Chromic acid

The pH (Sentron Argus X pH-meter, Fisher Scientific GMBH, Schwerte, Germany) and the osmolality (Osmomat 030, Gonotec, Berlin, Germany) of the immersion medium were measured before and after 24 h, 48 h and 72 h of pre-incubation. For each pre-incubation time 5 samples of each alloy were immersed in the immersion medium (α-MEM + 15% FBS + 1% p/s) under cell culture conditions. After immersion, the degradation products were removed by soaking samples in chromic acid (180 g/L in distilled water, VWR international, Darmstadt, Germany) for 20 min at room temperature. The degradation rate (DR) was calculated in unit mm/year by the following equation:
$DR=\frac{8.76\times \mathrm{\Delta }g\times {10}^4}{A\times t\times \times \rho }$
where Δg: weight change (g), A: sample surface area (cm²), t: immersion time (hours), ρ: density (g/cm³)

Before sample sterilization, the initial weight of the samples as well as the initial pH (SENTRON ARGUS X pH-meter, Fisher Scientific GMBH, Schwerte, Germany) and osmolality (Osmomat 030, Gonotec, Berlin, Germany) of the immersion medium were recorded. Six samples were immersed in 2 mL of immersion medium for total immersion times (with pre-incubation) of 4 and 14 days under cell culture conditions. The immersion medium was changed every 3–4 days to facilitate a semi-static immersion test and prevent saturation effects. The osmolality of the supernatant was measured after each change of the medium. After immersion, the subsequently formed products were removed by treating the corroded disk with chromic acid (180 g/L in distilled water, VWR international, Darmstadt, Germany) for 20 min at room temperature. Images of the samples before and after chromic acid treatment were taken with a Moticam10 instrument (Motic Deutschland GmbH, Wetzlar, Germany). The degradation rate (DR) was calculated in mm/year using the following equation:
$DR=\frac{8.76 \times {10}^4 \times \Delta g}{A \times t \times \rho }$
where Δg is the weight change in grams, A is the surface area of the sample in cm², t is the immersion time in hours, and ρ is the density in g/cm³.