
occurred preferentially at this aidliquid interface, even though the specimen diameter was often significantly larger in this region. To maintain the 37°C temperature a peristaltic pump circulated the saline solution from the corrosion cell to a larger bath equipped with an on-off heater activated by a signal from the thermistor probe (Yellowsprings Instruments, CA) located in the corrosion chamber, see Figure 1. The reference standard calomel electrode (SCE) Luggin probe tip was positioned within 1 mm of the center of the gage length of the specimen (also called the working electrode) and extended by a KCl salt bridge to a separate bath where the reference electrode was kept to avoid cross contamination by the isotonic saline. Lastly, a high-purity graphite counter electrode was located in the corrosion chamber, see Figure 1, to deliver the current necessary to maintain the selected voltages during the anodic polarization phases. All three electrodes were attached to a Wenking Potentiostat model LB75M (G. Bank Electronic, W. Germany) not shown in the figure. The current density was determined from the actual measured current being automatically divided by a preentered value of the gage length surface area. (If additional surfaces were created during rupture, such areas were not accounted for in the current density calculations.) The uniaxial tension fatigue tests were carried out at 10 Hz using load control of a sinusoidal wave form cycling from a minimum stress of 10 MPa (to maintain grip alignment) to maximum stresses of 600, 650, 700, 750, and 800 MPa at the narrowest cross section of the hourglass specimens. In this pilot study only one specimen was run for each maximum stress condition. The actual sequence employed in this study was to mount the specimen into the tensile grips, circulate the 37°C electrolyte, monitor the open circuit rest potential for 1 h, begin the cyclic loading, immediately apply the stimulating potential for 20 s at 800 mV vs. SCE, then lower to 200 mV vs. SCE and maintain at this anodic potential until fracture occurred. The circuit current density was continuously monitored throughout the cyclic Ioading portion of the test. For comparison, without imposing the electrochemical regimen, a new specimen at each maximum stress level for both room temperature air and 37°C isotonic saline solution was fatigued until failure. Fractography of all fatigued surfaces was done with a Super Mini scanning electron microscope (ISI, Mountain View, CA). Prior to the present study, ASTM F746 (ref. 13) was performed on three unloaded hourglass specimens coated at both ends with fingernail polish to expose only the gage length to determine both the open circuit potential, Elh, and the critical potential, Ecr,t,Lal. In 37°C isotonic saline the specified sequence of stimulating at 800 mV vs. SCE, then lowering to the nearest 50-mV step above E monitoring for passivation (see Fig. 2), then repeating stimulation and lowering to the next 50-mV step until repassivation no longer occurs, was carried out. Both in testing with ASTM F746 and the present pilot study, El,, was always between -220 and -180 mV vs. SCE. Therefore, the first lower potential step was selected as -150 mV vs. SCE. E,,,,,,,I for all three unloaded
TAIRA AND LAUTENSCHLAGER
1134
I current density
/-----
, /'\
stimulation
I
0/
Breakdown
/' Below Ecrit
\
Repassivation
specimens tested was greater than 200 and less than 250 mV vs. SCE, and thus, 200 mV vs. SCE was selected for this pilot study.
RESULTS
Figure 3 shows the maximum cyclic stress level vs. cycles to failure (S-N) curves for the three very different conditions; namely, (a) in ambient air with no applied potential, (b) in 37°C isotonic saline with no applied potential, and
104
105
lo6
Log N ( cycles Figure 3. Applied maximum stress vs. cycles to failure diagrams (S-N curves) under three different environmental conditions: (a) in ambient air; (b) in 37°C solution; (c) stimulated and 200 mV vs. SCE applied.
