Abstract: The study of 316L-type stainless steel reveals a significant anisotropy of nickel release that is dependent on the orientation of the test surface with respect to the casting and rolling direction. Cross-sectional specimens (transversal cuts with respect to the rolling direction) show a substantially higher sensitivity to corrosion phenomena compared with longitudinal cuts and they release nickel ions at rates 10-100 times higher. These findings indicate that orientation needs to be taken into account when interpreting test results, in particular when comparing different grades of austenitic stainless steel, as well as in product and production design.
Abstract: The role of nickel in the biological response to alloys used in medical devices is of immense significance with regard to toxicology and biological performance. There is now a tendency to take nickel out of alloys for medical applications. However, this needs careful evaluation since no compromise is acceptable with regard to mechanical properties, corrosion resistance or any other harmful consequences due to the nickel substitution. This paper analyses the corrosion behaviour and cations released for five austenitic steels, nominally "nickel-free". The analysis of electrochemical parameters, open circuit potential, polarisation resistance, Tafel slopes, corrosion current, breakdown potential, potentiodynamic polarisation curves, and coulometric analysis by zone, reveal that the new austenitic steels, nominally "nickel-free", do not behave in the same way. In the family of steels studied, quite a large dispersion is noted in the corrosion behaviour. With regard to the crevice corrosion behaviour, the steel grades studied can be classified into three groups, with crevice potentials of 600-650mV; 350-450mV and 100-150mV. The release of 18 cations (Al, Ba, Be, Cd, Co, Cr, Cu, Fe, Li, Mn, Mo, Ni, P, Pb, Sn, Sr, Ti, and V) was studied by extraction tests in artificial sweat and bone plasma fluid. The extraction tests reveal that the "nickel-free" steels indeed release only faint traces of nickel. Yet many other elements, some of them potentially harmful, are released in significant amounts. Generally, the amount of cations released is substantially higher in the artificial sweat solution than in the bone plasma.
Abstract: Precious metal based dental alloys generally exhibit a superior corrosion resistance, in particular enhanced resistance to pitting and crevice corrosion, compared to non-precious metal based alloys such as CoCr alloys. A new generation of Co-Cr alloys enriched with precious metals (Au, Pt, Ru) have now appeared on the market. The goal of this study was to clarify the effect of the precious metals additions on the corrosion behaviour of such alloys. Various commercial alloys with different doping levels were tested by electrochemical techniques in two different milieus based on the Fusayama artificial saliva and an electrolyte containing NaCl. Open circuit potentials, corrosion currents, polarization resistances, and crevices potentials were determined for the various alloys and completed by a coulometric analysis of the potentiodynamic curves. In addition, the microstructures were characterised by metallography and phase compositions analysed by EDX. The results show that the presence of precious metals can deteriorate the corrosion behaviour of Co-Cr alloys in a significant way. Gold doping, in particular, produces heterogeneous microstructures that are vulnerable to corrosive attack.
Abstract: The corrosion behavior of CoCrMo implants with rough titanium coatings, applied by different suppliers by either sintering or vacuum plasma spraying, has been evaluated and compared with uncoated material. The open-circuit potential, corrosion current and polarization resistance were determined by electrochemical techniques. The Co, Cr and Ti ions released from the samples into the electrolyte during a potentiostatic extraction technique were analyzed using ICP-MS. The Ti coatings from the different suppliers showed a different porous morphology, and the implants exhibited a distinct corrosion activity, underlining the importance of the coating process parameters. Among the titanium coated samples, the one with the sintered overcoat turned out to be the most resistant. Yet, on an absolute scale, they all showed a corrosion resistance inferior to that of uncoated CoCrMo or wrought titanium.
