Pitting corrosion is a damage mechanism that can occur with many metal alloys. The process can be defined as the accelerated local dissolution of metal occurring as the result of the breakdown of the otherwise protective passive film on the metal surface, leading to small cavities (‘pits’). Pitting is more difficult to detect, predict and design against in comparison to uniform corrosion and as such is considered to be more dangerous. The morphology of pits may vary considerably, in terms of geometric shape and depth-width ratio. Pits propagate with complex directionality, as a function of local stress, microstructure and chemistry. Pits can act as local stress raisers in a material and hence can participate in the failure of a material through fatigue and/or stress corrosion cracking (SCC) mechanisms.
Pitting corrosion is problematic for structures operating in marine environments, due to the presence of high concentrations of metal chlorides (NaCl, MgCl2 and CaCl2) which are aggressive to steel alloys. Consequently, research focussed on the condition for which pitting might occur during operational life and the determination of the probability of the pit to transit to crack transition has significant potential to improve material selection at the design stage and structural integrity in general.
Conventional approaches to assessment of corrosion pitting have centred on visual inspection and destructive metallographic techniques (e.g. light microscopy of polished cross-sections of specimens). Such approaches are not ideal and cannot easily resolve the complex geometries of pits and cracks and as well as their interaction with the existing microstructure. As a result, insights into the process of pit to crack transition and qualitative and quantitative prediction of damage progression had been slow.
The main aim of this project is to conduct in-situ studies of the pit-to-crack transition in stainless steel (austentic 316L grade) through use of an X-ray Computed Tomography microscope system. The approach is to make pre-defined pits and follow the evolution of localised corrosion and then apply in-situ environmental and stress loading and capture how the crack (and its origin) interacts with the local microstructure in real time. This information will be linked to other NDT inspection technologies with the objective being able to provide an informed tool for site application to enable the characterisation of pitting corrosion and risk of cracking according to pit morphology. (NaCl, MgCl and CaCl