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
A comparison of the strength of base metal and gold solder joints.
The purpose of this study was to evaluate the procedures involved in forming high- and low-fusing solder joints between two different base metal alloys and to compare the strength of these joints with the strength of those formed between specimens made from a high-gold content alloy. The conclusions of this investigation can be summarized as follows. All but 11 of the 120 specimens failed through the solder joint, which confirms that the solder joint is usually the weakest part of a fixed partial denture. The high-fusing joints formed between the base metal alloys showed higher relative bending strength values than the high-fusing joints formed between the gold alloy specimens. The low-fusing joints formed between the base metal alloys showed higher relative bending strength values than the low-fusing joints formed between the gold alloy specimens, and the low-fusing base metal joints exhibited the highest strength values in this study. The low-fusing joints formed between gold alloy specimens consistently bent before fracturing. Solder joints fail both adhesively (separation of the solder from the parent metal) and cohesively (fracture through the solder, parent metal, or a combination of both). A correctly formed joint between gold components should show cohesive fracture, whereas adhesive fractures are apparently acceptable with base metal alloys since the low-fusing base metal joints failed in this manner but still exhibited high strength values. Voids of different sizes and locations were observed in most of the solder joints, and within each group tested there was usually less strength exhibited by the joints with larger areas of porosity.
Evolution of carbide precipitates in Hastelloy N joints during welding and post weld heat treatment
Abstract The effects of welding and post weld heat treatment (PWHT) on the morphology, composition and lattice parameter of the precipitates in the welding joints of Hastelloy N alloy have been investigated. Joint samples were prepared using a gas tungsten arc welding process with ERNiMo-2 fillers, and then solid-solution treated at 1100 °C for 20 min. After welding, eutectic M6C carbides can be observed in the heat affected zones (HAZ) and weld zones (WZ). These eutectic M6C carbides become spheroidized during the following PWHT. The concentration of Si decreases during the transformation from the primary ones to eutectic ones, and then increases during the spheroidizing process. The total concentration of Si + Cr in M6C carbides remains stable during welding and PWHT. At the same time, the heat leads to the outward diffusion of Mo from the M6C carbides, which results in a decrease of the lattice parameter.