recently , serious concerns 
 have been reported regarding the mechanical 
 performance and safety of metal biomedical implants . among the many factors that can affect implant performance , metal - on - metal ( mom ) hip replacements , typically made of cocrmo 
 alloys , degrade because of the synergistic effects of mechanical wear 
 from metal  metal contact and electrochemical reactions with 
 salts , proteins , and other organic species . these 
 degradative processes release metal ions and nanoscopic wear particles 
 in vivo that often contribute to implant failure . however , 
 studies have shown that a tribofilm , composed of organic and metallic 
 constituents , can form under these same conditions on the surface 
 of cocrmo implants . implants with such 
 a tribofilm have demonstrated increased corrosion resistance ; the 
 film may also act as a lubrication layer , enabling better wear resistance . the organic constituents of this tribofilm are 
 presumably derived from proteins in synovial fluid , but mechanisms 
 causing the protein decomposition necessary for film formation are 
 unknown . the current theory suggests that frictional processes contribute 
 significantly to its development , with the combination of shear stresses 
 and a rise in local contact temperatures causing protein degradation . because it has been shown that tribofilms hold 
 co , cr , and mo ions , it is additionally 
 hypothesized that local variations in corrosion kinetics involving 
 proteins and metal ions contribute to the film formation . in 
 the study described here , we combined standard electrochemical 
 techniques with a quartz crystal microbalance ( qcm ) mass sensor to 
 delineate corrosion mechanisms of cocrmo and to investigate if tribofilm - like 
 layers can be generated electrochemically . the qcm enables in situ 
 quantification of surface changes accompanying corrosion and is very 
 sensitive to small changes in the total mass of a thin film . here , it was used to measure mass changes associated with the different 
 sublayers of the alloy surface , including the metal , metal oxide , 
 and any organic deposit , as depicted in the schematic in figure 1 . when an oscillating voltage is 
 applied across the electrodes , the quartz vibrates and its resonance 
 frequency can be measured . if the mass on the electrode surface changes , 
 so will the resonance frequency , providing detailed information that 
 quantifies mass changes down to the nanogram level . the qcm is also sensitive to viscoelastic properties that 
 introduce dissipation and reduce the quality factor of the resonance . although a variety of equivalent measures can 
 be used to quantify this dissipation , we use the half - width at half - maximum 
 of the conductance peak centered at the resonance frequency . the use 
 of this quantity , which we refer to as  , has been used extensively 
 by others and is discussed in more detail in a review article by johannsmann . schematic representation of a metal surface under oxidizing 
 conditions 
 in the presence of macromolecules that are able to form an organic 
 deposit at the metal / solution interface . the qcm has been used to study a variety of corrosion processes 
 in different materials , including fe  cr and au  pd  in 
 alloys . recently , valero vidal et al . employed an electrochemical qcm to investigate passive 
 film growth on cocrmo films in phosphate - buffered saline ( pbs ) . mass 
 loss was observed in all anodic potential ranges investigated , even 
 during oxide growth , but was accelerated upon transpassive dissolution . 
 in the present study , the corrosion behavior of cocrmo thin films 
 was measured in three different fluids that mimic different aspects 
 of the physiological environment . electrochemical qcm measurements 
 were employed to quantify mass changes associated with the corrosion 
 process . in protein solutions , it was found that an organic layer 
 deposits on the surface of the film at elevated potentials . a series 
 of potentiodynamic and potentiostatic tests indicated critical criteria 
 for deposition of this organic layer . additionally , similar corrosion 
 tests were conducted on thin films of the individual alloying elements 
 ( co , cr , and mo ) to determine if a particular ion is responsible for 
 the organic deposition . as we describe in the following sections , 
 it was found that a significantly thick organic film formed on the 
 mo films , suggesting that mo ions are essential to promoting organic 
 film formation on the alloy . working electrode 
 specimens were prepared on at - cut , 5 mhz quartz crystals with chrome 
 electrodes ( inficon , east syracuse , ny ) . cocrmo , cr , and mo films 
 were fabricated in a magnetron sputter deposition chamber ( aja orion , 
 n. scituate , ma ) at a base pressure of 10 torr 
 with ar as the working gas . cocrmo was deposited with a multistep 
 sputter method modified from nakagawa et al . , which was designed to 
 control the film s crystallinity and internal stresses . first , the chamber was heated to 300 c at a 
 rate of 14 c / min ; crystals were then cleaned with a plasma strike . 
