Dent Mater 12: 146-l 53, May. 1996
Corrosion of amalgams under sliding wear Hun Lian, Efstathios
State University, Materials Engineering Prop-cm, Mectm~~rcul Er~g~r~weringlhptlur/~~r~,-t/l. Baton Rouge, Louisiana, CJSA
ABSTRACT Objectives. During mastication, dental amalgams are simultaneously subjected to corrosion by the oral environment and to a sliding-wear process by biting forces. In the present study, the effect of sliding wear on the corrosion behavior of two high-copper dental amalgams was investigated. A&hods. An experimental apparatus was utilized that allows electrochemical testing under sliding-wear conditions. Corrosion potential measurements and anodic polarization scans were conducted in 0.1 M NaCl solution under sliding wear to characterize the behavior of two commerical, high-copper, single composition dental amalgams. In addition, long duration tests were conducted to assess possible corrosion and wear synergistic effects. Results. The results showed that sliding wear caused a sharp reduction in the corrosion potential, a significant increase in the corrosion rate and a decrease in the repassivation rate of both amalgams. These effects are due to the mechanical removal by the wear process of the surface protective film formed on dental amalgams. The simultaneous action of sliding wear and corrosion can also induce embrittlement that leads to cracking. Significance. The present evidence suggests that this cracking may be one of the major contributors to marginal failures of dental amalgam restorations. INTRODUCTION Dental amalgams interact in a complex manner with the oral environment; they are subjected to electrochemical, chemical, mechanical, biological and thermal forces (Marek, 1992). Electrochemical reactions are one of the most important forms of interaction between amalgam restorations and oral fluids. In the oral environment, dental amalgam fillings corrode and often show extensive degradation, significant localized penetrations, and marginal fracture (Espevik and Mjiir, 1979; Osborne et al., 1991). Retrieved restorations often show unusual extensive degradation and a substantial depth of corrosion under the occlusal surfaces. Espevik and Mjiir
146 Lian & Me/et&/Effect
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( 1979) reported that the corrosion damage reached a depth of’ about 200 pm in a 2.5 pear-old, low-copper amalgam restoration, and they cited the cracking that initiated from occlusal areas as the cause of marginal f&lure. This study indicated that additional factors along with corrosion may be involved in the deterioration process, as Port and Marshall ( 1985) suggested, based on clinical studies. The mechanism by which the degradation takes place IS not completely understood. A significant research effort has been devoted in past decades to determine the cause and mechanism of this type of degradation (Mahler. 1988). Possible mechanical contributions to this fract,ure include tensile overload, creep, and fatigue. Mechanical forces combined with corrosion ark expected to have a detrimental effect on dental amalgams, but work on this subject is limited. The acceleration of amalgam corrosion by plastic deformation was reported by Gjerdet and Espevik 11978). Similar effects on corrosion have been observed by creep and tensile strain iMahler PI’ 07.. 1970: 1982; Averette and Marek, 1983). Clinical studies on a wide variety of dental amalgams, however, demonstrated that it may be difficult to predict the amalgam marginal integrity by creep alone (Jordan et al., 1978: Osborne and Gale. 1979; Osborne d (I/.. 1980; Laswell et rrl., 1980). Tests by Her6 cf nl, ! 1983) showed that cyclic loading increased the corrosion rate by one or two orders of magnitude when compared to the rate without loading. Wright et al. ( 1980) also reported the acceleration of corrosion of a Co-Cr dental alloy by ultrasonically agitating abrasive particles in the electrolyte. Similar results were obtained in an interesting study conducted by Marek t 1984 1to investigate the effect ofabrasion on the corrosion behavior of dental amalgams. The relative motion of teeth during mastication can be best described by a sliding-wear process. This wear action on restorative amalgams is expected to continuously remove protective films and to locally deform the amalgam surface. It has been reported by Meletis et al. (1989! that these two
Pt Electrode 7
Bridge to Reference Electrode
fig. 1. Schematic illustration of the (a) casting mold and specimen geometry (Pl and P2 concave and convex plates, G ? ? guide plate and C = cover plate) and (b) wear/corrosion apparatus (A ? ? specimen, B ? ? specimen holder, C = shaft, D ? ? cylindrical end-piece, E ? ? loading-pin, F ? ? counter electrode, G ? ? connection bridge, H ? ?conducting leads, I ? ?gas dispersion tube, and J = magnetic stirrer).