CORROSION FATIGUE OF 316L SS
1135
(c) in the accelerated corrosion fatigue condition. All three conditions show that the fatigue lives increase monotonically with decreasing applied stress. However, the lives of the specimens run in air are definitely longer at all stress levels when compared to the other two conditions. Moreover, the lives of the specimens run in the environment of 37°C isotonic saline are definitely longer at all stress levels when compared to the accelerated corrosion fatigue condition, which entailed the application of 200 mV vs. SCE anodic potential following a 800 mV vs. SCE electrical stimulation while cyclicly loading in 37°C saline. Figures 4 to 6 are examples of the current density monitorings during the accelerated corrosion fatiguing at maximum stress levels of 600, 700, and 800 MPa, respectively. Table I presents data for all of the maximum stress levels tested, including EIh, cycles to failure, and failure mode. Below the 700-MPa yield point of the steel, fatigue fracture initiated at a single surface corrosion pit, and proceeded as a flat plane of striations perpendicular to the tensile axis. About three-quarters through the specimen, this gave way to a 45" shear fracture. This mode has been labeled "elastic." Above the yield point, multiple crack nucleation sites were observed along with "plastic" or permanent deformation upon failure. DISCUSSION
Anodic polarization is a conventional electrochemical technique, and has been widely used to determine the breakdown potential, Eb, of most implant
'
( uA/cm2)
200
600 MPa max. 150
100
M
0 S
ation
fracture I
cycles
Figure 4. Cycles vs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 600 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
TAIRA A N D LAUTENSCHLAGER
1136
I
I--
*O0
(/IA/cm2)
700 MPa max
150
50
0
I
I stimulation
fracture
cycles
Figure 5. Cycles vs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 700 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
I
1
4030,
I
( /IA/cm2)
800 MPa max. 3x0-
2000
-
1030
-
0 -
I
t”
stimulation
I
fracture
I
50000
I
100000
cyctes
Figure 6. Cyclesvs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 800 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
metals and dental alloy^.^^,'^ With 316L stainless steel this breakdown potential, where the current required to maintain the selected potential begins to rise very rapidly with only small increases in selected electrical potential, usually occurs from 300 to 500 mV vs. SCE.I7 This is much more noble than
CORROSION FATIGUE OF 316L SS
1137
TABLE I Corrosion Fatigue Parameters of 316L Cold Worked Stainless Steel Specimens Subjected to Cyclic Stress (10 MPa) to Selected Maximum Stress Levels (Sine Wave and 10 Hz) and Anodic Potential (+ZOO mV vs. SCE after Stimulation) ~
Maximum Cyclic Stress (MPa) 600 650
700 750 800
E ih (mV vs. SCE)
Cycles to Failure
Mode of Corrosion Fatigue Failure
-211 -190 -187 - 192 -199
177,460 72,510 52,950 53,470 32,560
Elastic Elastic Elastic Plastic Plastic
the 50- to 200-mV values normally present in the human body and mouth.",'* Therefore, since Eb should not be reached or exceeded, 316L stainless steel should never corrode within the human body. Yet, actual retrieval analyses of these orthopedic implants often belies this conjecture since corrosion does occur.'z The key may lie in the facts that: (a) Normal in vitro anodic polarization testing usually begins with a 1-h establishment of the open circuit potential followed by a slow raising of the anodic potential. This may give a specimen extra time to build up its oxide layer while being raised through the lower applied potentials. (b) The normal anodic polarization testing has no added mechanical stress component. Therefore, the original hypotheses to be tested in the present pilot study were (a) that following temporary breakdown by an 800-mV stimulation followed by an applied anodic recovery potential lower than E b could produce corrosion, and (b) if combined with mechanical load cycling this could result in significantly foreshortened fatigue life. Obviously, an E,,,t,,,l as low as 200 mV vs. SCE should be close to being clinically relevant but still well below Eb. As clearly shown in Figure 3, the application of this Ec,,t,c,lduring cyclic loading definitely reduced the fatigue life of 316L cold worked stainless steel. Also observed in this pilot study were some interesting, previously noted," current density changes as a function of fatigue loading time. In fact, it will now be speculated that these current density changes might be utilized to directly monitor the fatigue processes as they are taking place. As can be seen from cycles vs. current density transient curves in Figures 4 to 6, all cyclically stressed specimens repassivated to some degree following the 800-mV stimulation as was noted by their current densities rapidly returning to nearly zero. This implies that while small pits had now definitely been formed in the gage length surface, they were deactivated by repassivation. Then after a while, the combined mechanical and chemical processes initiated a crack or cracks. Since cracking means the rather sudden creation of new, unoxidized metal surfaces (see Fig. 7), this paper proposes that the corrosion-induced current density spikes observed in Figures 4 and 5 are indicators of such crack initiation occurring throughout the induction period. The fact that many of the spikes decreased back to zero might be a sign of crack initiation without propagation.
TAIRA AND LAUTENSCHLAGER
1138
Figure 7. Slip movement of the underlying metal due to cyclic tensile stress (after Ref. 19).