Abstract: The corrosion behavior of titanium with vacuum plasma sprayed titanium coatings and with anodized surfaces, both with and without polymeric bone cement were evaluated. Electrochemical extraction tests were carried out with subsequent analysis of the electrolyte by ICP-MS in order to verify our hypothesis of the ionic permeability of the polymer cement. The complexity of the situation resides in the existence of two interfaces: electrolyte-polymer and polymer-metal. The surface preparation (treatment of the surface) plays an important role in the corrosion resistance of titanium. The electrochemical magnitudes that were examined reveal that the plasma spray surfaces have the lowest corrosion resistance. The cement, in spite of having reduced electrical conductivity in comparison to metal, is an ionic transporter, and therefore capable of participating in the corrosion process. In the present study, we observed in fact crevice corrosion at the metal-cement interface. In the case of plasma spray surfaces, a process of diffusion of titanium particles in the electrolyte could accompany the crevice corrosion. In this study, we have shown that there is a corrosion process at the surface of the titanium through the cement which has as a consequence on the one hand the formation of titanium cations and on the other hand the growth of a passive layer on the titanium. In conclusion, we identified two principal factors that influence the corrosion process: [1] the type of surface treatment for the titanium, and [2] the ionic conductivity of the cement. There is indeed ionic transport through the cement; as evidenced by the presence of titanium in the electrolyte solution (ICP-MS analysis) and chloride at the surface of the titanium sample (EDX analysis).We show that the polymer cement is an ionic conductor and participates in the corrosion of the embedded titanium. We cannot deduce from our results, however, whether the polymer itself possesses corrosive properties. Long-term experiments will be necessary to study the degradation behavior of the polymer cement.
Abstract: The corrosion behavior of surgical implant CoCr alloy and REX 734 steel has been investigated. The pitting or crevice corrosion potentials have been determined to reach values as high as 500 mV vs. SCE for CoCr and 450 mV vs. SCE for REX 734. The galvanic corrosion behavior of CoCr/REX 734 couples has been evaluated with various electrochemical techniques. The measurement of the corrosion current of the galvanic couple as well as its prediction by applying mixed potential theories on measured potentiodynamic polarization curves revealed low galvanic currents in the range of nanoamperes.
Abstract: The corrosion behavior of combinations of materials used in an orthopedic implant: the spherical part (forged or forged and annealed) constituting the head, the weld (tungsten inert gas (TIG) or electron beam (EB) techniques), and the cylindrical part (annealed) constituting the shaft of a femoral prosthesis - has been investigated. Open-circuit potentials, potentiodynamic curves, Tafel slope, mixed potential theory and susceptibility to intergranular attack are electrochemical and chemical procedures selected for this work. Electrochemical measurements using a microelectrode have been made in the following zones: spherical part, cylindrical part, weld, and weld/sphere, and weld/shaft interfaces. To detect intergranular attack, the Strauss test has been used. At the interfaces, corrosion currents, measured (Icorr) and predicted (Icouple) are low, in the order of the pico- to nanoampere. The electrochemical behavior of the electron beam (EB) weld is better than that of the tungsten inert gas (TIG). Welds at interfaces can behave either anodically or cathodically. It is better if welds, which are sensitive parts of the femoral prosthesis, behave cathodically. In this way, the risk of starting localized corrosion (pitting, crevice or intergranular corrosion) from a galvanic couple, remains low. From this point of view, the sample with the EB weld offers the best behavior. All the other samples containing a TIG type of weld exhibit a less favorable behavior. The mechanical treatments (forged, and forged and annealed) of the steel sphere did not show any difference in the corrosion behavior. No intergranular corrosion has been observed at the weld/steel interface for unsensitized samples. With sensitized samples, however, a TIG sample has exhibited some localized intergranular corrosion at a distance of 500 microm along the weld/stainless steel (sphere) interface.
Abstract: The aim of this study was to compare the resistance of the corrosion of dental alloys in a solution containing oral bacteria named Actinomyces viscosus (ATCC19246). In this paper, we explain the choice of this precise species of bacteria, then specify its culture in artificial saliva and the experimental precautions needed to avoid the pollution by other bacteria. The electrochemical behaviour of two dental alloys (Ni-Cr alloy and gold-based alloy) was investigated by electrochemical means in sterile Fusayama artificial saliva (AS), AS enriched with sterile yeast extract (YE) and YE modified by introducing bacteria (AV). Open-circuit potentials, potentiodynamic curves, polarization resistance and impedance spectroscopy are the electrochemical procedures selected for this work. It has thus been shown that the open-circuit potential of the non-precious alloy is always lower than that of the gold precious alloy, and the colonization of metal surface by bacteria caused a drop in open circuit potential. The electrochemical impedance spectroscopy results have shown that the electrolyte resistance decreased between the AS, YE and AV milieu, in the presence of bacteria a slight decrease in polarization resistance was observed with the precious alloy and an increase with the non-precious alloy. The drop in the electrolyte resistance cannot explain the change in polarization resistance. The influence of Actinomyces viscosus might be essentially due to the consumption of oxygen at the metal/electrolyte interface of the specimen. For the non-precious alloy, the absence of oxygen (instigator of corrosion) led to an increase in polarization resistance whereas the slight decrease for the precious alloys might be justified by the organic and inorganic metabolites released by bacteria in to the electrolyte. The scanning electron micrography after electrochemical analysis, confirmed the absence of contaminants. These preliminary results demonstrate the unquestionable influence of this bacteria on the corrosion behaviour of the alloys studied, however, further studies are necessary.