processes contribute significantly to corrosion and that, more importantly, there may be a synergistic effect between corrosion and wear The purpose of the present work was to study the effect of sliding wear on the corrosion behavior of dental amalgams in an effort to understand the deterioration processes occurring in dental amalgams during mastication.
MATERIALS AND METHODS The apparatus developed to study the combined effects ofwear and corrosion is shown in Fig. 1. A detailed description of this apparatus was given previously (Meletis, 1989; Meletis et al., 1989). It consists ofthree components: an electrochemical cell, a driving system, and a dead loading device. The specimen (A) is arch-shaped and is used in conjunction with the specimen holder (B) that has a ring configuration. As shown in Fig. 1,
the specimen holder is attached firmly to the oscillating shalt (0 by tightening a cylindrical end piece (D). Current is transmitted from the specimen through the shaft to the potentiostatigalvanostat. The shaft is oscillated using an electric motor and a set of adjustable conversion linkages. Loading the specimen is performed by using a stationary pin (E) that can be made in various configurations. The pin shape determines the type of contact made with the specimen surface. For example, a hemispherical and cylindrical pin-end will produce a theoretical point and line contact, respectively Two commercial high-copper, single-composition dental amalgams were selected for the present, study, namely Tytin and Sybraloy Nominal compositions, along with the weight percent of mercury added during trituration, are given in Table 1. Both amalgams were nominally 100% spherical alloys. They were first triturated according to the manufacturer’s directions in a mechanical amalgamator (Crescent Wig-L-Bug, Crescent Dental MFG Co., Lyons, IL, USA) and condensed in a specially made stainless-steel mold using parameters ofADA Specification No. 1( 1974). The mold consisted of four components. Two plates with conforming concave (Pl) and convex (P2) surfaces were used to pressurize the amalgam and form the sample., a guide plate (G), and a cover plate (C) (Fig. 1). A universal testing machine (MTS, INSTRON, Canton, MA, USA) was used to apply a 30 MPa pressure on the triturated amalgam. The loading time was 60 s. This procedure resulted in reproducible arch samples 25 mm long, 4 mm wide, and 3 mm high. Following condensation, the samples were aged (15 d at 37°C) to stabilize the microstructure and then mechanically polished with 0.05 urn alumina powder as the final step. All wear/corrosion testing was performed in O.lM NaCl solution (pH = 7.0) at 25°C. The solution was deaerated for 30 min prior to each experiment by purging with N, gas. All potentials were measured with respect to a saturated calomel electrode (SCE). The wear action was provided by loading a cylindrical pin resulting in a theoretical line contact with the specimen. The pin was wide enough (5mm ) to wear the entire specimen surface. The pin was made from a ceramic material (Macor, Machinable Glass Ceramic. Corning Inc., Corning, NY, USA) with a Knoop Hardness of 250, elastic modulus of 6.4 x 104 MPa, and compressive strength of 345 MPa. The mechanical properties of this ceramic material are similar to those of human enamel (Craig et al., 1984). Tests were conducted under two loading levels that, through a cylinder-on-cylinder contact, resulted in 40 MPa and 115 MPa applied stresses on the sample, respectively Roark andYoung, 1975). Two oscillation (k 30” amplitude) velocities were used: 1.6 cm/s and 3.2 cm/s. It should be noted that a stress of 40 MPa and a sliding velocity of 1.6 (m-r/sare within the range Dentd Materials/May
