UDK 669.25:537.311.6 Izvirni znanstveni članek ISSN 1580-2949 MTAEC9, 39(5)133(2005) A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH ... RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH IN NJUNIH KOVINSKIH KOMPONENTAH Z ELEKTROKEMIJSKO IMPEDANČNO SPEKTROSKOPIJO STUDIES OF PASSIVE FILMS FORMED ON TWO COBALT-BASED ALLOYS AND THEIR METAL COMPONENTS BY ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY Aleksandra Kocijan1, Ingrid Milošev2 1 Inštitut za kovinske materiale in tehnologije, Lepi pot 11, 1000 Ljubljana, Slovenija 2 Institut »Jožef Stefan«, Jamova 39, 1000 Ljubljana, Slovenija aleksandra.kocijanŽimt.si Prejem rokopisa – received: 2005-07-18; sprejem za objavo – accepted for publication: 2005-08-24 Raziskovali smo tvorbo pasivne plasti na površini kobaltovih zlitin in kovinskih komponent z elektrokemijsko impedančno spektroskopijo (EIS) v simulirani fiziološki raztopini in v prisotnosti dveh kompleksantov: EDTA in citrata. Ugotovili smo, da je zlitina Co-Cr-Mo bolj korozijsko odporna, kar dokazujejo večje vrednosti polarizacijskih upornosti, predvsemv pasivnem območju. Ključne besede: kobaltove zlitine, impedanca, fiziološka raztopina The formation of passive layer on the surface of cobalt-based alloys in physiological solution and in the presence of complexing agents, EDTA and citrate was studied by electrochemical impedance spectroscopy. Studies showed better corrosion resistance of Co-Cr-Mo alloy due to higher Rp values, especially in passive region. Key words: cobalt-based alloys, impedance, physiological solution 1 UVOD 1 INTRODUCTION Človeško telo je zaradi visoke koncentracije klo-ridnih ionov in kompleksantov relativno agresivno okolje za kovinske materiale. Kljub dobrim korozijskim in mehanskimlastnostimkobaltovih zlitin prihaja do sproščanja kovinskih ionov in delcev v okoliško tkivo. Sproščeni kovinski ioni lahko tvorijo med drugim kloride, okside, hidrokside in organokovinske komplekse, ki se s telesnimi tekočinami izločijo iz telesa ali pa se raznašajo po organizmu in kopičijo v nekaterih organih, kot so bezgavke, pljuča, jetra in vranica 1. Povečna koncentracija kovinskih ionov v bezgavkah in omenjenih notranjih organih lahko povzroči nekrozo 2. Milijoni sproščenih mikro- in submikrometrskih delcev, ki nastajajo pri mehanski obrabi, se kopičijo v tkivu ob protezi, kjer lahko sprožijo proces fagocitoze. Fagoci-tirani delci v makrofagih sprožijo verižno reakcijo, ki privede do omajanja in posledično do menjave proteze. Zaradi navedenih razlogov so raziskave korozijskega vedenja kobaltovih zlitin in možnosti sproščanja korozijskih produktov v človeško telo zelo pomembne. Namen našega dela je bil raziskati elektrokemijske lastnosti pasivne plasti na površini zlitin Co-Cr-Mo in Co-Ni-Cr-Mo, zlitini medsebojno primerjati in poiskati skupne točke z njunimi kovinskimi komponentami, kobaltom, kromom, nikljem in molibdenom, v simulirani Surgical-grade Co-based alloys are widely used for the manufacture of prosthetic devices. Their extensive use is based on their superior resistance to corrosion and biodegradation and to their good mechanical properties, including high ultimate tensile and fatigue strengths combined with sufficient elongation at the point of fracture 1. Implanted metal was submitted to the simultaneous effects of mechanical and chemical interactions 2. In the course of time the alloy and its corrosion products react directly with the surrounding tissues, being transported by the bloodstreamto distant organs or being partially eliminated