Abstract
Transport paths for inhibitor release within a model strontium chromate (SrCrO4) inhibited/epoxy primer have been studied using a combination of tomography techniques. It has been found that the SrCrO4 particles form independent clusters within the model primer. The clusters have a range of fractal dimensions with the largest clusters (a few hundred microns in size) having a fractal dimension of 2.36. Leaching of the SrCrO4 from the primer appears to be initially through direct dissolution of particles in contact with the electrolyte but changes to diffusion through void pathways created by dissolution of the SrCrO4 phase. No evidence was found for the diffusion of chromate ions through the epoxy. Transport through such clusters does not follow Fickian diffusion, which has traditionally been employed to describe inhibitor release dynamics. Release kinetics typically follow a tm behaviour where t is time and m is an index which would be 0.5 for Fickian diffusion. Thus the overall release with time will evolve, being initially the result of direct dissolution, then at intermediate times, be dominated by transport through the fractal network and at the final stage go to zero since all the strontium chromate will be dissolved from the cluster connected to the surface. Clusters not connected to the surface remain undissolved and form additional reservoirs for further release in when local damage occurs in their vicinity. This new model of inhibitor transport creates new strategies for the development of self-healing properties for coatings.
Highlights
Porous structures through which material is transported are encountered extensively in nature
The depletion front does not evenly penetrate the primer showing a considerable variation between maximum and minimum penetration depths. These results are similar to those reported previously where the depletion depth appeared to reach a limiting value between 20 and 55 m after 7 days immersion and the leach rate had decreased by 3 to 4 orders of magnitude, indicating that the amount of material released in the first 7 days is close to the total amount that can be released even though a substantial amount of SrCrO4 remained in the primer [24]. This limitation to the release of chromate has been reported by Furman et al [19] who observed that most of the chromate that is going to be released is released within the first week even if the test is performed over two years
X-ray analysis of the Computed tomography (CT) volume reconstructions before (Fig. 6(a) and (c)) and after leaching (Fig. 6(b) and (d)). These analyses revealed that the primer contains three distinguishable regions comprising the epoxy resin, SrCrO4 particles and regions with lower absorption contrast than the epoxy that are considered to be low density epoxy (LDE) in green. (It should be noted that the sample is a long slither of irregular shape.) There was no evidence from EDS that chromate could diffuse through the LDE since the pattern of LDE was not reflected in the Cr X-ray maps, it may be an important transport path for water and small ions
Summary
Porous structures through which material is transported are encountered extensively in nature. Natural porous structures that are currently of great interest include rocks such as shales for the extraction of oil and gas resources [1,2,3,4,5]. Man-made porous materials are used in a wide range of applications [6] and the development of porous networks within these materials provides opportunities for designing structures using freeze-cast approaches [7] and biomimicry for bio [8] and self-healing materials [9] applications. Our understanding of the porous networks within these materials, the level of connectivity and the onset of percolation (the percolation threshold), is growing rapidly through integration of microstructure-based properties modelling [10] and 3D microstructural characterisation methods [2,11,12].
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