40 MPa 1.6cm/s
5 0 lb.4 NoCl Deaevakd
of average stress (Hero et al., 1983) and average sliding rates (Powell et al., 1975 1 applied on teeth during mastication. A higher level of stress (115 MPa) and velocity (3.2 cm/s) was used to create more severe conditions on the specimen surfaces for accelerated testing. Five specimens were tested for each experimen-421t 4' -194i9" 262+34c tal condition. Two types of electrochemical tests were performed to evaluate the effect of sliding wear on corrosion. The first involved corrosion potential measurements under wear and no-wear conditions. In these -568it15' -209* 8" 440+ 56!' tests, the open circuit potential was first allowed to stabilize and then the wear process was activated for 300 s, stopped to allow repassivation (for 1000 s) and reactivated, while the potential was recorded continuously This cycle was repeated several times during each experiment. In addition, some long-duration tests were conducted under 115 MPa applied stress and 1.6 cm/s slidingvelocity In these tests, the wear action was applied continuously for 90 min in order to evaluate possible wear/corrosion synergistic effects. Control tests of equal duration were also performed (a) under the same wear conditions (115 MPa, 1.6 cm/s 1 in air (no corrosion) and (b) in the same electrolyte under oscillation but with no applied load (“no-wear” condition). The second test involved anodic polarization scans under wear and no-wear conditions. The procedure first involved stabilization of the open-circuit potential under the weal action (E,.,), and then polarization scanning starting from :I potential approximately 150 mV below E,,_Lat a rate 01’ 0.5 mV/s. Corrosion current us.potential graphs were obtained and corrosion rates were calculated by using computer software (EG&G Corrosion Software 342C. EG&G Princeton Applied Research, Princeton, NJ, USA) that utilizes the Tafel slope extrapolation method. After testing, selected specimens were observed usmg scanning electron microscopy (SEMI. The SEM examinations were of a rather qualitative nature. Visual examination of’ images was used to assess the specimen surface morphology afler wear/corrosion testing. Semi-quantitative compositional analysis was conducted by using an x-ray energy dispersive analysis system using ZAF correction (System PV9400, EDAX. Mahwah, NJ, USA). Distances measured by SEM were calibrated by using standard copper grids of 70 pm size. Examinations of amalgam surfaces were conducted at 15-30 keV by using secondary and backscattered electron imaging (ISI-GOA,ISI, Pleasanton, CA USA) equipped with a Robinson backscattered detector.
Fig. 2. The effect of one sliding-wear cycle (300 s) on the corrosion potential of (a)Tytin and (b) Sybraloy. 148 Lian &
of corrosion and wear on amalgams
Preliminary tests showed that for both amalgams, the corrosion potential under no-wear (E,) stabilized after approximately 4 h of exposure in the electrolyte (deaerated. The potential-time response of the two amalgams under wear is shown in Fig. 2. In general, both amalgams exhibited a similar behavior The activation of the wear action caused a sharp reduction of several hundred milivolts in the corrosion
T NaCl ; Deaeroted
Solbtion: O.lM pti: 7.0
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0.06 zk 0.03”
0.08 f 0.04”
‘TIME, seconds Fig.3. Typical potential-time response ofTytin for three wear cycles.
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, ,,,,,,,, 1o-2
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Fig.4. The effect of sliding wear on the polarization scans of (a)Tytin and (b) Sybraloy.
