CoCrMo alloy has become one of the most important load-bearing materials in orthopedics. The influence of alloying components on the alloy’s electrochemical behavior (passivity, pitting corrosion, and transpassivity) has been investigated in situ under in vitro conditions in Hanks’ solution. It was shown that anodic oxidation provides specific alloy’s surface functionalization that imparts high corrosion resistance to the alloy, thus determining material’s biocompatibility and implantation applicability. The deep understanding of the electronic (semiconducting) properties of passive films anodically formed on CoCrMo alloy is needed to predict long-term corrosion resistance of implant materials in simulated body fluids. Mott-Schottky tests were used to probe electronic properties of the passive films formed on the alloy and pure chromium (as a result of the “chromium-like” alloys passivity). Both films were found to behave as p-type semiconductors, exhibiting the same flat-band potential, EFB, and the acceptor density values, NA, corrected for the frequency dispersion. One order of magnitude lower NA for the alloy, which corresponds to the point defects and/or film’s nonstoichiometry, was explained by interaction of cation vacancies, , with highly charged Mo-ions, segregated in the film during alloy’s anodic polarization. This complexation led to an increase in electronic conductivity of the surface film and in turn to an increased alloy’s resistance against general and pitting corrosion. It is important to point out that the semiconducting parameter EFB determines the accurate value of the transpassivity onset potential. At potentials E < EFB, the surface film as a p-type semiconductor is in depletion conditions, creating the Schottky barrier at the film/electrolyte interface that enhances passivity. At E > EFB, the surface film enters an accumulation mode and undergoes transpassive dissolution.
Electrochemistry of CoCrMo Implant in Hanks’ Solution and Mott-Schottky Probe of Alloy’s Passive Films
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M. Metikoš-Huković, J. Katić, Z. Grubač, I. Škugor Rončević; Electrochemistry of CoCrMo Implant in Hanks’ Solution and Mott-Schottky Probe of Alloy’s Passive Films. CORROSION 1 December 2017; 73 (12): 1401–1412. doi: https://doi.org/10.5006/2522
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