 subsequently , cr was sputtered for 200 s at 100 w power and 3 mtorr 
 pressure to produce an adhesion layer . using a wrought , low - carbon 
 cocrmo target ( astm f1537 ) , a cocrmo underlayer was deposited by sputtering 
 for 100 s at 180 w power and 0.25 mtorr . finally , cocrmo was sputtered 
 for 2000 s at 150 w power and 4 mtorr pressure . cr films were made 
 by sputtering at 300 c and 3 mtorr pressure for 3000 s at 180 w power 
 following the plasma cleaning step . following the plasma strike , a cr adhesion layer was 
 deposited by sputtering for 200 s at 100 w power ; mo was then sputtered 
 for 2000 s at 180 w power . co films were prepared using an edwards 
 auto 500 electron beam evaporator ( boc edwards , wilmington , ma ) evacuated 
 to a base pressure of 10 torr . a cr adhesion layer 
 ( 5 nm thick ) was first deposited at a rate of 0.08 nm / s at 40 ma beam 
 current . co ( international advanced materials , spring valley , ny ) 
 was deposited at the same rate at 100 ma beam current to a thickness 
 of between 130 and 300 nm . low - carbon cocrmo pins ( astm f1537 ) with 
 a diameter of 7 mm were utilized for bulk specimen tests . prior to 
 testing , pins were ground and polished with standard metallographic 
 methods to a mirror finish ( ra < 0.02 m ) . the 
 sputtered metal films on the quartz substrate acted as the working 
 electrode , with an exposed area of 2.85 cm . two electrical 
 leads connected the qcm holder to a qcm network analyzer and the potentiostat 
 ( biologic sp-150 , france ) , as shown schematically in figure 2 . mammalian ringer s salt solution ( electron 
 microscopy services , hatfield , pa ) , buffer solution ( ph 7.6 ) , or bovine 
 calf serum ( bcs ) served as the electrolyte . bcs is commonly used as 
 a testing fluid for wear tests of orthopedic bearings . its protein 
 content is comparable to that of natural joint fluid ( synovial fluid ) 
 with 30 g / l protein . ringer s solution was used as manufactured 
 and consisted of nacl , kcl , mgcl2 , cacl2 , nahco3 , and glucose . buffer solution was prepared with 0.154 m nacl , 
 0.223 m tris buffer , and 53.7 mm edta , adjusted to ph 7.6 with hcl . bcs was prepared by diluting newborn calf serum ( invitrogen , grand 
 island , ny ) in buffer solution . prior to each experiment , the 
 open circuit potential was measured for 3 min by allowing the potential 
 at the working electrode to equilibrate in the electrolyte . next , 
 the specimen was held at a cathodic potential of 0.9 v for 
 10 min to clean the surface and remove any oxide film that may have 
 formed in air . the open circuit potential was then measured after 
 the system was allowed to stabilize for 1 h. subsequently , potentiodynamic 
 and potentiostatic tests were conducted under varying conditions while 
 the qcm frequency response was measured at the first , third , and fifth 
 harmonics ( n = 1 , 5 mhz ; n=3 , 15 mhz ; n=5 , 25 mhz ) . in many cases , qcm data 
 from only the third harmonic are reported for clarity , but the data 
 showed good agreement among all three harmonics . as discussed in more 
 detail below , the experiments are generally conducted in the range 
 where a simple analysis based on only a single harmonic is sufficient . 