potential that remained at low levels (active region) as long as the wear process was operating. When wear ceased, the amalgams slowly repassivated but, in general, they assumed potentials (E’J lower than the initial open-circuit potential (E,) before wear. Tytin exhibited slower repassivation rates compared to Sybraloy Several wear cycles were performed to determine if and when the pattern of the potential response remained unchanged. The results showed that atIer three wear cycles, no detectable changes in the potential behavior were observed. Fig. 3 presents a typical potent,iaI response of Tytin for three wear cycles. The results of these corrosion potential measurements and the repassivation times are shown in Table 2. The effect of wear on the anodic polarization behavior is shown in Fig. 4. For both amalgams, the wear caused a shift of the anodic polarization curves to the active region corresponding to lower potentials and significantly higher current densities. The analysis of variance was pefiormed under the fixed effects model witha = 0.05. The increase of the corrosion rate due to wear (iJ is shown inTable 3. A significantly higher increase in the corrosion rate under wear was observed for Tytin compared to Sybraloy SEM examinations showed that both amalgams exhibited quite similar microstructures consisting mainly of spherical unreacted alloy particles surrounded by dispersed n’ phase (Cu,Sn,) in a y1 (Ag,HgJ matrix. Observations of Sybraloy surfaces that underwent corrosion testing under wear reveal& a type of attack that appeared to be non-random and might be associated with porosity However, there also seemed to be an association with the reaction zone surrounding unreacted spherical particles. If the latter is true, the corrosion could be associated with the q’phase since it was observed in locations associated with the q’ phase in untested samples (Fig. 5). In certain areas, the localized damage from wear and corrosion was more severe. As a result, these areas were usually associated with the location of larger clusters of wear/ corrosion products. Finally the region below the surface of the specimens that had been subjected to 90 min immersion testing under wear Dental Materials/May
Fig. 5. Scanning electron micrograph showing typical surface appearance in (a)Tytin and (b) Sybraloy after wear/corrosion testing. (Test Duration: 12 min,Wear Conditions: 115 MPa, 1.6 cm/s).
Fig. 6. SEM micrograph showing subsurface cracking in (a)Tytin and (b) Sybraloy after immersion testing under wear. (Test Duration: 90 min, Wear Conditions: 115 MPa, 1.6 cm/s).
was examined by preparing metallographic cross sections along the wear-track trace. SEM observations uncovered the existence of cracking just below the wear-track in these specimens (Fig. 61. Note: no cracking was observed in specimens tested in pure corrosion or pure water. Very limited, if any degradation was present in the specimens tested under the same wear conditions in laboratory air (pure mechanical wear) (Fig. 7a), or in the NaCl solution under “no-wear” (pure corrosion) (Fig. 7b). These subsurface cracks were found to propagate below the surface at a depth of approximately lo-15 um and 20-30 urn for Tytin and Sybraloy, respectively The cracking was of a mixed mode, propagating in the y, matrix, through the residual alloy particles and along their boundary with the matrix. It seemed that Sybraloy exhibited a more extensive cracking pattern (wider and longer cracks) compared to the Tytin specimens. The cracking exhibited a concave morphology by initiating and terminating at the
surface. In certain cases, cracks were observed to have initiated from the surface and then branched off and propagated within the amalgam (Fig. 8). Occasionally this particular cracking process resulted in removal of small amalgam portions producing irregularities at the surface (Figs. 6 and 8 1.
Lian & MeleWEffect
of corrosion and wear on amalgams
DISCUSSION The present study showed that sliding wear can have significant effects on the corrosion behavior of amalgams. The wear process was found to cause a sharp decrease in the corrosion potential, a significant increase in the corrosion rate and also a decrease in the recovery rate. These effects are consistent with the formation and subsequent removal of a protective surface film by the wear action. This pattern has been observed in other passivating materials (Mel&is, 1989). Acceleration of corrosion of dental alloys by mechanically
Fig. 8. SEM micrograph showing a crack (indicated by arrows) that developed in the surface region subjected to wear/corrosion and then branched off into theTytin specimen.
Fig. 7. SEM micrograph showing typlcal appearance of equivalent regions as those in Fig. 6 for specimens tested under (a) pure wear (Sybraloy, 115 MPa, 1.6 cm/s, 90 min) and (b) pure immersion (Tytin, 24 h).