by body fluids. During the long-term accumulation of metal corrosion products in internal organs, such as liver and kidney, the tolerance level can be exceeded, leading to conditions under which certain diseases can begin to develop 2. The natural, aggressive environment of the human body is difficult to simulate. In the present work Co-Cr-Mo and Co-Ni-Cr-Mo alloys and their individual metal components (Co, Cr, Ni, and Mo) were studied in a simulated physiological solution (SPS), with and without the addition of the complexing agents EDTA and citrate. The latter simulated the influence of bio-molecules, which in vivo can act as complexing agents for metal ions. The study was conducted using electro- MATERIALI IN TEHNOLOGIJE 39 (2005) 5 133 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH ... fiziološki raztopini, to je v Hankovi raztopini, pri pH = 7,8. Raziskovali smo tudi vpliv dodatka dveh komplek-santov: EDTA in citrata, na korozijsko vedenje omenjenih materialov. Pri tem smo uporabili elektrokemijsko impedančno spektroskopijo (EIS). Impedančna spektroskopija je zelo razširjena metoda za preučevanje elektrokemijskih sistemov, ki omogoča določitev ionske in elektronske prevodnosti ter študij osnovnih elementarnih procesov 3. Odlikujeta jo natančnost in možnost ločevanja posameznih elementarnih transportnih procesov, ki potekajo v materialih. Pri določevanju električnih lastnosti materialov navadno uporabljamo trielektrodne celice. V takih sistemih poteka več različnih transportnih procesov, npr. transport električnega naboja v notranjosti materiala, prenos naboja na fazni meji in v raztopini. Procesi se odzivajo v različnih frekvenčnih območjih, če potekajo "zaporedno", in z različnimi relaksacijskimi časi. V takih primerih lahko posamezne transportne procese ločimo med seboj. Elektrokemijski sistem vzbujamo z nizko napetostjo sinusne oblike, impedančni odziv pa dobimo z merjenjem razmerja med vzbujevalno napetostjo U in tokovnimodzivom I kot funkcijo krožne frekvence m. Vrednost Z(a>) pri izbrani frekvenci :\ Co -0,9 V _ / aA*1 mHz / °°űo 10 kHz citrat EDTA SFR 0 2 4 6 8 10 -Ga-0-,7V 10 15 20 25 30 0,0 0,5 1,0 1,5 2,0 2,5 Slika 1: Nyquistovi diagrami za Co pri (a) –0,9 V, (b) –0,7 V in (c) –0,3 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 1: Nyquist plot recorded for Co at (a) –0.9 V, (b) –0.7 V and (c) -0.3 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure 60 50 40 30 20 10 0 -10 500 400 300 200 100 0 0,5 0,4 0,3 0,2 (b) Cr-0,3 V 400 V 300 / 200 1 *5Č j!Čč 100 sč> 0 . f Čfky 100 200 300 0246 ° citrat EDTA " SFR Zreal / kOhm cm-2 Slika 2: Nyquistovi diagrami za Cr pri (a) –0,7 V, (b) –0,3V, (c) 0,1 V, (d) 0,6 V in (e) 0,8 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 2: Nyquist plot recorded for Cr at (a) –0.7 V, (b) –0.3 V, (c) 0.1 V, (d) 0.6 V and (e) 0.8 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure raztapljati (slika 2d). Impedančni odziv je v obliki odseka na abscisi in polkroga, ki pri nizkih frekvencah že nakazuje obliko polža, ki je značilna za korozijske procese. Opisali smo ga z nadomestno shemo, sestavljeno iz serijskega upora (Re) in enega RQ-elementa. Ta nadomestna shema natančno ne opiše procesa pri nizkih frekvencah, ker ne upošteva delnih anodnih in katodnih reakcij, vendar to ne vpliva na meritve polarizacijskih upornosti, ker so te neodvisne od oblike reaktivne komponente in bi bila uporaba kompleksnega modela nepotrebna 6. Polarizacijske upornosti so v primerjavi s pasivnim območjem mnogo manjše, in sicer: 6,5 k?