Chemical stability of gold dental alloys

Abstract

The bzocornpatibzlity and desirable working charactei istics ofgold-based alloys are wzdely recognized in dentisti y. Although there has been considerable activity in understanding such characteizstics as hardenability, there is peihaps more significance to the chemical stability ofa dental alloy sznce degradatzon ofa dental alloy duringservzce can inducea toxic response andpromote gingival tissue irritaion.Because of the recognized role of nobility of a dental alloy in determining chemical stability, various compositional specifications have been issued to help ensure successful service. However, it is recognized that alloys meeting these compositional specifications can prove unacceptable in service (1, 2). Conversely there are several alloys in service which appear to perform satisfactorily in spite of an apparently low nobility. These factors serve to illustrate the complex situation which exists concerning acceptance standards for dental alloys; performance standards are regarded as more reliable than compositional standards.This article describes astudy conducted to obtain quantitative tests and data for the tarnish and corrosion behaviour of gold-based dental alloys. The test data necessarily require clinical corroboration to ensure their validity as an indicatorof alloy suitability fororal use. However, significant progress has been made in determining the nature of a minimum, yet comprehensive, test battery. In addition, the procedures described in this report show quantitatively a capability of separating the performances of dental alloys in laboratory tests. Data obtained from five commercial alloys are given to illustrate the application of the testing approach. BackgroundThe chemical instability of dental alloys has long been recognized as a serious problem. Two distinct occurrences may alter the surface condition of a metal in the mouth; corrosion and tarnish. Corrosion involves the electrochemical dissolution of the metal or the formation of reaction products on the surface. Tarnish is the surface discolouration due to the formation of reaction products and is a specific form of corrosion. There have been several previous investigations of both forms ofattack and these have been reviewed by German (3).Because of the more visible nature of tarnish attack, there has been considerable emphasis on this subject in the previous dental alloy studies. Early research in a sulphurized oil bath by Souder and Paffenbarger (4) indicated that 75 weight per cent noble metal in an alloy was necessary to avoid tarnish. Swartz etal. (5) employed 13 different test solutions, using the as-received alloys in an undefined thermal condition, ranked the results in a subjective manner. The general conclusion was that discolouration occurred preferentially in the low nobility alloys when exposed to sulphide solu tions.The study by Tuccillo and Nielsen (6) demonstrated that on a microscopie scale tarnish attack has a galvanic nature. Silver-rich regions were preferentially attacked by the sulphide-containing solution used in...

Corrosion and tarnishing characteristics of low gold content dental casting alloys

“…The tarnish and corrosion performance of the alloy set in the various metallurgical states was assessed using the test battery developed by Treacy and German [4]. The degree of tarnish was quantified by daily spectrophotometriĉ Several methods are available for RSR processing.…”

“…Schematic diagram of the twin-roller rapid quench apparatus measurements of surface color changes over a 72-hour period. The test solutions employed were 0.5 per cent sodium sulfide and a standard artificial saliva [4]. In all cases, the solution temperature was kept constant at 37°C.…”

“…Thus, a quantitative ranking of tarnish resistance can be made using this technique [21]. The details concerning sample preparation and equipment specifications have been published elsewhere [4,8,9,21].…”

“…The chemical stability of dental casting alloys has long been a matter of concern to dentists because of the negative consequences of tarnish and corrosion [1][2][3][4][5]. In the past, the use of high-nobility alloys (those with a high content of gold or other noble metals) generally ensured resistance to attack in the human mouth.…”

“…In prior experiments, we have made progress in the identification of in vitro test procedures for evaluating the performance of dental casting alloys [4,8,9,21,22]. These tests have been applied to a variety of gold [23] and palladium-silver alloys [24] with good success.…”