destroying the surface film has also been reported previously (Wright et al., 1980; Marek, 1984). Thus, the sharp reduction in the corrosion potential upon application of the wear process (Table 2) can be attributed to the disruption and removal of the surface f&n allowing the fresh active amalgam surface to be exposed to the electrolyte. Under sliding wear, the corrosion potentials of the two amalgams ranged from Ek._C = -660 mV to -420 mV (Table 2). These potential values are consistent with the potentials of the electrochemical reactions corresponding to corrosion of Sn and formation of tin oxides and hydroxides @L&X and Greener 1975; Sarkar, 1981). The equilibrium potential for corrosion of Cu is about -190 mV and dissolution of Cu is not expected in the observed E,., range. The depletion of Cu from the amalgam surfaces after wear/corrosion testing can possibly be attributed to corrosion of Sn from q’producing a porous and weak particle that can easily be disrupted and removed by the wear process. In addition, SEM observations (Sarkar et al., 1979; Lin et al., 1983) have shown that Sn dissolves preferentially at the q‘lr, matrix interface and such a process could also aid in the dislodging of the q’ particles. It
has been documented by in uiuo and i;n vitro studies that microstructurally the r\’phase is the most corrosionsusceptible phase and is preferentially attacked (Meyer and Nallx 1977; Averette et al., 1978; Sarkar et al., 1978; Marek and Maller, 1979; Marshall et al., 1980). Detailed SEM and energy dispersive x-ray analysis by Linet al. ( 1983) established that Sn Tom the q’ phase oxidizes to form tin oxide. Thus, these previous fmdings are consistent with the SEM observations (Fig. 5) regarding preferential corrosion of the TJ’ phase. The present results also show that intensi&ation of the wear conditions in terms of applied load and sliding velocity caused a higher potential drop, higher corrosion rate, and longer repassivation time (Tables 2 and 3). These observations are consistent with the expectation that a higher applied load and/ or sliding velocity would result in more effective film removal and thus, lower potential, higher corrosion rate, and longer time for f&n repair A significant finding of the present study was that the corrosion rate can be accelerated by the wear process. Under the present experimental conditions, corrosion rates increased up to two orders of magnitude (Table 3). This increase is significantly higher than that observed on abraded amalgams (Marek, 1984). Since a much lower pressure (0.5 MPa) was applied onto the specimen surfaces by the abrasion pads in the latter study, the present results can probably be attributed to the effect of plastic deformation resul.ting fi-om the wear process. Acceleration of corrosion in amalgams caused by plastic deformation has been reported previously (Gjerdet and Espevik, 1978; Mahler et al., 1982;Averette and Marek, 1983; Hera et al., 1983). In view of the above, the observed increase in the corrosion rate by increasing the applied load in the present study (Table 3) can be attributed to a larger amount of plastic deformation expected at a higher applied load. Of great significance to this study were also the SEM observations that revealed the existence of cracking (Fig. 6) just below the wear-track region in the specimens tested for 90 min under wear/corrosion. For the present experimental conditions, no sign&ant degradation was observed for pure
wear or pure corrosion tests (Fig. 7). Thus, the occurrence of cracking indicates an interaction between sliding wear and corrosion (conditions prevailing during mastication) that can induce embrittlement. The present results suggest that the wear/corrosion action in areas subjected to occlusal forces can produce cracking that may contribute to marginal breakdown. The in uiuo studies of Espevik and Mjor (1979) reported cracking initiated from the occlusal areas and propagated to the margins, causing failure. Based on their observations, it was suggested that the marginal breakdown of amalgam restorations is due to stress corrosion cracking. Similar observations were made in the present study where cracks were found to initiate from the surface subjected to wear-l corrosion, and to propagate within the bulk of the amalgam (Fig. 6). It is interesting to note that the concave appearance of the observed cracking resembles the geometry of the tensile component of the stress field expected from a cylinder-oncylinder contact (Hamilton and Goodman, 1966). Stress analysis has shown that in this type of contact, one of the principal stresses is tensile near the edge of the contact. This stress acts in a radial direction and could account for the geometry of the observed concave crack. The exact reasons for the cracking observed in the present study are not known at the moment. It can be speculated, however, that the preferential attack of the n’ particles producing cavities can have two effects. First, it can increase the local stress intensity at the tip of the cavity Second, if the removal of the n’phase has progressed to a sufficient depth, an occluded oxygen concentration cell may develop that renders the surface within the cavity anodic, and/or the acidity in the cavity may increase, as suggested previously (Marek, 1983). Under these conditions, cracking can be produced by a corrosion-assisted or embrittlement process involving corrosion-fatigue (Hero et al., 1983), stress corrosion cracking (Espevik and Mjor, 1979), or even hydrogen embrittlement. It should be noted that the level of the applied stress in the present experiments (115 MPa) was significantly higher than the average stress applied during mastication; however, the implications of the observed cracking under wear/corrosion to the structural stability and durability of dental amalgam restorations are very important. As seen in the present results, under wear conditions, the corrosion potential of the two amalgams tested remains continuously in the active regime. Additionally this activated state is associated with significantly higher corrosion currents of a localized nature. After comparing chewing rates (Hero et al., 1983) and the repassivation characteristics exhibited by the two amalgams (Table 2), it seems evident that during mastication, corrosion rates will remain at high levels. Furthermore, over an extended period of time, such wear1 corrosion attacks can cause significant degradation of dental amalgam restorations. In addition, the simultaneous action of wear and corrosion can produce embrittlement leading to cracking. For these reasons, wear/corrosion processes should be seriously considered to explain observed deteriorations and marginal fractures of dental amalgam restorations in clinical studies.