/cm2 v SFR, 2,2 k?/cm2 v citratu in 1,8 k?/cm2 v EDTA (slika 7). Pri potencialu 0,8 V, kjer so gostote toka že precej visoke, je impedančni odziv v obliki polža zelo izražen (slika 2e). Opisali smo ga tako kot pri 0,6 V. Polarizacijske upornosti so v skladu z višjimi gostotami toka 4 še nižje: 1,7 k?/cm2 v SFR, 200 ?/cm2 v citratu in 110 ?/cm2 v EDTA (slika 7). Pri niklju je impedančni odziv pri potencialu polarizacije –0,75 V sestavljen iz odseka na abscisi in polkroga (slika 3a). Polarizacijske upornosti so 36 and the values of the polarization resistance were around 105 W/cm2 (Figure 7). The results at a potential of 0.1 V for Cr are similar to the case of -0.3 V; the same equivalent circuits were used (Figure 2c). Chromium was still in the passive region, and the values of the polarization resistance were around 105 W/cm2 (Figure 7). The addition of the complexing agents did not affect the impedance response due to the increasing thickness of the passive layer on the Cr. At a potential of 0.6 V the Cr entered into the region of transpassive oxidation where the dissolution of the passive layer started (Figure 2d). The impedance response consisted of a high-frequency intercept and a low-frequency arc, which bent into a spiral characteristic for the corrosion processes (Figure 2b). The equivalent circuit combined one RQ element in series with the Re. The scheme did not describe the processes at low frequencies that are related to partial anodic and cathodic reactions. However, that did not influence the measurements of the polarization resistance due to their independence of the form of the reactive component and, therefore, the application of a more complex model 4 16 12 4 -0,5 U 5 -0,1 0,0 0,4 0,8 1,2 Zreal / kOhm cm-2 MATERIALI IN TEHNOLOGIJE 39 (2005) 5 137 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH k?/cm2 v SFR, 35 k?/cm2 v citratu in 25 k?/cm2 v EDTA (slika 7). Pri potencialu –0,4 V prihaja do oksidacije niklja v NiO, kar je razvidno na cikličnih voltamogramih iz pojava vrha (slika 3b). Impedančni odziv je sestavljen iz odseka na abscisi in lepo izraženega polkroga. Vrednost polarizacijske upornosti v SFR je podobna kot pri –0,75 V, v prisotnosti kompleksantov pa se zmanjša, predvsem v EDTA, in je 17,5 k?/cm2 v citratu in 5,5 k?/cm2 v EDTA (slika 7). V prisotnosti kompleksantov pride do kompleksacije ionov Ni2+, ki nastajajo pri oksidaciji Ni, in s temposledično do tanjše zaščitne plasti. Pri potencialu 0 V je nikelj v pasivnemobmočju, impedančni odziv je sestavljen iz odseka na abscisi in polkroga (slika 3c). Polarizacijske upornosti so reda velikosti 105 ?/cm2, so višje kot pri drugih dveh polari-zacijskih potencialih, iz česar sklepamo na tvorbo debelejše zaščitne plasti. Pri molibdenu je impedančni odziv pri potencialu –0,7 V v SFR in citratu sestavljen iz odseka na abscisi in polkroga, v prisotnosti EDTA pa se pri nizkih frekvencah pojavi difuzija (slika 4a). Polarizacijska upornost je bila v SFR okrog 40 k?/cm2, v citratu 35 k?/cm2 in v EDTA 20 k?