Corrosion by galvanic coupling between carbon fiber posts and different alloys.

OBJECTIVES: The corrosion of metal type (NiCr, copper, etc.) posts in post and core restorations has been a well known fact over the last few years. A new generation of carbon fiber posts is now playing an extensive role in dental prosthetics, but these posts may, in some cases come in contact with alloys used for restoration purposes. The objective of this paper is to study the electrochemical behaviour of these posts in an artificial SAGF medium (Artificial Saliva Gal Fovet) for comparison with that observed for different alloys (gold, NiCr and amalgams) in the same medium, and to assess the coupling that might occur between these posts and alloys. METHODS: Posts of different diameters (1-2.1 mm) were molded in resin, thus forming 10 x 3 mm pads that were mounted on a rotating electrode. Pads of identical dimensions were also made for each alloy. Voltammograms (current-tension curves) were plotted for every material in the SAGF medium at 25 degrees C with normal aeration and low aeration (bubbling purified argon) using a potentiometer with three electrodes to determine the position of the zero current potentials. Evans graphs were then plotted once the anode part of the alloy curves and the cathode part of the post curves had been processed with computer software. The potentials and the maximum coupling current densities were estimated. RESULTS: Posts may be involved in galvanic coupling in the presence of oxygen. Galvanic activity in an aerated solution is highly probable upon contact with amalgams, but may also occur if the material in contact is NiCr alloy. Coupling between precious alloys and posts is improbable and can only induce very low currents in the presence of oxygen. SIGNIFICANCE: This study devoted to coupling of carbon fiber posts and metals in the mouth, attracts attention to the fact that a corrosion reaction can be set-up in the infiltration conditions relevant to the prosthetic restoration.     

Wetting of amalgam alloys by mercury.

The degree of wetting of amalgam alloy by mercury during trituration is an important variable in determining reaction rates and the degree of amalgamation. The purpose of this study was to measure the contact angle made by mercury on various phases present in amalgam alloy in air and on commercial alloys. During amalgamation, mercury is in contact with the oxides present on the amalgam alloy as well as the metallic amalgam phase. To measure the degree of wetting of the metallic phases, silver, tin, and copper were melted in such proportions as to give specimens of silver, tin, the alpha, beta, and gamma silver-tin phases, the eutectic in the silver-copper system. These were cast in the form of cylinders, sectioned, and the surfaces prepared by sandblasting. Oxide specimens of AgO, Ag2O, SnO, and SnO2 were prepared by compaction in a mold. Four commercial amalgam alloys also were tested. Mercury drops were then placed on the surfaces and the contact angles measured at different time intervals. The initial contact angles of mercury on the alpha, beta, gamma, and eutectic phases were found to be 90degrees, 112 degrees, 145 degrees, and 138 degrees, respectively. Mercury reacted with silver and tin, resulting in a change of contact angle with time. Contact angles measured on the commerical alloys were also high, on the order of 145 degrees. The gamma phase had contact angles closest to those of commercial alloys. Surface oxide phases are most likely responsible for this poor wettablility by mercury.     

Dental amalgam alloys.

Dental amalgam, which constitutes nearly 75% of all restorative materials used in dental practice, has been used successfully for almost 150 years. Dental amalgam is low in cost, easy to use, consistent, and durable. Although amalgams have been challenged by more esthetic restorative materials and by fears of mercury toxicity, amalgams continue to be the materials of choice by most practitioners. This review discusses the advantages and disadvantages of amalgam alloys and predicts that one day, amalgam probably will be replaced by more esthetic materials that truly bond to all hard tooth surfaces.     

Unusual in vivo extensive corrosion of a low-silver amalgam restoration involving galvanic coupling: a case report.

The basic mechanism of dental amalgam corrosion has been thoroughly studied during the last 50 years by various experimental techniques, most often carried out in vitro. Electrochemical methods were extensively employed, and it was recognized that a gradual dealloying of the more electroactive components, Zn, Sn, and to a lesser extent Cu, contributed to change the surface composition. It is also well known that, in all circumstances, galvanic coupling threatens the longevity of the restoration. Among the patients examined at the hospital attached to the Department of Dental Surgery of the Paris 5 University, the authors observed, in a few exceptional circumstances of defective restorations, a release of liquid metal droplets in the surrounding tissue. These particles were identified as elemental mercury. This unexpected phenomenon was the result of the use of a low-silver-content alloy, combined with an enhanced corrosion process due to galvanic coupling with a silver alloy crown and brass screw-posts. In the described clinical case, the tooth, which had to be extracted, was axially sliced for observation by scanning electron microscopy coupled with elemental analysis by X-ray fluorescence energy dispersive spectroscopy. The observations showed a dramatic corrosion of the amalgam matrix with the formation of porosity even in the bulk of the material. Superposition of the element mappings proved (1) the electrochemical coupling of the amalgam with the silver-based crown, (2) the decalcification of the dentin, and (3) the formation of tin oxychloride precipitates in the matrix porosities and the interfacial gaps. These experimental results corroborate the corrosion mechanisms described hereupon. They support the current national and international recommendations for the dental amalgam formulation and for a rigorous professional practice.     

