Study On Electrochemical Corrosion Of Three Commonly Used Dental Alloys In Different Media

At present, the materials used in prosthodontics include alloy, ceramic and resin. Dental casting alloys are widely used in the applications because of their high biocompatibility, low resistance to corrosion and cost. They exhibit sufficient physical and mechanical properties to ensure adequate function and structural durability over long periods of time.An alloy is any mixture of 2 or more metals. In dentistry, alloys usually contain at least 4 metals, and often 6 or more. Thus, dental alloys are complex metallurgically. In clinical the most commonly used dental alloys are nickel-chromium alloy, cobalt-chromium alloy, precious metal alloys, pure titanium and titanium alloys. Alloy compositions are diverse, and minor elements in alloys are even more diverse. The complexity and diversity of these alloys make understanding their biocompatibility difficult because any element in an alloy may be released and may influence the body.Abroad and domestic researches reported that corrosion resistance of metallic alloys was determined primarily by the composition of the passive film depending upon the alloy composition. The film would separate metal from medium mechanically, and its solubility was very small in medium, which made the metal in"steady state". Cr, Co, Mo, Mg, Ti and other elements could be oxidized and a thin film of corrosion products formed on a metal's surface spontaneously, acting as a barrier to further oxidation to reduce the release of ions. The presence of Co and Mo in Ni-Cr dental casting alloys was essential for the oxidation reaction through its ability to react with O2, leading to increase in the resistance of the corrosion in aqueous media. The presence of Be-containing oxide on the surface of Ni-Cr alloy was associated with high corrosion rate and susceptibility to accelerated corrosion process. The corrosion of Co-Cr alloys in neutral or acid solutions was found to proceed by selective dissolution of Co and the film composition changed into Cr oxide containing a small amount of Mo oxide to prevent the release of Co and Cr. For general applications in dentistry, Ti had the ability to passivate by the formation of a protective oxide layer, normally TiO2-based oxide, on metal, so they were more corrosion-resistant than any other dental alloys.Corrosion is fundamentally an electrochemical phenomenon, which might be defined as electrochemical or chemical destruction. Corrosion in aqueous environment and gaseous environment are electrochemical processes because corrosion is a kind of electron transfer process between the metal surface and the aqueous electrolyte solution. Electrochemical corrosion involves two reactions: the first is the anodic reaction, in which a metal is oxidized, releasing electrons into the metal. The other is the cathodic reaction, in which a solution species is reduced, removing electrons from the metal. Corrosion of alloys occurs when elements in the alloy ionize. Thus, the elements that are initially uncharged inside the alloy lose electrons and become positively charged ions as they are released into solution.Corrosion behavior is essentially determined by the composition of metals, but during the corrosion process the corrosive medium also has a major impact. Composition and chemical characteristics of the corrosive solution—such as pH and ionic composition, artificial saliva, cell culture medium, serum, etc.—were also of great significance. When pH decreases and the concentration of hydrogen ion increases, the oxide film on the surface of metal is not easy to be formed and speed of passivation slows down. Even if the oxide film is formed, it can easily be dissolved in acid. In addition, lactic acid and fluoride can also damage the passive film in order to speed up the release of alloy elements.In this experiment we used electrochemical methods to investigate electrochemical corrosion behaviors of Ni-Cr alloy, Co-Cr alloy and pure titanium in artificial saliva, cola, yogurt, alkaline ion water and fluoride mouthwash, while comparing the impacts of the different kinds of media which had close relations with oral cavity, then providing theoretical support for choice of clinical materials and guiding the patients'daily diet and lifestyles.In this study, 1000ml artificial saliva was made according to standard ISO/TR10271 with the pH of 6.8, and then the pHs of cola, yogurt, alkaline ion water and mouthwash were measured. In accordance with the casting performance of Ni-Cr alloy, Co-Cr alloy and pure titanium, 25 specimens with the size of 10mm×10mm×1mm were respectively embedded and casted. One surface connecting copper wire (10cm), four edges, as well the copper wire were embedded by self-curing plastic. Exposed surface (area of 10mm×10mm) was polished. The specimens were degreased with acetone, washed in distilled water under ultrasonic wave for 10 minutes, and then dried, randomly divided into groups of five in each. Specimens were immersed in different media for 24 hours. Electrochemical measurements were performed in a single compartment three-electrode cell with the prepared alloys as working electrodes, platinum foil as the counter electrode and a saturated calomel electrode(SCE) as the reference electrode. Corrosion potential (Ecorr) and polarization curves of a variety of metals in different media were measured. Software depicted Tafel curves, and computed corrosion current density (Icorr) and polarization resistance (Rp) automatically. The data was analyzed by SPSS17.0 package.The results showed that in artificial saliva, cola and alkaline ion water, Ni-Cr alloy, Co-Cr alloy and pure titanium's Ecorr had significant differences, and pure titanium>Co-Cr alloy>Ni-Cr alloy. Compared with the control group, Ni-Cr alloy, Co-Cr alloy and pure titanium's Ecorr in cola had obvious difference (Ecorr of pure titanium was maximum and Ni-Cr alloy's was minimum); besides Ecorr of pure titanium in yogurt, alkaline ion water and mouthwash were significantly different. In artificial saliva, yogurt and alkaline ion water, Ni-Cr alloy, Co-Cr alloy and pure titanium's Icorr were significantly different and pure titaniumNi-Cr alloy, pure titanium>Co-Cr alloy. Compared with the control group the Rp of Ni-Cr alloy and Co-Cr alloy in cola and yogurt and pure titanium in cola, yogurt, alkaline ion water and mouthwash had significant differences.The following conclusions can be drawn: 1. In artificial saliva, pure titanium's corrosive tendency was minimum, and corrosion resistance was the best, but corrosion resistance of Ni-Cr alloy was the poorest and Co-Cr alloy was in the middle of them. 2. In cola, yogurt, alkaline ion water and mouthwash, corrosion rate of pure titanium was the slowest and the corrosion resistance was the best, while corrosion rates both of Ni-Cr and Co-Cr were faster. 3. Three kinds of metal alloys were easier to be corroded and had faster corrosion rate and poorer corrosion resistance in acidic environment than in artificial saliva. 4. In acid or alkaline environment pure titanium had poorer corrosion resistance than that in artificial saliva; the lactic acid and the fluoride concentration could accelerate pure titanium to be corroded.