CORROSION PROTECTION FOR METALLIC SUBSTRATES

Abstract

A composition suitable for coating a metallic substrate that is susceptible to corrosion is disclosed. The composition comprises a carrier medium and graphene platelets in which the graphene platelets comprise between 0.002 wt % and 0.09 wt % of the coating, and the graphene platelets comprise one of or a mixture of two or more of graphene nanoplates, bilayer graphene nanoplates, few-layer graphene nanoplates, and/or graphite flakes in which the graphite flakes have one nanoscale dimension and 25 or less layers

Claims

1. A composition suitable for coating a metallic substrate that is susceptible to corrosion characterised in that the composition comprises a carrier medium and graphene platelets in which the graphene platelets comprise between 0.002 wt % and 0.09 wt % of the coating, and the graphene platelets comprise one of or a mixture of two or more of graphene nanoplates, bilayer graphene nanoplates, few-layer graphene nanoplates, and/or graphite flakes in which the graphite flakes have one nanoscale dimension and 25 or less layers.

2. A composition according to claim 1 in which the graphene platelets comprise between 0.002 wt % and 0.004 wt % of the coating, between 0.0026 wt % and 0.04 wt % of the coating, between 0.0026 wt % and 0.0035 wt % of the coating, between 0.006 wt % and 0.009 wt % of the coating, around 0.003 wt % of the coating, or around 0.03 wt % of the coating.

3. A composition according to claim 1 in which the packing density of the graphene platelets is sufficiently low that the majority of the graphene platelets are not in physical contact with any other graphene platelets.

4. A composition according to claim 1 in which the packing density of the graphene platelets is sufficiently low that the majority of the graphene platelets are not in electrical contact with any other graphene platelets.

5. A composition according to claim 1 in which the graphene platelets have a particle size distribution with a D50 of less than 30 m, less than 20 m, or less than 15 m.

6. A composition according to claim 1 in which more than 50% of the graphene platelets have an electrical conductivity greater than around 2.1510.sup.7 S/m at 20 C.

7. A composition according to claim 1 in which more than 50% of the graphene platelets have an electrical conductivity greater than around 3.510.sup.7 S/m at 20 C.

8. A composition according to claim 1 in which at least 50 wt % of the graphene platelets comprise graphite flakes with one nanoscale dimension and 25 or less layers.

9. A composition according to claim 1 in which the carrier medium is electrically non-conductive, and in which the carrier medium is selected from crosslinkable resins, non-crosslinkable resins, thermosetting acrylics, aminoplasts, urethanes, carbamates, polyesters, epoxies, silicones, polyureas, silicates, polydimethyl siloxanes, and mixtures and combinations thereof, and in which the carrier medium is plastically deformable once it has set/cured.

10-11. (canceled)

12. A composition according to claim 1 in which the composition further comprises a solvent.

13. A composition according to claim 1 in which the composition further comprises a dispersant.

14. A coating system for a metallic substrate that is susceptible to corrosion for creation of a first coating on the metallic substrate, and, subsequently, a second coating over the first coating characterised in that the first coating comprises a composition according to claim 1, and the second coating is formed from a second composition which comprises a carrier medium and 2D material/graphitic platelets in which the 2D material/graphitic platelets comprise more than 0.1 wt % of the second coating.

15. A coating system according to claim 14 in which the 2D material/graphitic platelets of the second composition one of or a mixture of two or more of graphene, graphene oxide, reduced graphene oxide nanoplates, hexagonal boron nitride, molybdenum disulphide, tungsten diselenide, silicene, germanene, Graphyne, borophene, phosphorene, a 2D in-plane heterostructure of two or more of graphene, graphene oxide, reduced graphene oxide nanoplates, hexagonal boron nitride, molybdenum disulphide, tungsten diselenide, silicene, germanene, Graphyne, borophene, phosphorene, bilayer graphene, bilayer graphene oxide, bilayer reduced graphene oxide nanoplatelets, few-layer graphene, few-layer graphene oxide, few-layer reduced graphene oxide nanoplatelets, graphite flakes in which the graphite flakes have one nanoscale dimension and 25 or less layers, layered hexagonal boron nitride (hBN), molybdenum disulphide (MoS.sub.2), tungsten diselenide (WSe.sub.2), silicene (Si), germanene (Ge), Graphyne (C), borophene (B), phosphorene (P), and or a 2D vertical heterostructure of two or more of the aforesaid materials.

