Electrochemical Behavior of Alloys for Fixed Partial Denture
The electrochemical behaviors of eight parent alloys and four post-solder alloys used in dental prostheses were specified in a (pH=7.4; (NaCl)=9g/L)-aqueous solution at 37°C. For each alloy, the free potential was followed during two hours, and a cyclic polarization between the cathodic domain and the solvent's oxidation was performed. The gal- vanic corrosion between each parent alloy and its usual post-solder was also studied. The free potentials stay in the immu- nity domain of the alloy's base-elements (precious metals) or in their passivation domain (nickel, chromium). A relatively strong oxidation of the alloys is possible only for high values of applied potential. Corrosion of the noblest alloys is al- most inexistent. It occurs for the nickel-base alloys but it is very slow. Most of the elements present can be oxidized, but only for high potentials which cannot be really achieved by dissolved oxygen. Galvanic corrosion can occur between the parent alloy and the post-solder alloy in some cases. The realization of frameworks for fixed partial denture (FPD) often involves the use of more or less noble metallic alloys and their assemblage by soldering. The latter is called pre-ceramic soldering or post-ceramic soldering, this de- pending on the step of the fabrication of the prostheses at which this soldering is realized. Soldering is well used to correct distortion of the framework which occurs during the casting process (1), to improve the seating accuracy (2) and to correct a possible movement of teeth before prosthesis cementation (3). The solder joint is often the weakest part of a fixed partial denture (4, 5). While some defects can be due to the process of fabrication (6), others can appear during the use of the prosthesis in the mouth, because of corrosion by saliva which can generate additional cavities on alloy sur- face. Indeed, the metallic framework may partly remain exposed to the buccal milieu, notably a part of the parent alloy and the post-ceramic solder which is not covered by ceramic. These are the reasons why the individual behaviors of main alloy of the framework and of the post-solder alloys in contact with saliva need to be better known, as well as the behavior of this parent alloy coupled with its usual post- solder. The purpose of this article is first to specify the separate behaviors of the parent alloy and of the post-ceramic solder, and second to study the possible galvanic corrosion which could be due to the association of the two materials, by using electrochemical techniques. The electrolyte used in this study, is characterized by a NaCl concentration of 9 g/L and
Electrochemical impedance spectroscopy study of high-palladium dental alloys. Part II: Behavior at active and passive potentials*
Electrochemical impedance spectroscopic (EIS) analyses were performed on three high-palladium alloys and a gold–palladium alloy at active and passive potentials in five electrolytes that simulated body fluid and oral environmental conditions. All four alloys were previously found to have excellent corrosion resistance in these in vitro environments. Before performing the EIS analyses, alloy specimens were subjected to a clinically relevant heat treatment that simulated the firing cycles for a dental porcelain. It was found that the EIS spectra varied with test potential and electrolyte. Diffusional effects, related to the dealloying and subsequent surface enrichment in palladium of the high-palladium alloys, along with species adsorption and passivation, were revealed at both active and passive potentials, although these effects were more evident at the passive potentials.
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