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Migration Of Molecules In Coatings

Periodic Reporting for period 1 - MOMIC (Migration Of Molecules In Coatings)

Reporting period: 2016-09-01 to 2018-08-31

Polymer coatings, composed of a polymer binder, functional pigments and non-functional fillers provide corrosion protection in three main ways: 1) forming a barrier to corrosive species, 2) blocking ionic transfer between anode and cathode areas on a metal surface, and 3) actively inhibiting corrosion at the metal surface by releasing anticorrosive species. Effective corrosion inhibiting pigments exhibit limited solubility in water minimising leaching and delivering long-term anti-corrosion performance during the entire service-life of the coating. Historically, the most efficient corrosion protective systems involved chromate-based pigments; however there is now a need to identify more environmentally-friendly, non-toxic alternatives. One serious obstacle to designing novel anti-corrosive systems is the limited understanding of the transport phenomena of ionic species (i.e. ions originating from the pigment particles) in polymer coatings [1].
The dominant theory in the literature proposes: (i) that ions can penetrate polymer matrices through conductive pathways formed as a result of environmental degradation; additionally, pigments present in polymer coatings may provide alternative pathways including: (ii) those formed after leaching of pigment particles and (iii) those via the interface of the matrix and filler particles [2,3]. The second mechanism based on pigment leaching has been shown to be important [3] however no strong experimental evidence for other mechanisms has yet been reported.
The goal of this project was to explore in more detail these hypotheses with two research approaches (namely ions-out and ions-in). The difference in chromate pigment leaching behavior (ions-out) between model coatings containing: (a) isolated pigment particles, and (b) clustered particles addressed the second and third hypothesis. Likewise, the possibility of ionic transport within the polymer coatings exposed to the solution with chromate ions (ions-in) was investigated using advanced analytical methods and addressed the first hypothesis.
[1] S.B. Lyon et al. Prog Org Coat 102 (2017) 2-7
[2] S. Sellaiyan et al. Prog Org Coat 77 (2014) 257-267
[3] A. E. Hughes et al. AIMS Materials Science 2 (2015) 379-391
In the first set of experiments two epoxy model coatings were prepared, containing chromate pigment with respectively, 1 % of pigment volume concentration (PVC) (isolated particles system) and 15 % of PVC (clustered particles system). These were exposed to sodium chloride solution and the leaching of chromate species (ions-out) was analyzed. SEM-EDS (Scanning Electron Microscopy-Energy Dispersive X-ray Spectroscopy) and STEM (Scanning Transmission Electron Microscopy)-EDS enabled the estimation of the particle depletion depth and the detection of changes of the particle composition. Focused Ion Beam (FIB)-SEM was used to analyze the structure of the coatings after pigment leaching by serial sectioning. This allowed direct observation of the 3D connectivity of the particles and voids left after leaching at high resolution. The results for the clustered system confirmed leaching with a non-uniform depletion depth while in the case of the separated particles system no significant changes were observed even after 12 months of exposure. This strongly suggested that the main transport mechanism was via connected pathways formed after leaching of the pigment particles.
In the second part of the investigation polymer films, with (filled) and without (unfilled) non-functional particles (fillers) of barium sulfate (BaSO4) or titanium dioxide (TiO2), were exposed to sodium chromate solution to study ion uptake and the formation of alternative migration pathways in the coating (ions-in). The chromium concentration in the unfilled polymer after exposure was determined by ICP-MS (Inductively Coupled Plasma-Mass Spectrometry). LA-ICP-MS (Laser Ablation-ICP-MS) was used to confirm chromate penetration into the films and to track the location and concentration of Cr on a micron scale. Furthermore, higher resolution (nm scale) data on the distribution of Cr in the films after exposure was delivered by TOF-SIMS (Time of Flight-Secondary Ion Mass Spectrometry) analysis. It was shown that chromate penetrated the polymer matrix only under specific conditions when the polymer was affected by temperature or solvent. Furthermore, Cr was confirmed to be located in specific channels inside the polymer film. SEM-EDS and STEM-EDS were used to investigate Cr location in the films with fillers after exposure. The addition of pigments (TiO2 or BaSO4) influenced the chromate transport in the coating. TiO2 particles did not actively contribute to the chromate transport by dissolution but transport pathways were available at the particle/matrix interface. BaSO4 particles however, were shown to actively contribute to the transport mechanism. Filler particles were partially dissolved under exposure conditions while chromate ions penetrated the polymer via the particle/matrix interface, resulting in precipitated barium chromate (BaCrO4) observed at the surface of the BaSO4 particles as shown in Figure 1.
Leaching of pigment species via connected pathways of pigment particles was confirmed as a dominant transport mechanism in active anti-corrosive coatings. Moreover, the experiments showed that non-functional filler particles influenced the ion species transport in coatings both actively and passively depending on the specific material and its stability in the test environment. This was one of the proposed explanations for anticorrosive pigment species transport in coatings, yet had previously never been proven experimentally [3]. This indicates that all components of the coating including those added as non-functional fillers may influence the performance and must be considered in coating design. This is also an important discovery for the general understanding of ion transport in coatings. It was shown that ions penetrate polymer coatings through the pigment/polymer interface which can possibly be also a fast (short-circuit) transport pathway for corrosive ionic species. Furthermore, a limited transport of chromate ions within the polymer matrix was shown under certain, more extreme, conditions (e.g. solvent swelling and/or temperature above glass transition of polymer).
The successful development of a robust methodology and analytical approach to study the transport of ionic species in coatings was an additional significant outcome which will support further research in the field.
Replacement of Cr (VI) based products in corrosion protection, driven by regulations and markets due to toxicity concerns is still a technological challenge, especially in aerospace industry. Without suitable replacements for Cr (VI) compounds in the products for corrosion protection, a considerable economic impact should be expected due to the reduction in lifetime of assets or the need for more frequent re-protection using less effective products. The results of the MOMIC project allow to better understand the transport of chromate in polymer coatings. This knowledge will accelerate development of Health, Safety and Environment (HSE)-friendly alternatives for chromate-based coatings with non-toxic replacements in the future.

[4] G. H. Koch et al.; FHWA-RD-01-156,
Illustration f different phases in the polymer coating with filler particles.