Research On Improving The Corrosion Resistance Of Porcelain-fused Nickel-chromium Alloys By Cryogenic Treatment And Sol-gel SiO₂ Coating

Fixed-partial dentures are welcomed by more and more dentists and patients for their extraordinary esthetics and comfort. Ni-Cr alloys, as the non-precious base-metal of porcelain-fused-to-metals, are and will be widely used in our country, especially in the inland, for their low cost. But a lot of researches showed that Ni-Cr alloys exhibit low resistance to corrosion, which led to release of metal ions, hyperplastic gingivitis, gray line and hypersusceptibility. Therefor, it is necessary to improve the corrosion resistance and clinic safety of Ni-Cr alloys.The purpose of this research was to evaluate the effect of two different methods on the corrosion resistance of Ni-Cr alloys, by the means of potentiodynamic polarization in artificial saliva. One is totally cryogenic treatment, the other is sol-gel SiO₂ coating on the surface. The study also analyzed the metallography of Ni-Cr alloys after cryogenic treatment and the component, crystallization, thickness, surficial form of the coating. The results of the study were showed as followings:1. Cryogenic treatment elevated the pitting corrosion potential and dropped the corrosion rate of Ni-Cr alloys. Following simulative porcelain sintering, as well as temper after cryotreat, slowed the corrosion rate. But the corrosion potential of Ni-Cr alloys fell after cryotreat and following simulative porcelain sintering. Temper after cryotreat did not affect the pitting corrosion potential.2. Much small secondary substance separated out and evenly distributed along the crystals of Ni-Cr alloys. Some changes also occurred in the primary crystals during cryotreat.3. Sol-gel SiO2 coating improved the pitting corrosion potential and reduced the corrosion rate. There was no statistical difference between 3-time coating and 6-time coating.4. The thickness of 3-times coating approximated 8.69μm. Neither pore nor aperture was found inside the coating or in the interface of coating and metal. The temperature in the sol-gel technics led to little crack and crystal in the 3-time coating. The coating was pure SiO2, without other elements.Conclusion:Cryogenic treatment, as well as sol-gel SiO2 coating, effectively improved the electrochemical corrosion resistance of Ni-Cr alloys. Temper following cryotreat was unnecessary. From the view of corrosion rate and pitting corrosion, the effect of cryotreat on the corrosion resistance of Ni-Cr alloys did not influenced by following porcelain sintering. 3-times SiO2 coating was more acceptable than 6-time coating.