Corrosion and creep of dental amalgam.

The influence of stress on corrosion was investigated by a potentiostatic technique. Amalgrams with low creep, both with and without gamma2-phase, revealed small increases in current with stress application while alloys with high creep exhibited large increases in current. Rupture of protective oxide due to increased strain for high creep alloys may explain the phenomenon.     

The new generation amalgam alloys. Clinical considerations

There is a good deal of current speculation that composite resin materials will shortly replace silver amalgam as routine posterior restoration systems. This article provides guidelines in the selection and clinical handling of the new generation high copper amalgam alloys.     

In vivo galvanic currents of intermittently contacting dental amalgam and other metallic restorations.

OBJECTIVE: It was the objective of this study to determine the magnitude of in vivo galvanic currents produced by simulating electrical contact between occluding metallic restorations, and to examine the influence of restoration age, difference in pre-contact corrosion potentials, and surface area. METHODS: A convenience sample of 106 human subjects was studied. A Ag/AgCl micro-reference electrode and a high impedance voltmeter were used to measure the pre-contact corrosion potentials. Galvanic couples (n = 194) were next formed by simultaneously contacting occluding restorations with gold-plated probe tips for 15 s. The resulting current-time transients were measured with a zero-resistance ammeter and recorded with an electronic data acquisition program. The vast majority of couples measured contained at least one dental amalgam restoration. RESULTS: Galvanic current-time transients were typically characterized by an immediate and rapid rise to a peak current, followed by an exponential decay to a much lower value at 15 s. For couples with a restoration < or = 12 mo old, median peak current and median current at 15 s were 2.26 microA (range 0.24-13.06 microA) and 0.57 microA (range 0.03-6.47 microA), respectively. For couples with a restoration >12 mo old, median peak current and median current at 15 s were 1.40 microA (range 0.24-12.09 microA) and 0.37 microA (range 0.00-3.05 microA), respectively. Couples with fresh amalgams (< or = 6 mo) generally had elevated currents (range 2.56-102.54 microA). SIGNIFICANCE: A wide range of galvanic currents resulted from electrical contact of restorations in vivo. These currents were influenced by restoration age and total surface area of the galvanic couple. For amalgam-amalgam couples, the difference in the pre-contact corrosion potentials may be useful in predicting galvanic currents, when the difference is at least 24 mV.     

钛种植体与冠修复金属电偶腐蚀电流的研究

目的 通过对HBIC型钛种植体与口腔科常用修复烤瓷合金在中性和酸性人工唾液中的电偶腐蚀的综合评价,筛选更适合种植体上部结构的冠修复材料.方法 选择HBIC型钛种植体(T-H)以及镍铬合金(Ni)、钴铬合金(Co)、金铂合金(Au)、铸造纯钛(Ti),每种金属制成直径4mm、高10mm的圆柱体电极各40个.采用电化学方法,在自行配制的pH =6.8、pH =5.6两种人工唾液中检测4种合金分别与T-H组成的电偶对的电偶电流密度(Id),并绘制出各自的电流时间曲线.结果 在中性人工唾液中四种电流密度依次为IdNi＞ IdCo＞IdTi＞IdAu,其中Ni组与Au组、Ti组间有统计学差异(P＜0.05),Co组与Ni组、Ti组、Au组,Ti组与Au组间无统计学差异(P＞0.05).在酸性人工唾液中四种电流密度依次为IdNi＞IdCo＞IdAu＞IdTi,其中Ni组显著大于Ti组、Co组、Au组,有统计学差异(P＜0.05),Ti组、Co组、Au组三组间无统计学差异(P＞0.05).扫描电镜观察,酸性环境中镍铬合金发生少量坑状腐蚀形貌,其余合金与中性环境相比无明显变化.结论 两种唾液中镍铬合金组电偶电流较大.酸性环境能增大电偶腐蚀,对镍铬合金影响较大,镍铬合金不适宜作为钛种植体上部冠修复材料.     