In addition, the current density profiles clearly indicate the duration of the induction period (time between stimulation and failure onset). Inspection of the figures shows the induction period decreasing with increasing maximum applied cyclic stress. It is here being speculated that this induction phase represents what classical fatigue theory terms crack initiation stage. At 800 MPa the induction phase nearly disappears completely. Throughout Figures 4 to 6, and indeed in all other current density monitorings not shown, there appears to be a definite ability of the current density monitorings to indicate crack initiation and subsidence, and to herald the onset and progress of the final fatigue rupture or crack propagation phase. This, coupled with the apparent deleterious trends observed with application of an accelerating corrosion regimen, suggests that this pilot should be developed into a full fatigue study. This study was supported in part by grants NIDRR H133E 80013 and NIH P60 AM 30692.
References A.C. Fraker and N.W. Ruff, “Metallic surgical implants-state of the art,” J. Metals, 29(5), 22-28 (1977). 2. H. S. Dobbs and J.T. Scales, ”Fracture and corrosion in stainless steel total hip replacement stems,” Corrosion and Degradation of Implant Materials, B.C. Syrett and A. Acharya (eds.), ASTM STP 684, ASTM Philadelphia, 1979, pp. 248-258. 3. P. Ducheyne, P. D. Meester, and E. Aernoudt, “Performance analysis of total hip prostheses: some particular metallurgical observations,” 1. Biomed. Mater. Res., 14, 31-40 (1980). P. Ducheyne, I?. D. Meester, and E. Aernoudt, “Fatigue fractures of the femoral component of Charnley and Charnley Muller type total hip prostheses,” 1. Biomed. Mater. Res. Symp., 6, 199-219 (1975). J.T. Scales and G. P. Winter, ”Corrosion of orthopedic implants,” 1.Bone Joint Surg., 418, 810-820 (1959). A. Weinstein, H. Amstutz, G. Pavon, and V. Franceschini, ”Orthopedic implants -A clinical and metallurgical analysis,” 1. Biomed. Mater. Res., 4, 297-325 (1973). 7. R.T. Gray, ”Metallographic examinations of retrieved intramedullary bone pins and bone screws from the human body,” J. Biomed. Mater. Res. Symp., 5, 27-38 (1974). 1.
CORROSION FATIGUE OF 316L SS
1139
G. D. Winter, “Tissue reactions to metallic wear and corrosion products in human patients,” J. Biomed. Muter. Res. Symp., 5, 11-25 (1974). 9. D. F. Williams and G. Meachim, “A combined metallurgical and histo-
8.
10. 11. 12.
13. 14. 15.
16. 17. 18. 19.
logical study of tissue-prosthesis interactions in orthopedic patients,” J. Biomed. Mater. Res. Symp., 5, 1-9 (1974). R. Duetman, T. J. Mulder, R. Brian, and J. P. Nater, ”Metal sensitivity before and after total hip arthroplasty,” 1. Bone Joint Surg., 59(A), 862865 (1977). J. E. Lemons, K. M.W. Nieman, and A. 8 . Weiss, ”Biocompatibility studies on surgical grade titanium, cobalt and iron-base alloys,” 1. Biomed. Mater. Xes. Symp., 7, 549-553 (1976). A.W. Weinstein, W.P. Spires, J. J. Klawitter, A. J.T. Clemow, and J.O. Edmunds, “Orthopedic implant retrieval and analysis study,” Corrosion and Degradation of Implant Materials, B.C. Syrett and A. Acharya (eds.), ASTM STP 684, ASTM, Philadelphia, 1979, pp. 212-228. Standard method of test for pitting and crevice corrosion of metallic surgical materials, ASTM-F746, Philadelphia, 1987. Source Book on Stainless Steels, Amer. SOC.for Metals, Metals Park, Ohio, 1976, pp. 172-173. M. S. Bapna, E. P. Lautenschlager, and J. 8. Moser, “The influence of electrical potential and surface finish on the fatigue life of surgical implant materials,” J. Biomed. Mater. Res., 9, 611-621 (1975). D.C. Mears, ”Metals in medicine and surgery, review 218,” Int. Metals Rev., 22, 119-155 (1977). E. H. Greener, J. K. Harcourt, and E. P. Lautenschlager, Materials Science in Dentistry, Williams and Wilkins, Baltimore, 1972, p. 363. G. Ewers and E. H. Greener, ”The electrochemical activity of the oral cavity,” 1. Oral Rehab., 12, 469-476 (1985). H. 0. Fuchs and R. 1. Stephens, Metals Fatigue in Engineering, Wiley, New York, 1980.
