Corrosion evaluation of recasting non-precious dental alloys.

The effect of recasting, up to four times, non precious Ni-Cr and Co-Cr commercial dental alloys on their corrosion behaviour in saliva and saline media was carried out. A potentiokinetic technique was utilised to analyse the electrochemical characteristics of the anodic polarisation curves of these alloys. A considerable anodic polarisation range (about 2.0 V vs SCE) was used to investigate the possibility of developing a passive regime at such high potential range. The most important feature noted was a rapid rise in current density above a certain critical potential called pitting potential, Ep. The experimental data show that the open circuit potential, Eoc does not enable differentiation between the corrosion resistance of the four alloys used in this study. However, Ep and the rupture potential Er does distinguish between them. Increasing the number of the successive recastings of Wirolloy leads to enhancing the pitting potential, thus, the corrosion resistance of Wirolloy (Ni-Cr) improves after remelting and recasting. It has been found that Wirolloy corrodes 26 times faster than Wironit alloy under the same solution. The alloys containing cobalt and molybdenum show higher corrosion resistance than those containing nickel. Additionally, their corrosion resistance was not affected by successive melting and recasting. In chloride solutions Ni-Cr alloys show a high susceptibility to pitting corrosion, while Co-Cr alloys show a noble behaviour. The corrosion resistance of the four non precious alloys were in the following order: Biocast > Wironit > Cobond > Wirolloy.