 during potentiodynamic tests , the potential was increased at a rate 
 of 0.25 mv / s from 0.8 to 1.0 v. all potentials are reported 
 against a saturated calomel reference electrode ( sce ) . the corrosion 
 potential ( ecorr ) and current ( icorr ) were extracted from the polarization curves 
 by the tafel method conducted with ec - lab 
 express software . qcm frequency shifts were related to the mass change per unit area 
 ( ma ) on the working 
 electrode surface with the sauerbrey equation1where zq is the theoretical acoustic 
 impedance of quartz ( 8.84  
 10 kg m s ) , n is the order of the measured harmonic ( 1 , 3 , or 5 ) , and f1 is the fundamental frequency ( 5 mhz ) . the 
 baseline frequency for a given harmonic corresponds to the resonance 
 frequency of the crystal at the outset of a potentiometric scan after 
 the system equilibrated for 1 h and the open circuit potential was 
 measured . the sauerbrey equation was originally applied to quartz 
 resonators in air or vacuum , but it has been subsequently shown that 
 the analysis can also be applied to mass deposition from a liquid 
 medium . the analysis holds in a liquid medium provided that the reference 
 frequency corresponds to the resonance frequency of the immersed crystal 
 prior to the deposition process and that the dissipation (  ) 
 due to viscoelastic effects is not very high . the thickness of the metal film prior to testing was also extracted 
 from eq 1 by measuring the resonance frequency 
 before and after film deposition and by assuming that the sputtered 
 density is the same as for the bulk material . x - ray photoelectron spectroscopy ( xps ) was conducted 
 with a thermo scientific escalab 250xi spectrometer ( thermo fisher 
 scientific ) using a monochromatized al k x - ray source at a 
 90 takeoff angle . survey spectra ( 01100 ev ) were obtained 
 at a 100 ev analyzer pass energy with a 1.0 ev energy step size . high - resolution 
 spectra of c 1s , o 1s , n 1s , co 2p , cr 2p , and mo 3d were obtained 
 at a 50 ev analyzer pass energy with a 0.5 ev energy step size . a 
 flood gun was employed to control the surface potential throughout 
 spectra acquisition . peak fitting was conducted with avantage software 
 ( thermo scientific ) using a shirley - type background and gaussian / lorentzian 
 peak shapes . binding energies were referenced by shifting the lowest - energy 
 c 1s peak ( hydrocarbon peak ) to 284.8 ev . five c 1s and o 1s and 1020 n 
 1s , co 2p , cr 2p , and mo 3d high - resolution scans were conducted for 
 each area . peak binding energies and atomic percentages are reported 
 as an average  one standard deviation . energy - dispersive 
 spectroscopy ( eds ) was conducted with a hitachi s-3400-nii scanning 
 electron microscope equipped with an oxford incax - act sdd energy dispersive 
 x - ray spectrometer . qcm measurements 
 indicated that sputtered cocrmo films ranged in 
 thickness from 200 to 300 nm . eds confirmed that the chemical composition 
 matched that of the bulk alloy , with approximately 66% co , 28% cr , 
 and 6% mo by weight . crystallinity was confirmed with transmission 
 electron microscopy ( tem ) in another study that showed the film to have longitudinal , nanocrystalline grains 
 and a hexagonal close - packed ( hcp ) lattice . although the microstructure 
 differs from that of the bulk alloy , it is comparable to the deformed 
 subsurface of mom hip joints , which also exhibits a nanocrystalline 
 hcp structure resulting from a strain - induced phase transformation . to determine how the electrochemical behavior 
 of the sputtered cocrmo thin film compared to that of the bulk alloy , 
 potentiodynamic tests were conducted on both specimens in a bcs electrolyte . 
