Corrosion Properties of CoCr Sintered Alloy vs. CoCr Cast Alloy

Methods: From Ceramill Sintron (Co66Cr28Mo5) and Girobond NB (Co62Cr26Mo5W5) 12 specimens were prepared for each corrosion test method respectively. For comparison half of them were exposed to ceramic firings. All corrosion measurements were performed according to ISO 10271. In the static immersion tests (0.1M sodium chloride/lactid acid, pH=2.3, 37°C) ion release was analyzed after 1, 4 and 7 days using ICP-OES (Optima 4300 DV, Perkin Elmer). In the electrochemical corrosion measurements (0.9% saline, 37°C) open-circuit-potential was recorded over 2 h followed by anodic polarization from -150~Ecorr to 1.0 V. Significance of results was evaluated with Student`s t-test (p<0.05).

Results: Without and with heat treatment total ion release after 7 d immersion was 1.7 and 1.5 µg/cm² (Co, Cr, Si) with Ceramill Sintron and 4.1 and 3.7 µg/cm² (Co, Cr, Nb, Si) with Girobond NB. Furthermore, Girobond NB showed a relatively high solubility of Ce (71.1 and 55.7 µg/cm²). All other elements were below the detection limit.

The open-circuit-potential Ecorr after 2 hours with and without heat treatment was -104±24 and -117±12 mV (Ceramill Sintron) and -176±17 and -177±19 mV (Girobond NB). The parameter i(300mV+Ez) with and without heat treatment was 3.0±0.3 and 3.3±0.6 µA/cm² (Ceramill Sintron) and 2.2±0.3 and 3.0±0.6 µA/cm² (Girobond NB). Pitting potential Ep was for all samples in the range of 650-675 mV.

Conclusion: Depending on the respective corrosion parameter Ceramill Sintron showed a marginally better or similar corrosion behavior compared to Girobond NB. Heat treatment improved the corrosion characteristics gradually, especially in the cast alloy.