Corrosion Behavior of New Dental Co-Cr-Nb-Mo-Zr Alloy
Authors: Drob Silviu Iulian , Vasilescu Cora , Andrei Mihai , Osiceanu Petre , Calderon Moreno Jose Maria , Demetrescu Ioana , Popa Monica 
Published: February 2020

This paper presents a complete and complex characterization of the new Co-Cr-Nb-Mo-Zr alloy in comparison with the commercial CoCrMo (Heraenium CE) alloy. The alloy microstructure was showed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS); the composition and thickness of the native passive film were identified by X-ray photoelectron spectroscopy (XPS). Also, the surface roughness was revealed by atomic force microscopy (AFM) and the contact angle evinced the alloys hydrophilic properties. The amounts of ions released in artificial Carter-Brugirard saliva were evaluated by the inductively coupled plasma-mass spectroscopy (ICP-MS) technique. The corrosion rate was obtained by the linear polarization method.
The cast new Co-Cr-Nb-Mo-Zr alloy is characterized by a multicomponent, fine dendritic structure with typical interdendritic distances of few microns. EDS spectra detected the alloying elements, Co, Cr, Nb, Mo, Mn, Si, etc. EDS elemental analysis quantified that Mo, Nb, Si and C are enriched in the interdendritic regions.
XPS survey spectrum for the new Co-Cr-Nb-Mo-Zr alloy surface showed a complex feature that included the peaks for Co 2p, Cr 2p, Nb 3d, Mo 3d, Zr 3d, o 1s; this film is composed from chromium, niobium, molybdenum, zirconium, manganese, silicon oxides and is thicker (6.5 nm – 8.0 nm), more compact and enhanced with oxides as compared with that of commercial Co-Cr-Mo alloy.
The topography and roughness parameters (AFM data) are similar for the two studied alloys and very proper for the cell adhesion.
An increase in the contact angle value for the new Co-Cr-Nb-Mo-Zr alloy has been observed as compared to the commercial Heraenium CE alloy. The increase from 75.920 to 86.790 values indicates that the both alloys have hydrophilic character but the stronger one is for the commercial alloy.
The quantity of ions (determined by ICP-MS method) released in artificial saliva for the both alloys is relatively small, but for the new alloy, the amount is significant lower. The quantified amounts in ppb (parts-per-billion, 10−9) sustain increased stability for the new alloy.
The evolution of open circuit potentials in time for the both alloys depicts a trend to steady state sooner after immersion in the both cases; it is a specific aspect for passivity due to the protective oxides formed on the surface of the both alloys. The open circuit potentials tend to more electropositive values, indicating an increase of passive film thickness.
The Tafel results showed that the new alloy is included in conventional stability scale „Perfect Stable” while the commercial alloy in “Very Stable” in the corrosion resistance group (corrosion classification). With the increase of immersion time, the penetration index, corrosion rate decreased. Also, the polarization resistance, Rp values for the new alloy increased in time as result of the passive film thickening; for the commercial alloy, polarization resistance decreased in time, denoting some dissolution and repassivation processes. Rp have higher values from about 8 times to 175 times than those of Heraenium CE alloy; correlating with the lower values of the corrosion current densities, it results a better corrosion resistance, a more resistant passive film for the new alloy due to its passive film that contains beside chromium oxide, niobium (Nb2O5, 49.8%) and zirconium oxides (ZrO2, 54.9%), as was showed by XPS analysis.
In conclusion, the new Co-Cr-Nb-Mo-Zr alloy displayed an increased corrosion resistance as compared with the commercial Heraenium CE alloy.