 as displayed in figure 3 , both materials exhibit 
 similar electrochemical behavior , which is consistent with the reported 
 literature data.ecorr and icorr of the bulk sample were 0.463 v and 1.85  10 a / cm , respectively , and ecorr and icorr of the thin films ranged from 
 0.585 to 0.318 v and 3.79  10 to 1.82  10 a / cm , depending 
 on the sample . the qualitative features of the potentiodynamic scan 
 for the bulk material can be used to divide the response into the 
 following regimes : ( 1 ) a cathodic regime below 0.6 
 v in which the current is determined by the reduction of water or 
 dissolved oxygen ; ( 2 ) a passive regime , extending from 0.2 
 to 0.5 v , where the current is nearly independent of the applied 
 potential , indicating the presence of a protective oxide ; and ( 3 ) 
 a transpassive regime above the breakdown potential at 0.5 
 v. the current peak and subsequent stabilization from 0.65 to 0.8 
 v is typically attributed to the oxidation of cr(iii ) to cr(vi ) , although 
 the electrochemical behavior in this potential regime is poorly understood . the electrochemical behavior above 1 v could not be studied 
 because 
 the thin film was destroyed under this aggressive oxidizing condition . overall , the electrochemical behavior is characteristic of behavior 
 previously reported for cocrmo alloys . the similarity between the bulk and thin - film responses is taken 
 as evidence that the thin films are an accurate model for studying 
 corrosion processes in cocrmo alloys . the additional information provided 
 by the qcm experiments with the thin films allows us to better understand 
 the processes occurring in the different potential regimes introduced 
 above and is the focus of the remainder of this article . mass changes on cocrmo films were measured 
 in situ during potentiodynamic tests in ringer s solution , 
 buffer solution , and bcs ; the results are displayed in figure 4 with the potentiodynamic response . mass changes 
 as a function of total charge released , obtained by integrating the 
 current over time , are displayed in figure 5 . according to faraday s law , the amount of mass lost during 
 a corrosion process should scale linearly with the charge . in our 
 notation , we write faraday s law in the following way2where ma is the 
 change in mass per unit area , qa is the charge density in equivalents 
 per unit area , and mq is the molar mass per charge ratio . our sign convention here is 
 that the current is positive during corrosion processes leading to 
 decreases in the mass . note that qa is obtained from the integrated charge by dividing by faraday s 
 constant ( 96  500 c / equiv ) . in situations where corrosion mechanisms 
 are changing , the behavior can not be characterized by a single value 
 of mq . in these cases , we specify an effective value of mq from the slope of a plot of the mass change versus charge:3changes in mqeff indicate a change in corrosion mechanisms or in the valence state 
 of the corroding species . mass changes and corresponding current density 
 on cocrmo thin films 
 during potentiodynamic scans ( scan rate : 0.25 mv / s ) in ( a ) ringer s 
 solution , ( b ) buffer solution , and ( c ) bcs . in each 
 electrolyte , specimens did not exhibit appreciable mass change below 
 0.4 v , although the current began to increase at 0.3 v. in ringer s 
 solution ( figure 4a ) , the alloy continuously 
 lost mass as the potential increased above 0.4 v. figure 5a illustrates that the alloy corroded with a constant 
 value of 15 g / equiv for mqeff . this value is significantly 
 lower than what would be expected from the ideal mass per charge ratio 
 of the alloy , which can be determined from the following equation4 in which fi , ni , and ai are the 
 mass fraction , valence state , and molar mass of each alloying element , 
 respectively . if the alloy is considered to have a composition of 
 co-29cr-6mo with valence states of 2 , 3 , and 4 for the respective 
 elements , then a value of approximately 
 24 g / equiv is expected for mq . in contrast , if the valence states of the corroding species 
 are considered to be 2 , 6 and 6 , respectively , then the theoretical mq ratio is 17 , which is significantly 
 closer to our measured value . the disagreement between the experimentally 
 determined value for mqeff and theoretically 
 expected values for mq likely results from the formation of an oxide film in solution , 
 which grows while the alloy is simultaneously corroding . these results 
 are consistent with what was reported by valero vidal et al . in pbs . change in mass on cocrmo thin films as a function 
 of charge released 
 during potentiodynamic scans in ( a ) ringer s solution , ( b ) 
 buffer solution , and ( c ) bcs . in buffer solution 
 ( figures 4b and 5b ) , 
 the film displayed a more distinct passive regime between 0.4 
 and 0.2 v but again began to lose mass at 0.4 v with mqeff  = 16 g / equiv . in contrast to the experiment in ringer s 
 solution , the film that was corroded in buffer solution exhibited 
 a current peak and subsequent stabilization between 0.65 and 0.8 v. 
