Creating new barriers with graphene

Despite excellent properties, steel is prone to corrosion that causes a degradation in properties and results in extensive maintenance or replacement of steel, at significant cost. This cost impact has been estimated at US$2.2 trillion or approximately 3% of global GDP² and attracts significant effort from coating manufacturers in an effort to develop coatings that provide protection against corrosion. Historically, the coatings industry has formulated systems with active inhibitors. Chromium-based coatings have been used extensively, given that they can act both as cathodic or anodic inhibitors. These have however come under pressure on health and environmental grounds forcing the development of a new generation of environmentally friendly systems. Metal phosphates find wide use with zirconia and titanium based fluoro-complexes, demonstrating comparable performance to chromium based pre-treatments.

Graphene, based on its exceptional electrical and mechanical properties, has attracted attention as a potential material to impart anti-corrosive performance. Bohm³ has proposed that graphene’s two dimensional platelet structure would enable excellent performance in barrier coatings based on its high surface area, electrical conductivity and impermeable nature and postulated that this enhanced performance might be explained by the combination of three processes:

• Graphene may make the path of water permeation more tortuous.
• The impermeability of graphene’s molecular structure will reduce penetration of oxygen, water and other corrosive materials.
• Graphene will provide an alternative path for electrons, breaking the electrochemical cell that is necessary for corrosion.

Graphene has subsequently been investigated to explore this potential. Work has included the doping of graphene with corrosion inhibitors³ and growth on substrates using chemical vapour deposition. Potentiometric analysis and traditional corrosion testing has suggested that graphene can provide a significant performance improvement.^{4, 5} A large part of this work has been done with discrete layers of graphene grown via vapour deposition or applied directly to surfaces. This however is a slow and expensive process and not applicable in the broader application requirements in the coating world.

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 to produce graphene nano-platelets (GNP). AGM’s manufacturing process uses sustainable raw material sources rather than graphite, which is inherently limited in supply.  The graphene nano-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 and can be incorporated into paint using standard dispersion equipment AGM’s nano-platelets have demonstrated true multi-functionality and development work is already underway to remove other less sustainable additives that improve properties such as electrical and thermal conductivity, wear, and fire resistance. This work explores the potential of nano-platelets distributed through a coating film to function in a comparable manner to a monolayer of graphene applied by vapour deposition and the comparative prevention of corrosion. Such an approach opens up the opportunity to coating manufacturers to utilise graphene in a convenient and manageable form when formulating coatings with improved performance.

Graphene in barrier coatings

AGM has worked with independent industry experts Paint Research Association (PRA) and TWI (The Welding Institute) to complete an evaluation of the company’s graphene platelets in an epoxy coating, with the aim of demonstrating how effective a graphene enhanced coating might be useful in preventing corrosion.

AGM evaluated two grades of graphene platelets: A-GNP 10 and A-GNP 35(T). A-GNP 10 is a medium density graphene with a rigid platelet structure and built-in oxygen functionality, which gives excellent dispersibility. A-GNP 35(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 that could be useful in preventing corrosion. A 2-pack epoxy system (epoxy equivalent weight [EEW] 184-190), cured with a proprietary unmodified aliphatic amine hardener system, was selected as being relevant for many of the epoxy primer systems that 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-GNP 10. A-GNP 35(T) was limited to a maximum loading level of 1.0 wt% due to the very high surface area of this graphene grade. Coatings were applied to mild steel panels hand-abraded (according to ISO 1514), cleaned with xylene and cured for seven days at 18 - 25°C to give a dry film thickness of 40 μm for testing using a draw-down method.

Salt fog testing

Cyclic corrosion resistance was tested under the guidelines of BS EN ISO 11997-2, modified to remove the UV light exposure. Duplicate specimens were exposed (using a repeated cycle of 60 mins.) to dilute electrolyte fog (0.35% ammonium sulfate, 0.05% sodium chloride) at 24 +/-3°C followed by 60 mins. 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 the positive results in the cyclic salt fog and continuous corrosion testing, AGM further investigated the corrosion preventing performance of the graphene-enhanced epoxy coatings. Steel panels were prepared in a similar manner, and were then subjected to a full immersion in synthetic sea water (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 oxidided, 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, panels were immersed in synthetic sea water 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 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 prevention performance of the epoxy. The proposed explanation for this has been that the graphene nano-platelets are acting as a barrier to diffusion of water and corrosive salts through the epoxy coating. The water vapour transmission rate (WVTR) through the epoxy coatings was measured 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-GNP 10 and A-GNP 35(T) were coated onto a paper substrate for this test.

Conclusions

AGM has shown that the addition of very low loadings of its A-GNP 10 and exceptionally low loadings of its A-GNP 35(T) graphene nano-platelets into epoxy coating systems can substantially improve the corrosion mitigation of these coatings. The graphene appears to offer an impressive barrier to the diffusion of corrosive elements into the underlying surface.

AGM believes that there is the potential for the A-GNPs to enable reduction of heavy metals and other anti-corrosive pigments, with the potential to remove other barrier additives and reduce coating weight. The inclusion of AGM’s A-GNP 10 and A-GNP 35(T) graphene nano-platelets into epoxy coating formulations offers the chance to significantly increase the lifetime of coated parts.

References

1. 'Steel contribution to low carbon future', Position Papers, World Steel Organisation, 2015.
2. HAYS, F. G., 'Now is the Time', World Corrosion Organisation, 2010.
3. BOHM, S., 'Graphene against Corrosion', Nat. Nanotechnology 2014, 9(10), 741-742.
4. RAMAN, R. S., et al., 'Protecting copper from electrochemical degradation by Graphene coatings', Carbon, 2012, 50(11), 4040-4045.
5. DENNIS, R. V., et al., 'Graphene nano-composite coatings for protecting low alloy steels from corrosion', Am. Ceram. Soc. Bull., 2013, 92(5), 18-24.