Inhibition of Corrosion-Driven Organic Coating Delamination on Cold-Rolled Steel By Graphene Nanoplatelets
Cathodic underfilm delamination occurs by the coupling of cathodic oxygen reduction at the delamination front with anodic iron dissolution, which remain localised to the defect. A thin, gel-like layer of electrolyte ingresses underneath the coating4. Loss of coating adhesion is attributed to polymer attack which occurs due to the highly alkaline-rendered environment at the cathode, in the case of iron this has been recorded to be as high as pH 145. Experimentally, the establishment of an electrochemical cell is achieved by placing an aqueous 5% w/v sodium chloride electrolyte onto an artificial penetrative coating defect.
Owing to the good electrical conductivity and high surface area of graphene, promising anticorrosion properties are reported. Three processes have been outlined in the available literature that contribute to the overall anticorrosion mechanism6. Firstly, excellent barrier properties are possible due to the impermeability of pristine graphene. Previous studies demonstrate a reduction in O2 and H2O permeability where polyaniline/graphene composites are used in place of both polyaniline/clay composites and neat polyaniline2,5. Secondly, an increase in the tortuosity of the pathway for permeating electrolyte can retard its ingression through the coating. Finally, the highly conductive nature of in-coating GNPs can provide an alternative pathway for electrons, thus preventing their inclusion at the cathodic site7.
In the current study, the time for delamination to begin (tdel) is shown to increase with increasing in-coating GNP content. Once a delamination cell is established, progression rates (xdel) are reduced with increasing GNP loadings. For coatings containing a 25% GNP loading, a tdel of 30 h and a 98% reduction in xdel was observed. A change from parabolic to linear delamination-rate kinetics is shown, where GNPs are present in the coating, suggesting an electron transfer blocking inhibition mechanism. Furthermore, the potential of the intact portion of the coated sample (Eintact) is found to progressively increase by up to 0.41 V vs. SHE with increasing loadings of in-coating GNPS. Whilst it is likely that the first two of the three processes (described above) contribute to the overall inhibition mechanism, the data indicates that the in-coating GNPs displace the O2 reduction reaction from the substrate to the coating. This eliminates the cathodic oxygen reduction reaction and delays the formation of an electrochemical cell.
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