 here , the rate of mass loss declined . as demonstrated in figure 5b , mqeff dropped as low as 
 3.6 g / equiv between 0.75 and 0.85 v. above 0.85 v , the rate of mass 
 loss began to increase again with the current , approaching 18 g / equiv 
 by the end of the potentiodynamic scan . as indicated 
 in figure 4c , the corrosion behavior in bcs 
 was similar to that in buffer solution until the potential reached 
 0.8 v. at this point , a substantial increase in the mass was 
 observed , despite the increased current that predicts mass loss . because 
 the behavior was not seen in ringer s or buffer solution and 
 was not reported by valero vidal in pbs , mass gain presumably results from protein deposition . this deposited 
 film persists to potentials up to about 0.93 v. at higher potentials , 
 the underlying cocrmo film corroded rapidly and was removed from the 
 quartz crystal surface . the plot of mass change with charge ( figure 5c ) shows that the alloy initially lost mass with 
 an mqeff of 13 g / equiv . the rate of mass gain 
 was not linear with charge , and the rate of final mass loss was not 
 consistent among the samples . s 
 and buffer solution , the dissipation remained relatively constant 
 until the film began to lose mass . neither the potential nor the degree 
 of mass loss at which dissipation increased was consistent among the 
 tests . in bcs , dissipation increased at approximately the same potential 
 at which mass gain occurred . the ratio of the dissipation shift to 
 frequency shift ( /f ) from a 
 potentiodynamic scan in bcs is shown in figure 6 . surface roughness can increase dissipation and cause an incorrectly 
 large mass gain reading on the qcm if surface cavities entrap fluid . however , frequency shifts resulting from roughness for samples immersed 
 in a liquid medium scale as n , which was not true of our samples . the presence 
 of a viscoelastic ( rather than rigid ) material on the qcm surface 
 can also enhance dissipation . if viscoelastic effects are small , meaning 
 /f  1 , then eq 1 holds and f scales with 
 the harmonic order , n. if viscoelastic effects are 
 significant , then ma as calculated from eq 1 will depend 
 on the harmonic and a viscoelastic correction factor must be included 
 in the analysis . in our experiments , as demonstrated 
 in figure 7 , the value of ma obtained from the sauerbrey equation 
 was the same for all three harmonics prior to mass deposition and 
 during the initial stages of the mass deposition process . above 0.85 
 v , /f became significant and 
 ma became dependent 
 on n. hence , we attribute the enhanced dissipation 
 to the viscoelastic character of this adsorbed layer , which supports 
 the hypothesis that the measured mass increase in bcs results from 
 protein deposition . additional information about the viscoelastic 
 properties of the deposited layer can be obtained from a more detailed 
 analysis of f and  in this highly 
 dissipative regime . ratio of dissipation 
 to frequency shift on cocrmo thin films during 
 potentiodynamic scans in bcs . ( scan rate : 0.25 mv / s . ) mass changes on cocrmo thin films during potentiodynamic 
 scans 
 in bcs , measured at three harmonics as a function of ( a ) potential 
 and ( b ) released charge . ( scan rate : 0.25 mv / s . ) to determine if there is a critical condition that promotes 
 mass deposition on cocrmo in bcs , the potential applied to the film 
 was increased from 0.8 v and held constant at various levels . the current density , total charge ( current integrated over time ) , 
 and potential were quantified at the points at which the initial mass 
 loss occurred , the rate of mass loss decreased , the mass increase 
 began , and the final mass loss began . no clear correlation existed 
 between these processes and the current density or integrated charge . however , the potentials at each point of mass change were consistent 
 and are provided in table 1 . mass deposition 
 begins at potentials near 0.77 v in all cases , always following the 
 current peak and stabilization that is characteristic of the transpassive 
 regime . thus , it is presumed that mass deposition results from reactions 
 between proteins and the ionic species that are formed near this potential . time dependence of the potential applied 
 to cocrmo thin films ( upper ) 
 and of the corresponding mass changes ( lower ) in experiments to determine 
 the conditions promoting deposition in bcs solutions . ( a ) potential 
 ramped from 0.8 to 1.2 v. ( b ) potential ramped from 0.8 
 to 0.83 v and held at 0.83 v. ( c ) potential ramped from 0.8 
 to 0.91 v and held at 0.91 v. the data also indicate that there is a maximum potential 
 for which 
 the deposited film will remain stable for extended periods of time . 
