Inhibition of Corrosion-Driven Organic Coating Delamination on Cold-Rolled Steel By Graphene Nanoplatelets
Authors: Glover Carol Frances , Subramanian Raman , Richards Calvin , Williams Geraint , McMurray Hamilton Neil 
Published: February 2020

Pressure on the coatings industry to remove toxic inhibitors based on highly effective, sparingly soluble chromium (VI) salts presents a need for an alternative1.  Graphene-based materials are of world-wide interest where applications for many fields of science and engineering are being explored2.  The current study employs a scanning Kelvin probe (SKP) to assess the influence of graphene nanoplatelets (GNP) on the kinetics and mechanism of the corrosion-driven delamination process affecting a polyvinyl butyral (PVB) coating adherent to cold-rolled steel substrates.  A systematic study into the effect of GNPs at various loadings is presented. PVB is a model coating used extensively in previous delamination studies for the incorporation of novel inhibitors3. 

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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