Time-lapse lab-based x-ray nano-CT study of corrosion damage

An experimental protocol (workflow) has been developed for time-lapse x-ray nanotomography (nano-CT) imaging of environmentally driven morphological changes to materials. Two case studies are presented. First, the leaching of nanoparticle corrosion inhibitor pigment from a polymer coating was followed over 14 days, while in the second case the corrosion damage to an AA2099 aluminium alloy was imaged over 12 hours. The protocol includes several novel aspects relevant to nano-CT with the use of a combination of x-ray absorption and phase contrast data to provide enhanced morphological and composition information, and hence reveal the best information to provide new insights into the changes of different phases over time. For the pigmented polymer coating containing nominally strontium aluminium polyphosphate, the strontium-rich components within the materials are observed to leach extensively whereas the aluminium-rich components are more resistant to dissolution. In the case of AA2099 it is found that the initial grain boundary corrosion is driven by the presence of copper-rich phases and is then followed by the corrosion of grains of specific orientation. Lay Description Time lapse x-ray computed tomography enables the collection of sequences of 3-D images. Few experiments have been carried out to date at submicrometre resolutions. Here we use an in situ cell to study the environmentally induced changes of two materials exposed to saline conditions. The first is a primer coating used to protect metal materials and the other is an aluminium alloy used in airplane (AA2099 aluminium alloy). For the primer coating, we followed the release of corrosion inhibitor (strontium aluminium polyphosphate) nanoparticles for 14 days. Using phase contrast imaging we were able to discern the inhibitor particle morphologies whereas attenuation contrast distinguished strontium- and aluminium-rich particles. We found that the strontium-rich particles dissolved rapidly whereas the aluminium-rich components were more resistant to dissolution. In the case of aluminium aerospace alloy, we tracked the progress of intergranular corrosion being driven by the presence of copper-rich phases at the grain boundaries of the alloy. This was then followed by the rapid corrosion of particular grains.