 when the potential was increased from 0.8 v at a rate of 0.25 mv / s and then held at 0.83 v , the film experienced comparable deposition 
 as during the complete potentiodynamic scan but maintained the layer 
 throughout the test , as displayed in figure 8a , b . in contrast , when the potential was held near the end of the 
 mass gain regime at 0.91 v ( figure 8c ) , the 
 film corroded in nearly the same manner as when the potential was 
 continuously increased to 1.0 v , even though the current density was 
 comparable to that at 0.83 v ( data not shown ) . potentiodynamic 
 tests were also conducted on the pure alloying elements in bcs to 
 determine if a particular ion is responsible for mass deposition on 
 the alloy . mass changes with potential variation are shown for each 
 metal in figure 9 . cobalt specimens ( figure 9a ) the mqeff ( figure 10a ) 
 averaged 29.3  4.2 g / equiv , approximately corresponding to the 
 co(ii ) oxidation state as expected . chromium specimens ( figure 9b ) gained approximately 3.7 mg / m mass 
 at 0.38 v but continuously lost mass beginning at 0.5 v. the mass 
 gain at 0.38 v is 1 to 2 orders of magnitude smaller than on the alloy 
 and corresponds to a stabilization in the current . hence , it is likely 
 attributed to an enhancement in the oxide thickness on the chromium 
 surface . the initial value of mqeff ( figure 10b ) was approximately 9 g / equiv , nearly corresponding 
 to the dissolution of cr(vi ) ions , but decreased to 6.7 g / equiv above 
 0.85 v. mass changes and corresponding current density of ( a ) co , ( b ) cr , 
 and ( c ) mo thin films during potentiodynamic scans in bcs . ( scan rate : 
 0.25 mv / s . ) molybdenum samples ( figure 9c ) exhibited 
 significantly different mass change behavior than cobalt or chromium . the metal film began to lose mass as the potential reached 0.16 
  0.02 v with mqeff  = 15 g / equiv 
 ( figure 10c ) , corresponding to the dissolution 
 of mo(vi ) ions . as the potential neared 0.06  0.02 v or the 
 current density reached approximately 5  10 a / cm , specimens gained at least 450 mg / m mass . the 
 qcm signal was lost at 0.10 v because of the large frequency 
 shift ; therefore , tests were stopped at this potential . the mass did 
 not increase on mo films during potentiodynamic tests in either ringer s 
 or buffer solution . our hypothesis based on these results is that 
 mo(vi ) ions are critical to the initiation of mass deposition on both 
 the mo and cocrmo surfaces . neutral mo oxidizes to mo(iv ) , forming 
 moo2 according 
 to the following reaction with water above the potential e0:56this reaction occurs spontaneously at all 
 ph values relevant to our experiment , thus our assumption is that 
 a thin layer of moo2 is always present at the metal surface . 