Received July 18, 1989 Accepted February 14, 1992
breakdown, and then (b) returning to a constant 200 mV vs. SCE anodic potential and maintaining that potential during cyclic loading until the specimens broke in two. During the anodic polarization by continuously monitoring the current it was possible to (a) observe the repassivation and corrosion behavior following stimulation, and (b) detect crack initiation, crack propagation and failure onset. The combined effects of accelerated corrosion and mechanical f a t i g u i n g disturbed the repassivation tendency and reduced the crack initiation times and the fatigue lives as compared to air and saline controls. A s the maximum cyclic load levels were increased, the fatigue lives were further foreshortened. 0 1992 John Wiley & Sons, Inc.
INTRODUCTION
Type 316L cold worked stainless steel has been successfully used for internal fixation devices of various designs for many years.’ Its use in medical application is, however, sometimes limited in duration in vivo because of localized corrosion and corrosion related Resultant corrosion products and loss of function of the implant can cause several clinical problems, including pain and implant loosening.” The objective of the present study was to investigate the combined effects of cyclic tensile stresses plus an imposed accelerated corrosive environment somewhat mimicking that found in ASTM Standard F 746.13While immersed in 37°C saline a large anodic potential is applied for 20 s to break down the *Now on the faculty of the Department of Dental Materials, School of Dentistry, Hiroshima University, Japan. ‘To whom correspondence should be addressed. Journal of Biomedical Materials Research, Vol. 26, 1131-1139 (1992) CCC 0021-9304/92/091131-09$4.00 0 1992 John Wiley & Sons, Inc.
TAIRA A N D LAUTENSCHLAGER
1132
oxide film. Then the 316L stainless steel is returned and maintained during cyclic loading at a much lower anodic potential, just below Ecritical,which is that potential at which the metal will no longer spontaneously passify. This is meant to be a pilot study with only one specimen per condition.
MATERIALS A N D METHODS
AISI 316L cold worked stainless steel was supplied as 1/4-in. rods (Carpenter Technology Corp., PA). This steel is known as ”consumer type 316L orthopedic implant stock” and had been vacuum melted, annealed, and cold worked. The average yield point and ultimate tensile strength of this material was measured to be 700 MPa and 1050 MPa, respectively. The 1/4-in. rod (6.35 mm diameter) was cut into 110-mm lengths and machined under cold air spray to an hourglass configuration over a 25.4-mm central gage length with the narrowest diameter occurring at the center and being 5.1 mm diameter (0.20 in.). After a 1-h 315°C treatment in air to eliminate the effects of machining in the lathe,’l the specimens were surface finished. They were sequentially polished along the long axis with dry 240-, 400-, and 600-grit Sic papers. Electropolishing followed (0.2 A, 2 V for 6 min) in an aqueous mixture of 600 mL of 20% H,PO, and 400 mL of 10% H2SO4.’’ The corrosion environmental chamber employed in conjunction with the dynamic lnstron machine model 1350 (Instron, Canton, Mass) was a rectangular Plexiglas box measuring 83 X 180 X 35 mm, see Figure 1. The top of the chamber was kept open to the atmosphere. The ends of the specimen beyond the hourglass gage length were coated with fingernail polish to confine the chemical attack to the gage length, and to seal off the aidliquid interface which occurs when the upper portion of the specimen leaves the 0.9 wt % NaCl isotonic saline solution to be inserted into the top grip of the fatigue machine. Our very early work showed that during combined fatigue and corrosion regimens, such as to be used in the present work, fatigue fracture 121
s o l u t i o n out
solution in
Figure 1. Schematic of the corrosion fatigue chamber and its accessories.