Corrosion behaviour of dental metals and alloys in different media

The corrosion tendencies of metals are related with their position in the electromotive series. These electrode potential degrees may change due to the compositions of the alloys, the surrounding media, or due to alterations in the composition because of recurrent casting. Therefore in this research, the electrode potentials and their changes over a period of time were measured in different pH media simulating the oral electrochemical conditions in vitro. The surface structure of the first and second castings of 29 different dental metals and alloys were examined under a scanning electron microscope and their composition in percentage weight was calculated by the Energy-dispersive X-ray Analysor system. Further the current–potential curves of the dental alloys were found by the potentiodynamic method in three different solutions and, in addition, the changes of corrosion potentials over time were also determined. The corrosion rates, corrosion potentials, their changes over time and their cathodic Tafel slopes were determined. All alloys tested showed ion leakage in corrosive media. Titanium exhibited the least, but alloys with tin and cobalt content displayed the greatest corrosion tendencies. Alloys with iron and copper corroded in the acid media, conversely alloys containing chromium, nichel and molybdenum proved to be resistant to corrosion. The recurrent castings were also corrosion resistant.     

An in vitro study of galvanic currents between amalgam and gold alloy electrodes in saliva and in saline solutions

The galvanic currents between amalgam and gold alloy electrodes were higher in saline solutions than in saliva. Conventional amalgam supported higher currents than high copper amalgams when short-circuited with a gold alloy. The galvanic currents were not affected or affected only to a very small degree when whole saliva from different persons were compared. Higher currents were recorded when short-circuiting conventional amalgam and gold alloy electrodes using parotid saliva than when using whole saliva collected from one person. However, removing molecules with a molecular weight higher than 10(4) from whole and parotid saliva did not seem to affect the galvanic currents.     

An in vitro study of galvanic currents between amalgam and gold alloy electrodes in saliva and in saline solutions

Abstract – The galvanic currents between amalgam and gold alloy electrodes were higher in saline solutions than in saliva. Conventional amalgam supported higher currents than high copper amalgams when short-circuited with a gold alloy. The galvanic currents were not affected or affected only to a very small degree when whole saliva from different persons were compared. Higher currents were recorded when short-circuiting conventional amalgam and gold alloy electrodes using parotid saliva than when using whole saliva collected from one person. However, removing molecules with a molecular weight higher than 10⁴ from whole and parotid saliva did not seem to affect the galvanic currents.     

Corrosion potential recovery of dental amalgam restorations following prophylaxis.

OBJECTIVE: Dental amalgam restorations are subjected to abrasion during selective prophylaxis that can damage or remove the protective oxide and result in increased rates of corrosion and chemical dissolution of mercury. It was the objective of this research to study the corrosion potential change of dental amalgam restorations to obtain an indication of the time required for in vivo repassivation following prophylaxis. METHODS: The corrosion potentials of 27 Class I and Class II amalgam restorations were measured pre- and post-prophylaxis using a high impedance voltmeter and a Ag/AgCl micro-reference electrode. Prophylaxis was performed for approximately 2s on each amalgam surface using a slow-speed handpiece with a rubber-cup and commercial abrasive paste. Subjects thoroughly rinsed before the post-prophylaxis corrosion potentials were measured. The data were analyzed using a confidence interval, a t-test and correlation analysis. RESULTS: The pre- and post-prophylaxis mean corrosion potentials were, respectively, -132 (27)mV and -126 (27)mV. The mean of the differences between the pre- and post-prophylaxis corrosion potentials was 6.1 (28)mV, with an associated 95% confidence interval of (-4.8, 17)mV. A t-test showed the mean absolute difference in corrosion potential was less than 50 mV (p<0.0001). SIGNIFICANCE: The results of this study show that the post-prophylaxis recovery of the corrosion potential of amalgam restorations occurred by at most 10-44 min, indicating that the period of elevated corrosion rate and elevated chemical dissolution rate of mercury, due to oxide damage or removal, may be short-lived.     

A 14-year clinical assessment of 12 amalgam alloys.

After 14 years, 40 patients from an in vitro/in vivo study of 12 amalgam alloys were evaluated. All these patients had been attending private practitioners over the past 10 years, and the clinical evaluation revealed that 320 restorations were intact and 47 (12.8%) had been lost. The majority (75.0%) of the lost restorations were lost to crown placement or the replacement with another amalgam restoration. The six high-copper alloys exhibited better clinical performance, in both loss rate and fracture at the margins, than did traditional low-copper materials. Fracture at the margins was a better predictor of the loss of a restoration than were mechanical properties.