Graphene and Corrosion Protection
Since the genesis of steel structures, those with maintenance responsibilities for them have needed to tackle the issue of corrosion. The World Corrosion Organization states that corrosion produces $2.5tr in damage to steel structures each year – around 3-4% of the yearly GDP of industrialized countries. To mitigate the damage and related costs, many alternatives exist, such as galvanization with zinc, chromium and other metals. Many of these metals are subject to intense pressure because of their REACH classification.
For very corrosive atmospheres, characterized in ISO 12944-2018 as CX environment, the most employed corrosion protection primers remain established on zinc-rich epoxy formulations (zinc content of above 80%), in various instances adapted with either with very complex binder systems and/or additional corrosion safeguarding pigments. For less corrosive conditions, passive corrosion safeguarding pigments such as phosphates are usually adequate.
Zinc is classed as extremely toxic to fish and will be released as zinc oxide, which is also toxic to fish, in the last phase of the life cycle of a corrosion protection coating throughout a corrosive attack, which will cause environmental contamination close to the steel construction.
In 2012, the Chinese government released a mandate for decreasing zinc in corrosion protection primers to accomplish an increased level of environmental safeguarding when these systems are employed.
Concurrently, Kirkland (Kirkland, N.T., Schiller, T., Medhekar.N.; Birbilis, N (2012). Exploring graphene as a corrosion protection”. Corrosion Science 56 (March 2012), 1 – 4) proved that graphene could be an option as corrosion safeguarding agent.
This study, together with other studes encouraged The Sixth Element to accept the challenge from the Chinese government in 2012 to create a corrosion protection primer with less zinc by combining zinc powders with graphene for improved performance.
Employing a specially-created kind of graphene, known as SE1132, and extremely big zinc powder particles (D50 greater than 10 µm), The Sixth Element showed that combining 25 weight % zinc powder and one weight % of this kind of graphene, all constructed on dry film, leads to enhanced corrosion safeguarding performance in a 2k-epoxy primer.
3000 hours was achieved in usual salt spray testing environments (50 +/- 5 g of sodium chloride per liter of water at 35°C +/- 2°C, 50 µm dry film thickness)( see figure 1b). This finding is now safeguarded by a patent awarded in China and the US (China patent No. CN201210108771.3, US Patent No. US9982142B2).
Explanation of the Success:
- Graphene possesses an increased lateral size (in xy-direction, some µm each) with extremely low thickness in nano-meter scale (< 3 nm). This kind of particle creates a large pigment capacity concentration applying the density of 2200 kg/m³. For the calculation, the volume of every particle must be determined on the basis of the dimensions. Conventional formulas are unsuitable. The consequence of this elevated pigment volume concentration is that despite one weight % having a very large amount of particles present, substantially expanding the barrier properties of the coating technique (see Figure 1 a)
- Because of its electrical conductivity, graphene additionally supports zinc in its cathodic safeguarding behavior by constructing a denser electrical network.
Figure 1. (a) Schematic of rGO dispersed in a coating layer, compared to spherical nanofillers or without coating; (b) Salt spray testing indicates the better anti-corrosive performance for SE1132-containing epoxy primer compared to conventional primer; (c) A rack in Nantong, China, applied with SE1132-enhanced anti-corrosive primer. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
Apart from SE1132, The Sixth Element has shown that graphene types SE1133 and SE1233 can be employed as corrosion safeguard additives in primer systems. Consumers in China and in Europe have introduced primer formulations, including graphene, to the market successfully (shown in Figure 2).
Figure 2. Graphene powder products and graphene coatings products. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
For the first time in the world, a graphene-based corrosion protection primer has been employed for coating the inner and outer wall of the steel construction of an off-shore wind energy plant in December 2014. Many paint inspections of the steel construction, conducted in 2015 and 2019, corroborated the remarkable performance of the coating system comprised of the corrosion protection primer.
Figure 3. The graphene anti-corrosion coating system used in the wind turbine in 2014 to 2019. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
In addition to this, The Sixth Element moved to significantly smaller zinc particles (D50 between 3 µm and 7 µm). In this case, it was enough to merge 0.6 – 0.8 weight-% of SE1132 with 25 weight-% to 28 weight % of zinc powder (all based on dry film) to gain superb outcomes in corrosion safeguarding.
A 2k water-based epoxy formulation with 45 % zinc and 1 % of S1233 (both based on dry film) proved to pass 2400 in neutral salt spray testing easily.
Figure 4. 2K water-based epoxy system containing zinc and graphene. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
A further demonstration how effective graphene can enhance properties of corrosion protection primers is a case in which SE1233 with a concentration of 0.45 % was combined with 33.5 % of zinc in solvent based 2k epoxy system. 4000 hours of neutral salt spray testing was passed.
Figure 5. 2k solvent based epoxy with 33.5 % zinc, 0.45 % SE1233 . Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
Graphene is famous also for its thermal conductivity. Why not combining corrosion protection with thermal conductivity to enhance the thermal dissipation capacity of metal material in heat exchangers, radiators etc. To balance mechanical, corrosion protection and thermal conductivity properties, The Sixth Element has developed the ready to use systems SE5133/5143. Good corrosion protection and heat dissipation values are achieved.
Figure 6. Thermal conductive coating – 100 µm primer with SE5133, 40 µm top coat with SE5143. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
Figure 7: total dry film thickness – 140 µm, primer with SE5133, top coat with SE5143. Image Credit: The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
This information has been sourced, reviewed and adapted from materials provided by The Sixth Element (Changzhou) Materials Technology Co.,Ltd.
For more information on this source, please visit The Sixth Element (Changzhou) Materials Technology Co.,Ltd.