CORROSION FATIGUE OF 316L SS
1133
occurred preferentially at this aidliquid interface, even though the specimen diameter was often significantly larger in this region. To maintain the 37°C temperature a peristaltic pump circulated the saline solution from the corrosion cell to a larger bath equipped with an on-off heater activated by a signal from the thermistor probe (Yellowsprings Instruments, CA) located in the corrosion chamber, see Figure 1. The reference standard calomel electrode (SCE) Luggin probe tip was positioned within 1 mm of the center of the gage length of the specimen (also called the working electrode) and extended by a KCl salt bridge to a separate bath where the reference electrode was kept to avoid cross contamination by the isotonic saline. Lastly, a high-purity graphite counter electrode was located in the corrosion chamber, see Figure 1, to deliver the current necessary to maintain the selected voltages during the anodic polarization phases. All three electrodes were attached to a Wenking Potentiostat model LB75M (G. Bank Electronic, W. Germany) not shown in the figure. The current density was determined from the actual measured current being automatically divided by a preentered value of the gage length surface area. (If additional surfaces were created during rupture, such areas were not accounted for in the current density calculations.) The uniaxial tension fatigue tests were carried out at 10 Hz using load control of a sinusoidal wave form cycling from a minimum stress of 10 MPa (to maintain grip alignment) to maximum stresses of 600, 650, 700, 750, and 800 MPa at the narrowest cross section of the hourglass specimens. In this pilot study only one specimen was run for each maximum stress condition. The actual sequence employed in this study was to mount the specimen into the tensile grips, circulate the 37°C electrolyte, monitor the open circuit rest potential for 1 h, begin the cyclic loading, immediately apply the stimulating potential for 20 s at 800 mV vs. SCE, then lower to 200 mV vs. SCE and maintain at this anodic potential until fracture occurred. The circuit current density was continuously monitored throughout the cyclic Ioading portion of the test. For comparison, without imposing the electrochemical regimen, a new specimen at each maximum stress level for both room temperature air and 37°C isotonic saline solution was fatigued until failure. Fractography of all fatigued surfaces was done with a Super Mini scanning electron microscope (ISI, Mountain View, CA). Prior to the present study, ASTM F746 (ref. 13) was performed on three unloaded hourglass specimens coated at both ends with fingernail polish to expose only the gage length to determine both the open circuit potential, Elh, and the critical potential, Ecr,t,Lal. In 37°C isotonic saline the specified sequence of stimulating at 800 mV vs. SCE, then lowering to the nearest 50-mV step above E monitoring for passivation (see Fig. 2), then repeating stimulation and lowering to the next 50-mV step until repassivation no longer occurs, was carried out. Both in testing with ASTM F746 and the present pilot study, El,, was always between -220 and -180 mV vs. SCE. Therefore, the first lower potential step was selected as -150 mV vs. SCE. E,,,,,,,I for all three unloaded
TAIRA AND LAUTENSCHLAGER
1134
I current density
/-----
, /'\
stimulation
I
0/
Breakdown
/' Below Ecrit
\
Repassivation
specimens tested was greater than 200 and less than 250 mV vs. SCE, and thus, 200 mV vs. SCE was selected for this pilot study.
RESULTS
Figure 3 shows the maximum cyclic stress level vs. cycles to failure (S-N) curves for the three very different conditions; namely, (a) in ambient air with no applied potential, (b) in 37°C isotonic saline with no applied potential, and
104
105
lo6
Log N ( cycles Figure 3. Applied maximum stress vs. cycles to failure diagrams (S-N curves) under three different environmental conditions: (a) in ambient air; (b) in 37°C solution; (c) stimulated and 200 mV vs. SCE applied.
CORROSION FATIGUE OF 316L SS
1135
(c) in the accelerated corrosion fatigue condition. All three conditions show that the fatigue lives increase monotonically with decreasing applied stress. However, the lives of the specimens run in air are definitely longer at all stress levels when compared to the other two conditions. Moreover, the lives of the specimens run in the environment of 37°C isotonic saline are definitely longer at all stress levels when compared to the accelerated corrosion fatigue condition, which entailed the application of 200 mV vs. SCE anodic potential following a 800 mV vs. SCE electrical stimulation while cyclicly loading in 37°C saline. Figures 4 to 6 are examples of the current density monitorings during the accelerated corrosion fatiguing at maximum stress levels of 600, 700, and 800 MPa, respectively. Table I presents data for all of the maximum stress levels tested, including EIh, cycles to failure, and failure mode. Below the 700-MPa yield point of the steel, fatigue fracture initiated at a single surface corrosion pit, and proceeded as a flat plane of striations perpendicular to the tensile axis. About three-quarters through the specimen, this gave way to a 45" shear fracture. This mode has been labeled "elastic." Above the yield point, multiple crack nucleation sites were observed along with "plastic" or permanent deformation upon failure. DISCUSSION
Anodic polarization is a conventional electrochemical technique, and has been widely used to determine the breakdown potential, Eb, of most implant
'
( uA/cm2)
200
600 MPa max. 150
100
M
0 S
ation
fracture I
cycles
Figure 4. Cycles vs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 600 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
TAIRA A N D LAUTENSCHLAGER
1136
I
I--
*O0
(/IA/cm2)
700 MPa max
150
50
0
I
I stimulation
fracture
cycles
Figure 5. Cycles vs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 700 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
I
1
4030,
I
( /IA/cm2)
800 MPa max. 3x0-
2000
-
1030
-
0 -
I
t”
stimulation
I
fracture
I
50000
I
100000
cyctes
Figure 6. Cyclesvs. current density transient curve of the corrosion fatigue experiment, under cyclic stress (10 to 800 MPa, sine wave and 10 Hz) and anodic potential (200 mV vs. SCE after stimulation).