Abstract: The purpose of this study was to investigate, in different experimental conditions, the galvanic corrosion phenomena which can exist between a dental suprastructure and a dental implant. The electrochemical behavior of 7 alloy superstructures with titanium and titanium alloy (Ti6A14V) implants was investigated by electrochemical means in Fusayama-Meyer de-aerated saliva and Carter-Brugirard (AFNOR) non de-aerated saliva. Different techniques were used to obtain the value of the galvanic coupling current and potential for each couple. All showed very low corrosion rates, ranging from 10(-6) to 10(-8) A. Surface analysis confirmed these results.
Abstract: Fluoride ions are the only aggressive ions for the protective oxide layer of titanium and titanium alloys. Thus their presence may possibly start a localized corrosive degradation by pitting and crevice corrosion processes. Since hygiene products like toothpastes and prophylactic gels contain fluoride ions, the present study has been completed to evaluate the effect of fluoride ions on titanium and dental alloys used, for example, as dental implants and superstructures. Two different milieu based on the Fusayama artificial saliva and an electrolyte solution containing NaCl, with and without fluoride ions, have been used for the electrochemical tests, in a pH range of 6.15 to 3.0. Open circuit potentials, potentiodynamic curves, Tafel slopes, galvanic couplings and crevice potentials are the electrochemical procedures selected for this work. Based on the experimental results obtained with these procedures, the mixed potential theory was applied to predict couple potentials and couple currents. It has thus been shown that: (a) with and without fluoride ions, galvanic currents are weak (10E - 7 to 10E - 8 A cm(-2)) within a pH range of 6.15 to 3.5; (b) titanium submitted to anodic polarization in an electrolyte, even one containing fluoride, merely develops an oxide layer and does not corrode within that same pH range of 6.15 to 3.5; (c) in confined areas where fluoride ions are present, titanium and the dental alloys tested undergo as corrosive process, in the form of crevice and pitting, as soon as the pH drops below 3.5.
Abstract: Cobalt-based alloys (Co-Cr-Mo) are usually used in dentistry as frameworks for removable partial dentures. In their basic form these structures function successfully. However, modifications or repairs of the frameworks may reduce their resistance to corrosion and, as a consequence, may provoke biologic reactions in the soft tissues. These reactions may be the result of different types of alloys that contact each other and, in the presence of saliva (based on potential differences), produce a galvanic cell. In this study, a clinical situation after repair of a removable partial denture was examined. The metallographic study of the prosthesis revealed a brazed zone where a gold braze was joining the Co-Cr-Mo framework with a Co-Cr-Ni type alloy (without Mo). The latter revealed signs of corrosion. Various electrochemical parameters (Ecorr, Ecouple, icorr, icouple) of these alloys were analyzed in the laboratory. The Co-Cr-Ni alloy had the lowest nobility and underwent galvanic corrosion in a galvanic couple with gold braze.
