3D XRD imaging of corrosion in steel

The corrosion of steel-based mechanical components is said to be responsible for the loss of about 3% of annual global GDP. Cracks can appear in stainless steel components when stress or strain is combined with a corrosive environment that attacks sensitive grain boundaries. In nuclear power plants, certain grain boundaries can become sensitive during heat treatments or during fast neutron irradiation. It is important to observe how these cracks grow in detail, because they have been identified as the primary cause of several critical system failures. At the European Synchrotron Radiation Facility (ESRF), Grenoble, France, Dr. A. King and his colleagues recently revealed how growing cracks interact with the 3D crystal structure of stainless steel. The sample was a wire of 0.4 mm in diameter, and 40 keV X-rays were employed. By using diffraction contrast tomography, the research group could observe the shapes, positions, and orientations of 362 different grains with some 1600 grain boundaries without destroying the sample. They put the wire into a corrosive liquid, K2S4O6, and applied a load to cause microcracks to grow between the grains. As the cracks grew, 3D tomographic scans (of 30 minutes each) were made at intervals of between several minutes and two hours to follow the progress of the cracks. It was found that the cracks grew along the boundaries between the grains. The technique has enabled visualization of the cracks as they grow and of certain special boundaries that resist cracking. Information on this method is given in the following papers; "X-ray diffraction contrast tomography: a novel technique for three-dimensional grain mapping of polycrystals. I. Direct beam case", W. Ludwig et al., J. Appl. Crystallogr. 41, 302 (2008) and "II. The combined case", G. Johnson et al., J. Appl. Crystallogr. 41, 310 (2008). For more information on the present research, see the paper, "Observations of Intergranular Stress Corrosion Cracking in a Grain-Mapped Polycrystal", A. King et al., Science, 321, 382 - 385 (2008).