/cm2 (slika 7). would be unnecessary 6. The values of the polarization resistance were lower compared with the passive region, i.e., 6.5 kW/cm2 in the SPS, 2.2 kW/cm2 in the citrate and 1.8 kW/cm2 in the EDTA (Figure 7). At a potential of 0.8 V the current densities were higher than at a potential of 0.6 V; therefore, the impedance response in the form of a spiral was more pronounced (Figure 2e). The equivalent circuit combined one RQ element in series with the Re. The values of the polarization resistance were, in accordance with higher current densities 4, even lower, i.e., 1.7 kW/cm2 in the SPS, 200 W/cm2 in the citrate and 110 W/cm2 in the EDTA (Figure 7). The impedance response for Ni at a potential of -0.75 V consisted of a high-frequency intercept and a low-frequency arc (Figure 3a). The polarization resistance values were 36 kW/cm2 in the SPS, 35 kW/cm2 in the citrate and 25 kW/cm2 in the EDTA (Figure 7). At a potential of -0.4 V the oxidation of Ni to NiO started. The impedance response consisted of a high-frequency intercept and a low-frequency arc (Figure 3b). The polarization resistance values in the SPS were similar to the values at -0.75 V, with the addition of the complexing agent the values decreased, especially in the 50 100 150 200 250 300 Z / kOhm cm-2 Slika 3: Nyquistovi diagrami za Ni pri (a) –0,75 V, (b) –0,4 V in (c) 0 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 3: Nyquist plot recorded for Ni at (a) –0.75 V, (b) –0.4 V and (c) 0 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure (a) Mo -0,7 V /% 1 mHz f S Ą -ČXČ\ ° citrat f ČČČČH EDTA i R* ¦ SFR - *10 kHz I 0 5 10 15 20 25 30 35 20 01234 Z / kOhm cm-2 Slika 4: Nyquistovi diagrami za Mo pri (a) –0,7 V, (b) –0,4 V in (c) –0,1 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 4: Nyquist plot recorded for Mo at (a) –0.7 V, (b) –0.4 V and (c) -0.1 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure 6 12 4 30 0 138 MATERIALI IN TEHNOLOGIJE 39 (2005) 5 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH . 300 250 -200 150 100 50 0 0 2 4 6 8 10 12 14 0 50 100 150 200 250 300 300 200 0,6 0,4 0,2 -0,0 -0,2 0 100 200 300 400 500 -0,4 0,0 0,4 0,8 1,2 1,6 citrat EDTA SFR 0,00 0,05 0,10 0,15 0,20 0,25 Zreal / kOhm cm-2 Slika 5: Nyquistovi diagrami za zlitino Co-Ni-Cr-Mo pri (a) –0,7 V, (b) –0,3 V, (c) 0,1 V, (d) 0,6 V in (e) 0,8 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 5: Nyquist plot recorded for Co-Ni-Cr-Mo alloy at (a) –0.7 V, (b) –0.3 V, (c) 0.1 V, (d) 0.6 V in (e) 0.8 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure 200 0,06 0,04 -0,02 0,00 0,00 0,04 0,08 0,12 0,16 Zreal / kOhm cm-2 Slika 6: Nyquistovi diagrami za zlitino Co-Cr-Mo pri (a) –0,7 V, (b) –0,3 V, (c) 0,1 V, (d) 0,6 V in (e) 0,8 V v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata z ustreznimi nadomestnimi shemami Figure 6: Nyquist plot recorded for Co-Cr-Mo alloy at (a) –0.7 V, (b) –0.3 V, (c) 0.1 V, (d) 0.6 V in (e) 0.8 V in SPS and SPS containing 50 mM EDTA or citrate. The equivalent circuit models are inserted in the figure Pri potencialu –0,4 V so vsi trije impedančni odzivi v obliki polkroga (slika 4b). Polarizacijske upornosti so višje kot pri –0,7 V, kar kaže na rast pasivne plasti, in so 75 k?/cm2 v SFR, 60 k?/cm2 v citratu in 25 k?/cm2 v EDTA (slika 7). Pri potencialu –0,1 V molibden prehaja v območje transpasivne oksidacije, kar je razvidno tudi iz impe-dančnega odziva v obliki polkrogov, kjer so vrednosti polarizacijskih upornosti precej manjše kot v preostalih dveh primerih, in sicer 3750 ?/cm2 v SFR, 400 ?/cm2 v citratu in 325 ?/cm2 v EDTA (slika 4c). Pri zlitini Co-Ni-Cr-Mo je impedančni odziv pri potencialu –0,7 V sestavljen iz odseka na abscisi in dveh polkrogov (slika 5a). Polkrog pri višjih frekvencah smo pripisali elektrodni reakciji, vrednosti polarizacijske upornosti so reda velikosti 1000 ?