Increasingly researchers are looking to bring together perspectives across multiple scales, or to combine insights from different techniques, for the same region of interest. To this end, correlative microscopy has already yielded substantial new insights in two dimensions (2D). Here we develop correlative tomography where the correlative task is somewhat more challenging because the volume of interest is typically hidden beneath the sample surface. We have threaded together x-ray computed tomography, serial section FIB-SEM tomography, electron backscatter diffraction and finally TEM elemental analysis all for the same 3D region. This has allowed observation of the competition between pitting corrosion and intergranular corrosion at multiple scales revealing the structural hierarchy, crystallography and chemistry of veiled corrosion pits in stainless steel. With automated correlative workflows and co-visualization of the multi-scale or multi-modal datasets the technique promises to provide insights across biological, geological and materials science that are impossible using either individual or multiple uncorrelated techniques.

Pitting corrosion kinetics of type 304L stainless steel have been obtained using a quasi in-situ X-ray computed tomography (X-ray CT) approach. A miniature electro-chemical cell was constructed to allow imaging during potentio-dynamic polarization of a wire specimen in chloride solution. The formation of three discrete pits was observed, allowing comparison between real pit geometry and different geometrical assumptions to estimate pit growth kinetics. The pit volumes obtained by X-ray CT showed a good fit with the volume of metal dissolution calculated using Faraday's law. Large fluctuations of the mean current density were observed during the pit nucleation stage, followed by pit growth with mean current densities of 1-3 A.cm −2 . Stability products associated with these pits were on the order of 0.3-0.6 A.m −1 , with a diffusivity parameter (D . C) of 1. Pitting or crevice corrosion can occur on stainless steels exposed to halide containing environments. The breakdown of the passive surface film results in the nucleation of local attack, which is followed by pit growth, with a number of mechanisms proposed for each stage. 1,2Electrochemical polarization above the critical pitting potential leads to formation of pits, in the form of localized metal dissolution, with corrosive electrolyte developing inside the nucleated pit due to hydrolysis of dissolved metal ions, and chloride ions being attracted from bulk solution to maintain charge neutrality.3 At high applied electrochemical potentials, pits grow with a polished inner surface morphology, associated with salt film formation, whereas pits grown at lower applied potentials display more irregular etch morphologies of the internal pit surface. 4 An undercutting mechanism was suggested for pit growth at high electrochemical potentials, leading to the development of dish shaped pits covered by metal lacy covers. 5-10Pit propagation is controlled by diffusion of species through the salt film at the inner surface of pits. A high anodic dissolution rate, which facilitate critical ion concentrations inside a pit, is the criterion for pits to propagate. 11 The anolyte concentration inside a pit influences pit growth kinetics and the pit shape. For example, a 3 M concentration of dissolved metal ions was proposed as the minimum concentration to turn an open hemispherical meta-stable pit into a stable pit. However, pits developed with lacy metal covers grow at far lower metal ion concentrations, since these covers are believed to provide an effective diffusion barrier 12 and an increased ohmic resistance between pit interior and the bulk electrolyte.1 Based on these observations for the transition between meta-stable and stable pits, the "pit stability product" (0.3 A. m −1 ≤ i.r ≤ 0.6 A. m −1 ) was proposed, with the latter based on the evolution of pit current density (i) and pit depth (r). 12To sustain stable pit growth under anodic polarization, a minimum current density is required, with typical values for stainless steel in the region of 1-5 A.cm −2 . 13,14 For ...

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