Creating new barriers with graphene

In its purest form, graphene possesses an unsurpassed combination of electrical, mechanical and thermal properties, which gives it the potential to replace existing materials in a wide range of applications and, in the long term, to enable new applications. Graphene’s unique two-dimensional structure in the nanoplatelet form results in very high aspect ratio, high surface area materials, which are particularly suited for use as multi-functional additives in paints and coatings formulations.

Applied Graphene Materials (AGM) is a leading innovator in the production and application of graphene. AGM has developed and patented a unique graphene synthesis process. AGM’s manufacturing process uses sustainable raw material sources, rather than graphite, which is inherently limited in supply. The graphene platelets produced using AGM’s process are ‘dispersion ready’, which means there is no need to add any intermediate energy consuming functionalisation step to aid in dispersing the platelets. A-GNPs have demonstrated true multi-functionality, further development is already underway to remove other less sustainable additives used to improve properties such as electrical and thermal conductivity, wear, and fire resistance.

AGM’s ability to manufacture graphene to various platelet grades enables the optimisation of loading levels in the final formulation to very low percentages, these represent a step change in technology and underlines commercial viability.

Graphene in barrier coatings

It has been theorised that graphene’s two dimensional platelet structure would enable excellent performance in barrier coatings. Applied Graphene Materials has worked with independent industry experts to complete an evaluation of AGM’s graphene platelets in an epoxy coating, with the aim of demonstrating how effective a graphene enhanced coating might be in preventing corrosion. AGM evaluated two grades of graphene platelets, A-GNP10 and A-GNP35(T). A-GNP10 is a medium density graphene with a rigid platelet structure and built-in oxygen functionality, which gives excellent dispersability. A-GNP35(T) is an ultra-low density, high surface area graphene which has a flexible, crumpled sheet morphology. These materials were selected because they each have properties which could be useful in preventing corrosion.

A two pack epoxy system, Epikote 828 with Epikure 3234 hardener, was chosen as this would be relevant for many of the epoxy primer systems, which are used to protect steel and aluminium structures. The graphene was dispersed directly into the resin, at loading levels, which ranged from a low of 0.1 wt% to as high as 5 wt% for A-GNP10. A-GNP35(T) was limited to a maximum loading level of 1.0 wt% due to the very high surface area of the graphene. Coatings were applied to mild steel Q-panels for testing using a draw down method, and the thickness of the coatings was monitored before testing.

Salt fog testing

AGM engaged with established paints and coatings experts PRA, formerly the Paints Research Association, to test the performance of the graphene loaded epoxy coatings under salt fog conditions. Cyclic corrosion resistance was tested under the guidelines of BS EN ISO 11997-2, with the modification to remove the UV light exposure.

Duplicate specimens were exposed using a repeated cycle of 1 hr dilute electrolyte fog (0.35% ammonium sulfate, 0.05% sodium chloride) at 24 to -3°C followed by 1 hr dry with temperature rising to 35°C for a total of 1000 hrs. Panels were checked regularly to monitor progression of corrosion. The panels were rated for defects such as blistering and rusting at three and six weeks under the guidelines of EN ISO 4628 parts 2, 3 and 8 (blistering, corrosion and corrosion/delamination around a scribe).

Immersion testing

Following positive results in the cyclic salt fog corrosion testing, AGM worked with globally recognised corrosion experts TWI Ltd to further investigate the corrosion protection properties of the graphene enhanced epoxy coatings. Steel panels were prepared in a similar manner, and were then subjected to a full immersion in synthetic seawater, prepared to standard ASTM D1141 – ‘Standard Practice for the Preparation of Substitute Ocean Water’, at ambient 20 – 30°C for a duration of 30 days. Upon completion of the immersion testing, samples were cross sectioned and imaged by SEM.

Electrochemical testing

Electrochemical monitoring of a substrate during immersion testing can provide useful information about how well the coating is protecting the steel panel. Corrosion is an electrochemical process, as the metal in the substrate is oxidised, which produces an electrical current. It is possible to monitor this electrical current to quantify the amount and rate of corrosion occurring. In these experiments, also carried out by TWI Ltd, panels were immersed in synthetic seawater and measurements taken using a three electrode system. The corrosion current for each sample was monitored over the 30 days of immersion. The first observation is that the corrosion current recorded for the graphene-loaded samples is roughly 1000 times smaller than for the graphene-free epoxy control sample. This very low corrosion current correlates well with the conclusions from the visual assessment and the SEM analysis, and confirms that the addition of graphene to the epoxy is drastically improving the corrosion protection offered by the epoxy coating.

Water vapour permeation

The graphene enhanced epoxy coatings have been demonstrated to significantly improve the corrosion protection performance of the epoxy. The proposed explanation for this has been that the graphene platelets are acting as a barrier to diffusion of water and corrosive salts through the epoxy coating. AGM and PRA investigated this by measuring the water vapour transmission rate (WVTR) through the epoxy coatings following ASTM D 1653-03 using Test Method B (wet cup method) condition A (23 C, 50% RH). Samples of graphene-free epoxy and epoxy loaded with A-GNP10 and A-GNP35(T) were coated onto a paper substrate for this test.

The results of the WVTR testing showed a clear reduction in the diffusion rate of moisture through the graphene loaded samples. The data showed that the epoxy control had a WVTR of approximately 200 g/m²/d. The addition of the smallest amounts of graphene tested, at 0.1 wt% reduced this diffusion rate by a factor of almost 100. WVTR of approximately 5 g/m²/d were recorded for all graphene loaded samples.

This data confirms that the graphene platelets are forming a very effective barrier to diffusion, by effectively forming a ‘tortuous path’ and greatly increasing the time it would take for corrosive elements to migrate through the coating to the substrate. The very effective barrier properties of the graphene platelets in the epoxy coatings explains the impressive corrosion mitigation observed on the steel panels in both immersion testing and cyclic salt fog testing.

Conclusion

Applied Graphene Materials has shown that the addition of very low loadings of their A-GNP10 and exceptionally low loadings of their A-GNP35(T) graphene platelets to epoxy coating systems can drastically improve the corrosion mitigation of these coatings. The graphene offers an impressive barrier to the diffusion of corrosive elements to the underlying surface. The dispersion compatible nature of A-GNPs, and the very low loadings levels required, points towards non-disruptive access to this new technology and underlying commercial viability. AGM believes that there is the potential for the A-GNPs to facilitate the formulation for removal or reduction of heavy metals and other anti-corrosive pigments, but also generally showing the potential to remove other barrier additives and reduce coating weight and thicknesses. The inclusion of Applied Graphene Materials’ A-GNP10 and A-GNP35(T) graphene platelets into epoxy coating formulations offers the chance to significantly increase the lifetime of coated parts.

Read the article online at: https://www.worldpipelines.com/special-reports/02112016/creating-new-barriers-with-graphene/