 at higher potentials , mo(iv ) further oxidizes to mo(vi ) according 
 to the following reaction:7 the equilibrium reduction 
 potential ( vs sce ) for this reaction 
 is8no other reaction is predicted 
 until water is oxidized to evolve o2 . change in mass as a function of charge released during 
 potentiodynamic 
 scans in bcs on ( a ) co , ( b ) cr , and ( c ) mo thin films . a useful estimate of the molybdate ( moo4 ) ion concentration at the point at which mass 
 deposition occurs 
 can be obtained by assuming that all of the mass loss up to this point 
 is due to the release of mo from the surface as moo4 . from figure 9c we see that 
 mass deposition occurs when 400 mg / m of mo has 
 been released into solution , corresponding to a surface molybdate 
 concentration , [ moo4 ] , of 4  10 mol / m . these 
 ions are concentrated within a thin layer of width ,    ( dt ) , where d is the diffusion 
 coefficient of the ions 
 in solution and t is the time that the ions have 
 had to diffuse . for the molydate ions , we expect d  3  10 m / s , a value 
 comparable to the diffusion coefficient of similarly sized ions in 
 water . mass deposition occurs about 15 
 min after molybdate ions are first released in solution ( figure 9c ) . taking t = 900 s , we obtain    5  10 m and [ moo4]s/  
 8 mol / m , corresponding 
 to a molar concentration of 8  10 m. the measured potential at which oxidation to mo(vi ) occurs is consistent 
 with eq 8 provided that possible changes in 
 the local ph are accounted for . for example , if we assume [ moo4 ] = 8 mm and a ph equal to the starting 
 ph of the bcs solution ( 7.6 ) , then we obtain e0 = 0.6 v from eq 8 . at the opposite 
 extreme , we can assume that the bcs solution has no buffering capacity 
 so that the eight protons released by the combination of the reactions 
 shown in eqs 5 and 7 contribute to the decrease in the local ph . in this case , the ph 
 is reduced to 1 and we obtain e0 = 0.182 v from eq 8 . not surprisingly , the 
 measured potential range over which molybdate ions are released into 
 solution is between these two extremes . kinetic effects will also 
 play a role here because the equilibrium conditions for which eq 8 can be applied quantitatively are not established 
 under the corrosion conditions relevant to our experiments . it is likely that any change in local ph that occurs upon oxidation 
 of molydenum will induce changes in the surface charge of the mo specimen 
 and proteins in solution , leading to mass deposition from electrostatic 
 adsorption . however , the magnitude of mass increase measured by the 
 qcm is significantly greater than what would result from a monolayer 
 of adsorbed proteins . for example , ithurbide et al . found that the 
 adsorption of albumin onto a passivated chromium specimen increased 
 the mass on the surface by only 4 mg / m , which is 2 to 
 3 orders of magnitude smaller than what was measured in this study . thus , whereas the local protein / substrate interactions 
 may play an important role in the adhesion of the interfacial layer 
 to the metallic substrate , the measured mass gain can not be explained 
 by adsorption processes that by definition are confined to the immediate 
 vicinity of the metallic surface . it is more appropriate to think 
 of the mass gain as resulting from the formation of some sort of interfacial 
 phase that forms upon the release of mo(vi ) ions ( presumably in the 
 form of molybdates ) into a solution that contains appropriate macromolecules . 
 on the cocrmo alloy , the mass gain occurs at potentials within the 
 secondary passive regime where the electrochemical behavior is believed 
 to be controlled by the oxidation of cr(iii ) to cr(vi ) . however , because 
 mass deposition occurred only on the mo films and not on co nor cr , 
 we conclude that a release of mo(vi ) ions rather than cr(vi ) is necessary 
 to forming the deposit on the cocrmo surface . xps was used to assess the composition of the deposited layers on 
 qcm specimens that exhibited material deposition without subsequent 
 mass loss . the cocrmo specimen was prepared by conducting a potentiodynamic 
 scan in bcs from 0.8 to 0.83 v at 0.25 mv / s and then holding 
 the potential at 0.83 v for 30 min . the mo sample was prepared by 
 conducting a potentiodynamic scan in bcs from 0.8 to 0.08 
 v. subsequently , specimens were rinsed once in deionized water and 
 twice in methanol and dried with a compressed air canister to remove 
 any serum that may have simply adhered to the film . the peak binding energy and atomic percentage of each element is included 
 in table 2 . data confirm that the deposited 
 layer on both metals is primarily organic and derived from proteins : 
