Dark field imaging of the in vitro corrosion of biodegradable magnesium screws

Orthopedic magnesium implants offer a unique solution for the repair and internal fixation of bones since they are biodegradable, i.e., eventually absorbed by the body. In this contribution, we analyse screws from the magnesium alloy MX1 (1% Mn, < 0.5% Ca). The test sample is represented by an ISO M3 x 16 screw which was analysed before and after controlled corrosion in a flow cell. We use X-ray microcomputed tomography (XCT), dark-field and phase contrast XCT, and scanning electron microscopy (SEM) to analyse microstructural changes during degradation. While microcracks in corroded parts can only be visualized at very high XCT scanning resolutions or SEM imaging, DFC images show a clear indication of corroding regions. Since DFC delivers morphological information in the sub-pixel regime it is possible to quantify microcracks in relatively large samples whereas single microcracks are visible in SEM images but not in low-resolution attenuation contrast images, e.g. at a voxel size of 10 μm. X-ray dark-field imaging improves defect detection in corroded Mg samples without the necessity of small sample dimensions, hence providing a new approach to quantify degradation rates in corroding Mg implants in three dimensions.