/cm2 (slika 8). Pri nižjih frekvencah se pojavi difuzija, kar je razvidno tudi iz vrednosti difuzijskih koeficientov (n2) okrog 0,5, ki so značilne za difuzijski proces, v temprimeru gre verjetno za difuzijo ionov Co2+. Pri potencialu –0,3 V in 0,1 V je zlitina Co-Ni-Cr-Mo v pasivnem območju, impedančni odziv je EDTA, i.e., 17.5 in the citrate and 5.5 kW/cm2 in the EDTA (Figure 7). In the presence of the complexing agent, complexation of the released Ni2+ ions fromthe oxidation of Ni occurs, and consequently a thinner passive layer is formed. At a potential of 0 V the Ni is in the passive region, the impedance response is combined from a high-frequency intercept and a low-frequency arc (Figure 3c). The values of the polarization resistance are around 105 W/cm2; they are higher than at the other two potentials and, accordingly, a thicker passive layer is formed. In the case of Mo at a potential of -0.7 V the impedance response consisted of a high-frequency intercept and a low-frequency arc in the SPS and in the citrate (Figure 4a). In the presence of the EDTA the diffusion occurs at low frequencies. The polarization resistance values were 40 kW/cm2 in the SPS, 35 kW/cm2 in the citrate and 20 kW/cm2 in the EDTA (Figure 7). The impedance response for Mo at a potential of -0.4 V consisted of a high-frequency intercept and a low-frequency arc in all three solutions (Figure 4b). The 12 8 4 4 0 500 300 400 0,08 0,04 0,00 -0,04 MATERIALI IN TEHNOLOGIJE 39 (2005) 5 139 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH 60 50 Co . — citrat — EDTA \ —•— SFR 40 / 30 / 20 / 10 - \Č-- 0 1e+3 1e+3 8e+2 6e+2 4e+2 2e+2 0 -0,8 -0,6 -0,4 E / V vs. NKE -0,4 0,0 0,4 0,8 E / V vs. NKE 350 300 250 200 -150 100 -50 0 -0,8 -0,6 -0,4 -0,2 E / V vs. NKE -0,6 -0,4 -0,2 0,0 E / V vs. NKE Slika 7: Vrednosti polarizacijskih upornosti v odvisnosti od potenciala polarizacije za kovine Co, Cr, Ni in Mo v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata Figure 7: Evolution with the potential of polarization resistance values obtained for Co, Cr, Ni and Mo in SPS and SPS containing 50 mM EDTA or citrate sestavljen iz odseka na abscisi in enega polkroga (slika 5b, 5c). Vrednosti polarizacijske upornosti so visoke, reda velikosti 105 ?/cm2, kar potrjuje prisotnost izredno stabilne pasivne plasti, zato je tudi vpliv kompleksantov na impedančni odziv zanemarljiv. Pri potencialu 0,6 V preide zlitina Co-Ni-Cr-Mo v območje transpasivne oksidacije, kjer pride do raztapljanja pasivne plasti (slika 5d). Impedančni odziv je značilen za korozijske procese in ima obliko polža. Polarizacijske upornosti se zmanjšajo za 4-krat (slika 8), kar potrjuje rezultate anodne polarizacije, kjer gostota toka močno naraste pri potencialih, večjih od 0,4 V. Pri potencialu 0,8 V je impedančni odziv v obliki polža še bolj izražen, polarizacijske upornosti so še manjše, kar kaže na nadaljnje raztapljanje pasivne plasti (slika 5e). Rezultati impedančnih meritev za zlitino Co-Cr-Mo so podobni tistim za zlitino Co-Ni-Cr-Mo (slike 6a–e). Vrednosti polarizacijskih upornosti so nekoliko višje od tistih za zlitino Co-Ni-Cr-Mo, kar kaže na večjo stabilnost zlitine Co-Cr-Mo (slika 8). Obe zlitini sta najbolj stabilni v SFR in najmanj v EDTA, kar potrjujejo znižane vrednosti polarizacijskih upornosti v prisotnosti kompleksantov. 