Electrochemical corrosion behaviour of uncoated and DLC coated medical grade Co28Cr6Mo

The electrochemical corrosion resistance is one essential factor for determining the biocompatibility of metals. Although surgical alloys obtain an extraordinary high electrochemical corrosion resistance in the body environment, additional reduced metal ion release is expected to further improve the biocompatibility. In the following study, diamond-like carbon coatings with two different thickness were deposited on polished Co28Cr6Mo-samples (cast Protasul*-2, ASTM standard F75, ISO standard 5832/4) by a plasma deposition method (direct current discharge) using gaseous hydrocarbon as precursor. The electrochemical behaviour of DLC is investigated in 0.89 wt.% NaCl solution and compared with the behaviour of the uncoated medical grade CoCrMo. Potentiodynamic polarization tests were carried out starting from the rest potential into the anodic direction up to a potential of +1500 mV, and subsequently, into the cathodic direction. The polarization curves are evaluated and correlated to the thickness of the DLCs. Additional information is given by hardness measurements (loads 4 mN, 10 mN) as well as characterization of the nano- and microstructure (Raman spectroscopy). The shift of the break down potential to more positive values and the reduction of the current densities give evidence that DLC coatings provide an useful measure for reducing the corrosion attack of the cast Co28Cr6Mo-alloy in 89 wt.% NaCl. In this study, the 2.7-μm thick coating offers further improved protection in comparison with the 0.8-μm thick DLC.