Corrosion and Surface Analyses of Ni-Cr Alloy in Bleaching Agents

Objective: The aim of this study was to evaluate the corrosion behavior of a Ni- Cr dental casting alloy subjected to 10% hydrogen peroxide (HP) and 10% carbamide peroxide (CP) solutions and to determine the composition of surface oxide layer formed on alloy samples.

Methods: Cylinder shaped (4 mm in diameter x 25 mm in height) 10 specimens were cast from a Ni-Cr alloy (Wiron 99) and divided into 2 groups (n = 5). A potentiodynamic polarization test was used to compare the corrosion rates of samples in HP and CP (pH = 6.5). The open circuit potentials (Ecorr) and the current densities (Icorr) were determined from the corrosion curves. Differences in Ecorr and Icorr were determined using one-way ANOVA and Dunnett T3 test (a = 0.05). Before and after polarization tests, a SEM was used to analyse the surface morphology and surface characterization of passive film formed on alloy samples was also performed by using XPS.

Results: In this study, bleaching agents had an effect on the anodic process for two groups. Although no statistical difference was identified between the groups for the both corrosion parameters, results indicated that the effect of CP on the corrosion behavior was less than that of HP. These results agreed with the SEM observations. XPS data showed that the amounts of oxides formed on CP treated samples were higher than HP treated ones. Also, Mo rates were increased with CP application comparing to HP.

Conclusions: The comparison of the effects of the two bleaching agents at 10% showed that the alloy suffered less corrosion rates in presence of CP than HP. This result was also confirmed by the SEM and XPS datas. The presence of Mo on the oxide layer effected the oxide layer, leading to a better corrosion behavior.