4 DISKUSIJA Rezultati elektrokemijskih meritev so pokazali, da pri zlitini Co-Cr-Mo v SFR ni klasičnega maksimuma pri prehodu med aktivnim in pasivnim stanjem. Na 1e+3 8e+2 6e+2 4e+2 2e+2 0 1,2e+3 1,0e+3 8,0e+2 6,0e+2 4,0e+2 2,0e+2 0,0 -0,4 0,0 0,4 E / V vs. NKE -0,4 0,0 0,4 0,8 E / V vs. NKE Slika 8: Vrednosti polarizacijskih upornosti v odvisnosti od potenciala polarizacije za zlitini Co-Ni-Cr-Mo in Co-Cr-Mo v SFR in v prisotnosti 50 mM EDTA ali 50 mM citrata Figure 8: Evolution with the potential of polarization resistance values obtained for Co-Ni-Cr-Mo and Co-Cr-Mo alloy in SPS and SPS containing 50 mM EDTA or citrate polarization resistance values were higher than at -0.7 V, which pointed to the growth of the passive layer, i.e., 75 kW/cm2 in the SPS, 60 kW/cm2 in the citrate and 25 kW/cm2 in the EDTA (Figure 7). At a potential of -0.1 V Mo entered the transpassive region, which was shown from the impedance response in the form of two well-defined arcs (Figure 4c). The polarization resistance values were lower than at the other two potentials, i.e., 3750 W/cm2 in the SPS, 400 W/cm2 in the citrate and 325 W/cm2 in the EDTA (Figure 7). The impedance response for the Co-Ni-Cr-Mo alloy at a potential of -0.7 V consisted of a high-frequency and a low-frequency arc (Figure 5a). The high-frequency arc was related to the electrochemical reaction, the values of the polarization resistance were around 1000 W/cm2 (Figure 8). At low frequencies diffusion was present, which is evident from the diffusion coefficients (n) around 0.5 that are characteristic for diffusion processes. At potentials of -0.3 V and 0.1 V the Co-Ni-Cr-Mo alloy is in the passive region, the impedance response consisted of a high-frequency intercept and a low-frequency arc (Figures 5b and 5c). The values of the polarization resistance were around 105 W/cm2, confirming the formation of an extremely protective passive layer; hence, the influence of the complexing agents was negligible. At a potential of 0.6 V the Co-Ni-Cr-Mo alloy moved to the region of transpassive oxidation and underwent dissolution of the passive layer (Figure 5d). The impedance response was, as is characteristic for corrosion processes, in the form of a spiral. The values of the polarization resistance were reduced four times (Figure 8), which confirms the results of the anodic polarization where the current density rapidly increases for potentials higher than 0.4 V 4. The impedance response at a potential of 0.8 V was in the form of a spiral, the values of the polarization resistance were lower, which confirms a further dissolution of the passive layer (Figure 5e). -0,8 -0,8 0,8 -1,0 -0,2 -U,8 80 BO 40 20 0,0 -0,8 140 MATERIALI IN TEHNOLOGIJE 39 (2005) 5 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH potencialih, nižjih od potenciala pasivacije (–0,3 V), zlitina še ni pasivna, kar je razvidno iz meritev elektrokemijske impedančne spektroskopije, kjer so vrednosti polarizacijske upornosti nizke in je prisoten difuzijski proces. Prehod v pasivno območje je povezan s povečanjempolarizacijske upornosti in pojavom polkroga na Nyquistovih diagramih, gostota toka se ne spreminja več s povečanjem potenciala. Vzpostavitev pasivne plasti je posledica povečanja debeline plasti Cr2O3. V notranjemdelu pasivne plasti pa postane izrazita tudi tvorba CoO in MoO34. Pasivno območje je stabilno do 0,4 V, ko začne gostota toka intenzivno naraščati, istočasno se zmanjša tudi polarizacijska upornost. Vpliv kompleksantov na elektrokemijsko vedenje čistih kovin Co, Ni in Mo je zelo izražen. Stopnja vpliva kompleksantov na posamezne kovinske ione je odvisna od konstant stabilnosti kovinskih kompleksov 7. Nikelj in kobalt tvorita močne komplekse z EDTA, zato opazimo naraščanje gostote toka. Istočasno se zmanjšujejo vrednosti polarizacijskih upornosti. Premik korozijskega potenciala lahko razložimo z vplivom dodatka komplek-santa na ravnotežni potencial oz. koncentracijo raztopljenih kovinskih