The corrosion of nickel-based dental casting alloys

Objectives: To assess the microstructure of two commercially available Nickel (Ni)-based alloys before and after casting and heat treatments on Ni2+ ion concentration. The response of the alloys to cellular function including proliferation, protein synthesis, and ATP levels was also assessed. Methods: Two alloys with different Ni and Chromium (Cr) contents were chosen and disc-shaped specimens cast. Microstructural characterisation was achieved using an optical microscope and the elemental distribution was studied utilising EDX. All electrochemical measurements were performed using a computer controlled potentiostat with a 3-electrode cell. 3T3 mouse fibroblasts (passages 12-15) were cultured to compare the cell proliferation rates. The cells were exposed indirectly by prior incubation of the culture medium (3 ml) with disc-shaped specimens immersed for nine days which provided sufficient medium to replace after three days whilst maintaining a constant dose of metal ions to the cultures. The cells were then seeded at 5x103 cells/cm2 into a 12-well culture dish containing 1 ml of exposed medium. The number of viable cells was determined by trypan blue staining and counted using a haemocytometer at various time points up to six days. All the cell experimental investigations were performed in triplicate and statistical significance was determined by a one-way analysis of valiance (ANOVA) compared with a control culture. Results: The alloy with a lower Cr content (12.6 wt%) had a lower corrosion resistance which was further reduced at pH 2.5 where crevice corrosion was observed. Furthermore, the proliferation of mouse fibroblasts was reduced when exposed indirectly to this alloy. The alloy containing a high Cr content (25 wt%) maintained high levels of Cr within the microstructure and showed superior corrosion resistance. Conclusions: Cr is crucial for the formation of a passive oxide film to provide sufficient protection from corrosion in the variable conditions of the oral environment.