 the n 1s peak near 400 ev corresponds to the binding energy of amine 
 or amide bonds , and the n to total c ratio ( 0.22  0.02 on cocrmo , 
 0.23  0.07 on mo ) is the same as that reported for bovine serum 
 albumin powder . additionally , the decomposition 
 of the c 1s peak ( figure 11b , d ) shows evidence 
 of carbon in an organic binding environment . for cocrmo , the c 1s 
 peak was decomposed into two peaks ; the peak at 284.8  0.03 
 ev corresponds to adventitious carbon , and the peak at 287.3  
 0.34 ev corresponds to carbon in an amine or amide bond . in addition to the adventitious carbon peak , a peak 
 is present at 286.1  0.06 ev , attributed to c  o and c n 
 bonds , and at 287.7  0.06 ev , attributed 
 to peptide bonds . high - resolution scans at cr 
 2p ( 570595 ev ) and mo 3d ( 220240 ev ) on the cocrmo 
 specimen did not detect either element . our primary conclusion from 
 these results is that the deposited layer is primarily organic , with 
 a very small metallic component . an electrochemical quartz crystal microbalance was utilized to 
 investigate the corrosion of cocrmo thin films in ringer s 
 solution , buffer solution , and bovine calf serum . films were prepared 
 in a manner such that their microstructure and electrochemical behavior 
 model those of mom hip joint surfaces . the key findings emerging from 
 this work are as follows : ( 1 ) if proteins are present in solution , 
 then the qcm measures a mass increase on the alloy surface during 
 corrosion at elevated potentials . ( 2 ) typical values for the mass 
 of the deposited film range from 50 to 200 mg / m . ( 3 ) material 
 is deposited at a critical potential , 0.77 v , and can be maintained 
 for some time at slightly higher potentials , suggesting that deposition 
 results from redox reactions between oxidized alloying elements and 
 proteins in solution . ( 4 ) dissipation of the qcm signal increases 
 with mass gain , providing evidence of a viscoelastic layer . ( 5 ) qcm 
 measurements at multiple harmonics scale with the sauerbrey equation 
 prior to large increases in dissipation , indicating that mass gain 
 measurements are real and not an artifact of roughness effects . ( 6 ) 
 during the corrosion of co , cr , and mo films in protein solutions , 
 mass deposition was found only on the mo films under conditions corresponding 
 to the release of mo(vi ) ions into solution . from this , we infer that 
 the mass gain results from interactions between mo(vi ) ions and proteins 
 in solution . ( 7 ) xps analysis of cocrmo and mo films after mass gain 
 reveals a primarily organic surface , further supporting conclusions 
 that mass gain results from protein deposition . xps survey scans and 
 decomposed c 1s spectra of cocrmo and mo surfaces 
 following mass deposition in bcs . ( a ) cocrmo survey , ( b ) cocrmo c 
 1s , ( c ) mo survey , and ( d ) mo c 1s . the organic film is reminiscent of tribochemical reaction 
 layers 
 that form on the surface of mom hip bearings , suggesting the existence 
 of local electrochemical pathways that promote the protein degradation 
 essential for their formation . because of the reproducibility of the 
 experiments , these purely electrochemical experiments are very well 
 suited for fundamental investigations of tribochemical processes that 
 also involve mechanical forces . further investigations are being undertaken 
 in order to more completely understand the mechanisms involved in 
 the formation of these electrochemically induced organic reaction 
 layers in vivo and their effects on the tribological performance of 
 cocrmo alloys .
the corrosion of cocrmo , an alloy 
 frequently used in orthopedic 
 implants , was studied with an electrochemical quartz crystal microbalance 
 ( qcm ) in three physiologically relevant solutions . 
 mass changes were 
 measured during potentiodynamic tests , showing material deposition 
 in protein solutions at potential levels that caused mass loss when 
 the proteins were not present . 
 x - ray photoelectron spectroscopy ( xps ) 
 data indicated that the deposited material was primarily organic and 
 therefore was most likely derived from proteins in the electrolyte . 
 material deposition consistently occurred at a critical potential 
 and was not dependent on the current density or total charge released 
 into solution . 
 corrosion studies on pure co , cr , and mo in protein 
 solutions found material deposition only on mo . 
 we hypothesize that 
 organic deposition results from the interaction of mo(vi ) with proteins 
 in the surrounding solution . 
 the organic layer is reminiscent of tribochemical 
 reaction layers that form on the surface of cocrmo hip bearings , suggesting 
 that these types of layers can be formed by purely electrochemical 
 means .