zvrsti. To potrjujejo tudi povečane koncentracije kovinskih ionov v raztopini v prisotnosti kompleksantov 8. Slabšo korozijsko odpornost Co, Ni in Mo v prisotnosti kompleksantov potrjujejo tudi impe-dančne meritve, kjer so vrednosti Rp pri vseh polariza-cijskih potencialih manjše kot v SFR. Kompleksanti vplivajo na obe zlitini. V pasivnem območju je ta vpliv zanemarljiv zaradi tvorbe izredno stabilne pasivne plasti Cr2O3. Dobro korozijsko odpornost kroma potrjujejo tudi nizke koncentracije kromovih ionov v raztopini, izmerjene s polarografijo 8. EDTA tvori zelo močne komplekse tudi z ioni Cr3+ 9, vendar zaradi zelo počasnega raztapljanja močno zaščitne plasti Cr2O3 je koncentracija Cr(III) zvrsti, ki je na voljo za kompleksacijo, izredno majhna 8. Pri drugih potencialih sta obe zlitini korozijsko manj stabilni v prisotnosti kompleksantov, kar je razvidno iz nižjih vrednosti Rp, ker prihaja do tvorbe kompleksov Co2+, Ni2+ in Mo6+ z EDTA ali citratomin posledično do slabše pasivacije. 5 SKLEPI Iz dobljenih rezultatov lahko sklenemo, da se obe zlitini vedeta precej podobno v simulirani fiziološki raztopini, vendar pa je zlitina Co-Ni-Cr-Mo kljub vsemu nekoliko manj korozijsko odporna, predvsem v bolj agresivnemokolju v prisotnosti kompleksantov, kar se kaže v nekoliko tanjši pasivni plasti in večjemvplivu kompleksantov na elektrokemijske karakteristike te zlitine. Rezultati so tako potrdili naše domneve, da je zlitina brez niklja in z večjimdeležemkroma bolj korozijsko stabilna. MATERIALI IN TEHNOLOGIJE 39 (2005) 5 The results of the impedance measurements for the Co-Cr-Mo alloy were similar to those for the Co-Ni-Cr-Mo alloy (Figures 6a–e). The values of the polarization resistance were somewhat higher than in the case of the Co-Ni-Cr-Mo alloy, which indicates a better corrosion resistance of the Co-Cr-Mo alloy (Figure 8). Both alloys were the most stable in the SPS and the least stable in the EDTA, which is confirmed by the reduced values of the polarization resistance in the presence of the complexing agents. 4 DISCUSSION The results of the electrochemical measurements show that for the Co-Cr-Mo alloy there is no classical maximum at the crossing between the active and the passive states. At potentials lower than the passivation potential, i.e., -0.3 V, the alloy is not yet in the passive state, which is evident fromthe impedance response, where values of the polarization resistance are low and the diffusion process is present. Transfer to the passive region is related to the increase of the polarization resistance and to the formation of the arc in the Nyquist plots. The current density no longer alters with increasing potential. The formation of the passive layer is a result of the thickening of the passive layer of Cr2O3. In the inner part of the passive layer CoO and MoO3 are also present 4. The passive region is stable up to a potential of 0.4 V, where the current density starts to increase abruptly, and simultaneously the polarization resistance decreases. The electrochemical characteristics of pure metals, i.e., Co, Ni and Mo, are significantly affected by the addition of the complexing agents. The level of influence on the particular metal ions depends on the stability constants of the metal complexes 7. Nickel and cobalt formstrong complexes with the EDTA, consequently an increase of the current density simultaneously with a decrease of the polarization resistance is observed. The shift of the corrosion potential is explained by the influence of the complexing agent on the