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
Microstructure of the solder-casting zone in bridges of dental gold alloys.
The composition and structure were studied in the cast and in the solder of bridges which had failed clinically. Both materials were gold alloys but with different contents of Pt, Ag and Zn. The metallographic investigation revealed defects mostly in the solder, situated in the subsurface layer. SEM studied of the fracture surface revealed large porosites and a structure of dense parallel lines--striations--, indicating that the material had failed from fatigue. The materials in the fractured bridges were identified by micro-probe measurements. It was stated that cast material and solder material used together, were of different composition.
Measuring Sheet Resistances of Dielectrics Using Co‐Planar Hydrogel Electrochemical Cells with Practical Applications to Characterize the Protective Quality of Paints on Sculptures
Electrical properties of thin dielectric films at the solid-liquid phase boundary are an important performance characteristic of many devices, coatings and sensors. In this paper, co-polymeric hydrogels of polyacrylic acid co-sulfonic acid, swollen with a salt solution to act as the solid electrolyte, were used to assess interfaces using electrochemical impedance spectroscopy (EIS) in a co-planar geometry. Silane-modified glasses were characterized by the co-planar hydrogel EIS cell and found to be distinguishable based on their surface monolayer chemistry. EIS measurements were also made on primed and painted metal substrates, using both test panels and an outdoor sculpture, Tony Smith's Stinger. The panels were then exposed to accelerated and outdoor weathering and showed degradation on the surface paint layers, which was observable electrochemically using EIS and confirmed visually by SEM. Electrochemical spectral features were compared between data from a standard paint-test cell versus this co-planar hydrogel cell; both cell types were able to measure coating capacitance, providing useful information about the condition of the bulk coating. Yet, sheet resistance (Rs) was a spectral feature seen only by the co-planar hydrogel cell. Thus, it can be concluded that the use of such co-planar hydrogel cells can provide an earlier warning sign to coating degradation and such cells provide a new type of spectral information that is not assessable by the standard geometry.