Creating New Barriers with Graphene Authors: Richard Akam, Lynn Chikosha & Tim von Werne Introduction Graphene was first isolated in 2004 by Andre Geim and Konstantin Novoselov at Manchester University. They used sticky tape to remove single layers from the surface of graphite and deposit them on a silicon wafer. The isolation of graphene was a breakthrough for which Geim and Novoselov ultimately won the 2010 Nobel prize for Physics. 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 functionalization 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 theorized 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 2-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 60 minutes dilute electrolyte fog (0.35% ammonium sulfate, 0.05% sodium chloride) at 24+/- 3 C followed by 60 minutes dry with temperature rising to 35 C for a total of 1000 hours. Panels were checked regularly to monitor progression of corrosion. The panels were rated for defects such as blistering and rusting at 3 and 6 weeks under the guidelines of EN ISO 4628 parts 2, 3 and 8 (blistering, corrosion and corrosion/delamination around a scribe). Figure 1. Epoxy coated steel panels before (A, B, C) and after (D, E, F) 1000 hrs salt fog testing. (A) 0% Graphene Epikote 828 control at 0 hrs. (B) 0.5% A-GNP35(T) in Epikote 828 at 0 hrs. (C) 5.0% A- GNP10 in Epikote 828 at 0 hrs. (D) 0% Graphene Epikote 828 control at 1007 hrs. (E) 0.5% A- GNP35(T) in Epikote 828 at 1007 hrs. (F) 5.0% A-GNP10 in Epikote 828 at 1007 hrs. Figure 1 shows representative images of a selection of panels before and after the 1000 hour salt fog testing. It is clear from the results that addition of even the smallest amount of graphene significantly enhanced the corrosion protection of the epoxy coating. It was found that the graphene-free epoxy control sample rapidly developed rust spotting and clear evidence of corrosion after only a couple of days, and the rusting became quite severe during the period of the test. The
graphene loaded epoxy coated samples remained corrosion free for up to 12 days, and then only showed very small, localized spotting. This localized corrosion appeared to be limited to regions where there had been defects or pitting in the coating. There was also an increase in the time it took for onset of corrosion (appearance of black rust spots) with increasing loadings of graphene. The best performance was achieved with 5% A-GNP10 and 0.5% A-GNP35(T). Immersion testing Following the positive results in the cyclic salt fog corrosion testing, AGM worked with globally recognized 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 20-30C for a duration of 30 days. Upon completion of the immersion testing, samples were cross sectioned and imaged by SEM. Figure 2. Epoxy coated steel panels before (A, B, C) and after (D, E, F) 30 days immersion testing in synthetic seawater. (A) 0% Graphene Epikote 828 control at 0 days. (B) 1.0% A-GNP35(T) in Epikote 828 at 0 days. (C) 1.0% A-GNP10 in Epikote 828 at 0 days. (D) 0% Graphene Epikote 828 control at 30 days. (E) 1.0% A-GNP35(T) in Epikote 828 at 30 days. (F) 1.0% A-GNP10 in Epikote 828 at 30 days. Figure 2 contains photographs of the epoxy coated steel panels before and after the 30 day immersion in synthetic seawater. It is clear again that the graphene-free control epoxy has suffered severe corrosion and rusting, while the graphene loaded epoxy samples are virtually corrosion free. The graphene significantly enhanced the corrosion mitigation of the epoxy coating even at loading levels as low as 0.1 wt%. Typically, the corrosion mitigation improved as the loading level of
graphene increased. Both A-GNP10 and A-GNP35(T) offered improved corrosion protection relative to the base epoxy. Figure 3. SEM Micrographs of cross sectioned epoxy coated steel panels after 1000 hrs salt fog testing. (A) 0% Graphene Epikote 828 control showing growth of corrosion products under the epoxy coating. Inset red box marks area examined using EDX. (B) 0.5% A-GNP35(T) in Epikote 828 showing no corrosion of steel substrate under coating. SEM imaging of the panels was conducted to characterize the corrosion of the substrate under the epoxy coating, which are shown in Figure 3. The left hand image is a cross section of the graphenefree epoxy coated panel after 30 day immersion testing. It is clear that the seawater has ingressed to the steel substrate surface and the steel has started to corrode and rust. The red box marks an area that was analysed using the EDX function, which can give the elemental composition of a material. The EDX spectrum is shown in Figure 4, and highlights the presence of iron oxide which then confirms rusting of the substrate. The right hand image in Figure 3 is a representative cross section of one of the graphene enhanced epoxy coated panels. The coating has remained intact, and there is no rusting or corrosion present at the surface of the substrate as the diffusion of water and salts to the surface has been prevented. This confirms that the graphene has greatly improved the corrosion mitigation of the epoxy coating.
Figure 4. Energy dispersive X-ray (EDX) spectrum of corrosion products detected in cross sectional analysis of 0% graphene Epikote 828 control after 30 days immersion in synthetic seawater. Presence of Fe, O and Cl indicate rusting. 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 oxidized, 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 3 electrode system. The corrosion current for each sample was monitored over the 30 days of immersion, and examples of the data are shown in Figure 5. 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. Figure 5. Corrosion current recorded during 30 day immersion testing of epoxy coated steel panels in synthetic seawater. (A) 0% Graphene Epikote 828 control. (B) 1.0% A-GNP35(T) in Epikote 828. (C) 1.0% A-GNP10 in Epikote 828 at 0 days. Note different scale on Y axis in plot A. 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, presented in Figure 6, shows that the epoxy control had a WVTR of approximately 200 g/m 2 day. The addition of the smallest amounts of graphene tested, at 0.1 wt% reduced this diffusion rate by a factor of almost 100. Water vapour transmission rates of approximately 5 g/m 2 day were recorded for all graphene loaded samples.
Figure 6. Water vapour transmission rate data for Epikote 828 epoxy control and 0.1% and 1.0% A- GNP10 and A-GNP35(T) in Epikote 828 epoxy coatings. 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. Conclusions 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 anticorrosive 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. Applied Graphene Materials UK Ltd The Wilton Centre, Redcar, Cleveland TS10 4RF, United Kingdom +44 (0)1642 438214 info@appliedgraphenematerials.com www.appliedgraphenematerials.com