equilibrium potential or on the concentration of the dissolved metal species, which is confirmed by the increased concentration of metal ions in the solution in the presence of the complexing agents 8. The inferior corrosion resistance of Co, Ni and Mo in the presence of the complexing agents is confirmed by the EIS, where the Rp values in the presence of the EDTA and the citrate are lower than in the SPS at all polarization potentials. Both alloys are influenced by the addition of the complexing agents. In the passive region the influence is negligible due to the formation of an extremely protective layer of Cr2O3. The excellent corrosion resistance of chromium is confirmed by low concentrations of Cr ions in the solution, measured by polarography 8. The EDTA forms very strong complexes with Cr3+ ions 9; however, due to an extremely slow dissolution of the highly protective layer of Cr2O3, the 141 A. KOCIJAN, I.MILOŠEV: RAZISKAVE PASIVNIH PLASTI NA DVEH KOBALTOVIH ZLITINAH 6 LITERATURA/REFERENCES 1 K. Merrit, S. A. Brown, N. A. Sharkey, The binding of metal-salts and corrosion products to cells and proteins in vitro, J. Biomed. Mater. Res. 18 (1984), 991–1004 2 N. J. Hallab, C. Vermes, C. Messina, K. A. Roebuck, T. T. Glant, J. J. Jacobs, Concentration- and composition-dependent effects of metal ions on human MG-63 osteoblasts, J. Biomed. Mater. Res. 60 (2002), 420–433 3 D. Raistrick, J. R. Macdonald, D. R. Franceschetti v: Impedance Spectroscopy Emphasizing Solid Materials and Systems, John Wiley & Sons, New York 1987, 2 4 A. Kocijan, I. Milošev, B. Pihlar, Cobalt-based alloys for orthopaedic applications studied by electrochemical and XPS analysis, J. Mater. Sci., Mater. Med., 15 (2004), 643–650 5 A. Kocijan, I. Milošev, Electrochemical and XPS studies of cobalt-based alloys in a simulated physiological solution, Mater. tehnol. 38 (2004) 6, 339–343 6 N. Bonanos, B. C. H. Steele, W. B. Johnson, W. L. Worell, D. D. Macdonald, M. C.H. McKurbe v Impedance Spectroscopy Emphasizing Solid Materials and Systems, John Wiley & Sons, New York, 1987, 4 7 I. Milošev, Effect of complexing agents on the electrochemical behaviour of orthopaedic stainless steel in physiological solution, J. Appl. Electrochem. 32 (2002), 311–320 8 A. Kocijan, I. Milošev, B. Pihlar, The influence of the complexing agent and proteins on the corrosion of stainless steels and their metal components, J. Mater. Sci., Mater. Med., 14 (2003), 69–77 9 Critical Stability Constants, Plenum Press, New York, London, 1977, 3 concentration of Cr(III) species, in option for complexation, is very small 8. Both alloys are less corrosion resistant in the presence of complexing agents at other polarisation potentials, which is evident from lower Rp values due to the formation of the complexes Co2+, Ni2+ and Mo6+ with the EDTA or citrate, and therefore the passivation is inhibited. 5 CONCLUSIONS An electrochemical study of Co-Ni-Cr-Mo and Co-Cr-Mo alloys showed that both alloys act rather similarly in simulated physiological solutions; however, in spite of this, Co-Ni-Cr-Mo alloy is somewhat less corrosion resistant, especially in the more aggressive environment of complexing agents. This results in a thinner passive layer and the pronounced influence of complexing agents on the electrochemical characteristics of the alloy mentioned above. The results thus confirmed our predictions that the alloy without nickel and with a larger chromium content displays superior electrochemical characteristics. 142 MATERIALI IN TEHNOLOGIJE 39 (2005) 5