Abstract: Forty-four dental casting alloys and pure metals have been evaluate for corrosin resistance in an artificial salavie. The electrochemical tests included measurement of the rest potential and tracing of polarization curves. Qualitative comparrisons of the normalized polarization curves and quantitatve determination of corrosion resitance, and of corrosion and breakdown potentials, provided complementary means of evaluation. Alloys were assigned two families: alloys for crown and bridges (C&B alloys), and alloys for ceramic veneering (PFM alloys). Among thesse two families, groups were based on the gold-content (high and low), and onthe main non-gold component: silver for C&B alloys, and palladium for PFM alloys. Pure metals and somme non-noble alloys have been included as references. From this study, it clearly appesrs that the PFM alloys, having a higher content of noble matals, are more corrosion resistant than the C&B alloys containing copper and silver. Some relationships between noble-metal content and electrochemical parameters such as rest potential and corrosion resistance, have been identified, but the strenghth of the correlation is somewhat lowered by the influence of additional parameters, such as the presence of non-noble components or the nature of the microstructure. The present investigation is part of a larger study including other electrochemical parameters as well as various biocompatibility tests on the same set 44 alloys
Abstract: OBJECTIVES: The corrosion resistance of a series of 51 dental alloys and pure metals was evaluated by coulometric analysis. METHODS: The method consisted of dividing the anodic polarization curves into three separate zones: zone I, from the open-circuit potential (after 24 h immersion) up to +300 mV (SCE): zone II, from 300 mV to 700 mV (SCE): zone III, +700 mV to 1000 mV (SCE). The surfaces below the curves are integrated for each zone and the results expressed in millicoulombs (mC). RESULTS AND CONCLUSIONS: Although the intensity of degradation in the mouth may vary from patients to patient, we think that a potential danger may occur in zones I and II in particular. Coulometric analysis reveals that: (i) the alloys for the porcelain-fused-to metal (PFM) technique have a better corrosion resistance (2.58 mC on average for the sum of the three zones, against 283.7 mC for conventional alloys): (ii) the elements Cu, Ag and Ni reduce the corrosion resistance of gold-based alloys: (iii) within the same class, single-phase alloys resist better than two-phase alloys.
Abstract: The infiltration of saliva into the multi-metallic structures on titanium implants brings different types of alloys into temporary or permanent contact. In this way a galvanic cell is established as a result of their potential difference. The galvanic cell phenomenon is compounded by another type of corrosion resulting from the geometry of the assembly: localized crevice corrosion. Fifteen galvanic couples (Ti/gold-based alloys, Ti/palladium-based alloy and Ti/non-precious alloys) were studied. Various electrochemical parameters (Ecorr, Ecommon, Ecouple corr, Ecrevice, icorr, icouple corr and Tafel slopes) were analysed. The galvanic currents measured are of the same order of magnitude (except Ti/stainless steel). They remain low. Application of the mixed-potential theory shows that titanium in coupling with the alloys studied will be under either cathodic or anodic control. According to the results obtained, an alloy that is potentially usable for superstructures in a galvanic coupling with titanium must fulfil a certain number of parameters: in a coupling, titanium must have a weak anodic polarization; the current generated by the galvanic cell must also be weak; the crevice potential must be markedly higher than the common potential.
Abstract: An alloy intended for medical application is subject to preclinical and biological evaluation in order to ensure compliance with the specification regarding patients safety. The preclinical evaluation includes biological evaluations described in ISO 10993-1 (cytotoxicity, sensitization, irritation, toxicity, genotoxicity, implantation, haemocompatibility) on one hand, as well as physico-chemical tests concerning localized corrosion and the realease of metallic cations on the other hand. In recent years, a comprehensive study was conducted on localized corrosion of and the release of cations, particularly nickel, from stainless steels commonly used for medical devices and consumer products in permanent contact with the human body. We present here selected results for stainless-steel grades used in applications such as medical instruments (AISI 431), hypotubes for angioplasty (AISI 304L), and implants (AISI 316L - ASTM F138 97, MP35N). The same grades are also used for watches and similar products that come into contact with the skin. In this context European Directives were decreed imposing a limit to the tolerable rate of nickel release concerning products that are into direct and prolonged contact with the skin.
The study reveals a significant anisotropy of localized corrosion and nickel release, depending on the orientation of the test surface with respect to the casting and rolling direction of the steel. Cross-sectional specimens (transversal cuts with respect to the rolling direction) show a substantially higher sensitivity to corrosion phenomena compared to longitudinal cuts, and they release nickel ions at rates 10 to 100 times higher.
These findings indicate that orientation needs to be taken into account when interpreting test results, in particular when comparing different grades of steel, as well as in product and production design.