metals and dental alloy^.^^,'^ With 316L stainless steel this breakdown potential, where the current required to maintain the selected potential begins to rise very rapidly with only small increases in selected electrical potential, usually occurs from 300 to 500 mV vs. SCE.I7 This is much more noble than
CORROSION FATIGUE OF 316L SS
1137
TABLE I Corrosion Fatigue Parameters of 316L Cold Worked Stainless Steel Specimens Subjected to Cyclic Stress (10 MPa) to Selected Maximum Stress Levels (Sine Wave and 10 Hz) and Anodic Potential (+ZOO mV vs. SCE after Stimulation) ~
Maximum Cyclic Stress (MPa) 600 650
700 750 800
E ih (mV vs. SCE)
Cycles to Failure
Mode of Corrosion Fatigue Failure
-211 -190 -187 - 192 -199
177,460 72,510 52,950 53,470 32,560
Elastic Elastic Elastic Plastic Plastic
the 50- to 200-mV values normally present in the human body and mouth.",'* Therefore, since Eb should not be reached or exceeded, 316L stainless steel should never corrode within the human body. Yet, actual retrieval analyses of these orthopedic implants often belies this conjecture since corrosion does occur.'z The key may lie in the facts that: (a) Normal in vitro anodic polarization testing usually begins with a 1-h establishment of the open circuit potential followed by a slow raising of the anodic potential. This may give a specimen extra time to build up its oxide layer while being raised through the lower applied potentials. (b) The normal anodic polarization testing has no added mechanical stress component. Therefore, the original hypotheses to be tested in the present pilot study were (a) that following temporary breakdown by an 800-mV stimulation followed by an applied anodic recovery potential lower than E b could produce corrosion, and (b) if combined with mechanical load cycling this could result in significantly foreshortened fatigue life. Obviously, an E,,,t,,,l as low as 200 mV vs. SCE should be close to being clinically relevant but still well below Eb. As clearly shown in Figure 3, the application of this Ec,,t,c,lduring cyclic loading definitely reduced the fatigue life of 316L cold worked stainless steel. Also observed in this pilot study were some interesting, previously noted," current density changes as a function of fatigue loading time. In fact, it will now be speculated that these current density changes might be utilized to directly monitor the fatigue processes as they are taking place. As can be seen from cycles vs. current density transient curves in Figures 4 to 6, all cyclically stressed specimens repassivated to some degree following the 800-mV stimulation as was noted by their current densities rapidly returning to nearly zero. This implies that while small pits had now definitely been formed in the gage length surface, they were deactivated by repassivation. Then after a while, the combined mechanical and chemical processes initiated a crack or cracks. Since cracking means the rather sudden creation of new, unoxidized metal surfaces (see Fig. 7), this paper proposes that the corrosion-induced current density spikes observed in Figures 4 and 5 are indicators of such crack initiation occurring throughout the induction period. The fact that many of the spikes decreased back to zero might be a sign of crack initiation without propagation.
TAIRA AND LAUTENSCHLAGER
1138
Figure 7. Slip movement of the underlying metal due to cyclic tensile stress (after Ref. 19).
In addition, the current density profiles clearly indicate the duration of the induction period (time between stimulation and failure onset). Inspection of the figures shows the induction period decreasing with increasing maximum applied cyclic stress. It is here being speculated that this induction phase represents what classical fatigue theory terms crack initiation stage. At 800 MPa the induction phase nearly disappears completely. Throughout Figures 4 to 6, and indeed in all other current density monitorings not shown, there appears to be a definite ability of the current density monitorings to indicate crack initiation and subsidence, and to herald the onset and progress of the final fatigue rupture or crack propagation phase. This, coupled with the apparent deleterious trends observed with application of an accelerating corrosion regimen, suggests that this pilot should be developed into a full fatigue study. This study was supported in part by grants NIDRR H133E 80013 and NIH P60 AM 30692.
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Received July 18, 1989 Accepted February 14, 1992
