Co-Cr-Mo Alloy surface features and composition prior and after mechanical polishing and corrosion in fluids simulating oral conditions

It has been well-documented that metal ions are released from all dental alloys, which may be of a clinical concern and may be a potential health problem. Corrosion behaviour in oral fluids is strongly influenced by microstructural characteristics and surface properties of dental alloy. Aim of the Study was to examine the surface of CoCrMo alloy prior and after exposure into different solutions simulating oral conditions, as well as prior and after different mechanical polishing procedures. The cast and electropolished specimens of CoCrMo alloy (EP) (Wironit®, extra hard) were examined by SEM with EDS, by optical microscope and by AFM prior and after immersion into two solutions simulating saliva and dentobacterial plaque for 30 days at 37 oC. The effect of polishing at the surface of CoCrMo alloys has also been studied. After electropolishing procedures (EP, 6 samples) another samples were further mechanically polished using green rubber discs and afterwards a high shine polishing paste and a rotating black brush (BB, 6 samples), or using green rubber discs, a high shine polishing paste and a rotating deer leather brush (DL, 6 samples). The AFM analysis provided usefull information about the surface morphology. AFM imaging revealed that the surface of EP specimens prior the contact with corroding solutions was characterized with at least 3-4 different phases: undulating surface corresponding to dendrites in the optical microscope images, clusters of nanocrystallites emerging from the undulating surface (islands of nanocrystallites) corresponding to the interdendritic phase in the optical microscope images and some high conical crystallites emerging from both, undulating surface and the islands of nanocrystallites. Also some very small grain-like crystallites were also observed from AFM images. The undulating surface was twice richer with Co than Cr, while in the interdendritic phase (clusters of nanocrystallites forming islands and emerging from the undulating surface) Co was almost equal to Cr, which was different from the declared alloy's composition. However, some cristallytes were dissolved after corrosion, but some new phases (dense sprinkles of very small crystallites) appeared. The respective alloy peaks in the EDS spectrum revealed that these phases were a result of a corrosion - primarily of metal dissolution and salt formation. The presence of some elements from the corroding solution was registered at the alloy surface, as well. The effect of mechanical polishing was clearly evident at CoCrMo surface. The clusters of nanocrystallites forming islands (emerging from the undulated surface) were smoothened and/or thorn away by further mechanical polishing, while the islands' perimetar was unaltered. Mechanical polishing also produced scratches in both, BB and DL samples. The scratches were observed not only at the undulating surface, but also at the smoothed islands. The Z range (distance from the highest to the lowest peak) of EP samples revealed significantly higher values than that of BB and DL (p 0.05), compared to the mechanically polished BB and DL samples. AFM imaging revealed at least four different phases at the surface of CoCrMo electropolished specimens. The most dominant phases were: undulating surface and clusters of nanocrystalites emerging from the undulating surface forming islands, but some high conical crystallites and some small grain-like crystallites have also been observed. Mechanical polishing of EP samples smoothed islands of nanocrystallites, but also formed scratches at the surface, which were more prominent in DL than BB samples. During the corrosion process of EP samples probably metal salts (anions from corroding solutions) were binded at the surface as a new phase. Electrochemical behaviour of CoCrMo alloys is clearly affected by pH and composition of the fluids. It is obvious that some complex mechanisms involving reactions between ionic species are dominating the interfacial processes, which in turn affect the morphology and the composition of the phases formed.