ACKNOWLEDGMENTS This investigation was supported in full by USPHS Research Grant DE07964 from the National Institute of Dental Research, Bethesda, MD 20892 USA. The authors would 152 L/an & Me/et/s/Effect
of corrosion and wear on amalgams
like to acknowledge Professors M. Marek from Georgia Institute of Technology and N. K. Sarkar from LSIJ School of Dentistry for their valuable comments during the course of this work. Received April 16,199s
/Accepted February 251996
Address correspondence and reprint requests to: E. I. Meletis Louisiana State Cniversitj Mechanical Engineering Department Baton Rouge, LA 70803-6413
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Marshall GW, Jackson BL, Marshall SJ (1980). Copper-rich and conventional amalgam restorations after clinical use. JAm DentAssoc 100:43-47. Meletis EI (1989). Wear-corrosion behavior of ion-plated thin fihns. J Mater Eng 11:159-167. Meletis EI, Gibbs CA Lian K (1989). A new dynamic corrosion test for dental materials. Dent Mater 5:411-414. Meyer JM, Nally NJ (1977). Early and delayed corrosion resistance of dental amalgams. ZADR Z’rogr 56, Abstr. No. 241. Osborne Jw, Gale EN (1979). Failure rate ofamalgams with a high content of copper. Oper Dent 4:2-8. Osborne Jw, Leinfelder Kl$ Gale EN, Sluder TB (1980). Two independent evaluations of ten amalgam alloys. JProsthet Dent 43:622-626. Osborne Jw, Norman RJI, Gale EN (1991). A 14-year clinical assessment of 12 amalgam alloys. Quintessence Znt 22:857863. Port RM, Marshall Jr, GW (1985). Characteristics of amalgam restorations with variable clinical appearance. JAm Dent Assoc 110:491-495. Powell JM, Phillips RW Norman RD (1975). In vitro response
of composite resin, amalgam and enamel. J Dent Res 54:1183-1195. Roar-k RG,Young WC (1975). Formulas for Stress and Strain. New York: McGraw-HiII, 5 17. Sarkar NK (1981). Application of potentiokinetic hysteresis technique to characterize the chlorine corrosion of highcopper dental amalgams. In: Mansfeld, I? and Bertocci, U., editors. ElectrochemicaI CorrosionTesting, ASTM STP 727: Philadelphia, 283-289. Sarkar NK, Greener EH ( 1975). Electrochemistry of the saline corrosion of conventional amalgams. J Oral Rehb 2:49-62. Sarkar NK, Fuys RA, Stanford JW (1978). Corrosion of high copper silver-tin dental amalgam. ZDR Z’rogr 57, Abstr. No. 358. Sarkar NK, Fuys RA, Stanford JW (1979). Application of electrochemical techniques to characterize the corrosion of dental alloys. In: Syrett BC, Acharya A, editors. Corrosion and Degradation of Implant MateriaIs,ASTM STP 684:277294. Wright SR, Cocks FH, Pear&I GW, Gettleman L (1980). An ultrasonic abrasion simulation test method applied to dental alloy evaluation. Corrosion 36:101-103.