16. A coating system according to claim 14 in which the 2D material/graphitic platelets of the second coating comprise between 0.1 wt % and 20 wt % of the coating, between 0.1 wt % and 6.0 wt % of the coating, or between 0.1 wt % and 0.5 wt % of the coating.

17. A coating system according to claim 14 in which more than 50% of the 2D material/graphitic platelets of the second coating have an electrical conductivity which is less than the electrical conductivity of more than 50% of the graphene platelets of the first coating.

18. A coating system according to claim 14 in which more than 50% of the 2D material/graphitic platelets of the second coating have an electrical conductivity of around or less than 2.010.sup.5 S/m at 20 C.

19. A method of treatment of a metallic substrate in which the substrate is coated with a composition according to claim 1.

20. A method of treatment of a metallic substrate in which the substrate is treated with the system according to claim 15.

21. A method of treatment according to claim 19 in which the substrate is aluminium, an aluminium alloy, or a magnesium based alloy.

22. A method of treatment according to claim 20 in which the substrate is aluminium, an aluminium alloy, or a magnesium based alloy.

Description

EXPERIMENTAL RESULTS

[0050] Various loadings of graphene nanoplatelets of varying conductivity as shown in Table 1 were incorporated into an epoxy system in the quantities shown in Table 2. The epoxy system used was an epoxy resin with and epoxy equivalent weight of 171-175 g/eq and the Grade 1 and Grade 2 graphene platelets had a particle size with a D50 of less than 50 m. Control A is the epoxy system with no graphene included, and Control B is no formulation at all. That is Control B is bare, untreated metal.

[0051] The epoxy system and graphene platelets were weighed out using a 4 decimal place analytical balance.

[0052] Each of Formulations 1 to 6, Control A and Control B were applied to two aluminium panels. The aluminium panels were each made of aluminium 5005 alloy, an alloy with the following composition Magnesium (Mg) 0.50-1.10 wt %, Iron (Fe) 0.0-0.70 wt %, Silicon (Si) 0.0-0.30 wt %, Zinc (Zn) 0.0-0.25 wt %, Manganese (Mn) 0.0-0.20 wt %, Copper (Cu) 0.0-0.20 wt %, Others (Total) 0.0-0.15 wt %, Chromium (Cr) 0.0-0.10 wt %, Other (Each) 0.0-0.05 wt %, Aluminium (Al) Balance. Each Formulation and Control was applied by spray application, using a conventional gravity-fed gun, through a 1.2 mm tip, resulting in dry film thicknesses ranging from 40-60 m on the aluminium panels. The panels were cured for 1 week at ambient temperature, before commencing testing.

[0053] One panel for each Formulation and Control was scribed with a 225 mm scribe using a knife. Care was taken that the scribes were as consistent as possible throughout due to relatively small surface area of study. The panels for each Formulation and Control were tested in duplicate in both scribed and unscribed forms. Scribed samples were studied because they offer an immediate study of a bare metal surface which may be contacted with an electrolyte without having to observe the lengthy breakdown/degradation of the film coating e.g. due to water uptake.

[0054] All electrochemical measurements were recorded using a Gamry 1000E potentionstat in conjunction with a Gamry ECM8 multiplexer to permit the concurrent testing of up to 8 samples per experiment. Each individual channel was connected to a Gamry PCT-1 paint test cell, specially designed for the electrochemical testing of coated samples.

[0055] Within each paint test cell, a conventional three-electrode system was formed, the bare aluminium, epoxy coated aluminium, and scribed coated epoxy aluminium panels were the working electrode, a graphite rod served as a counter electrode and a saturated calomel electrode (SCE) served as the reference electrode. All tests were run using a 3.5 wt % NaCl electrolyte.

[0056] For all samples, electrochemical testing consisted of corrosion potential measurements (E.sub.corr) followed by potentiodynamic polarisation scans. Since this work is focussed on the change in electrochemical properties over time, each cycle of experiments was conducted at approximate intervals of 2 hours over a period of 1 week for all samples.

[0057] Potentiodynamic polarisation scans were carried out in order to generate Tafel polarisation curves. These curves were produced as a result of applying a potential of 250 mV from the open circuit potential (500 mV sweep) at a scan rate of 0.5 mV/second with a sample period of 1 second, over a sample area (working electrode area) of 14.6 cm.sup.2. Data fitting to the Tafel region was carried out using the Gamry Echem Analyst software in order to extract values for the anodic and cathodic Tafel constants, E.sub.corr, and corrosion rate. These values were then plotted for the time duration of the experiment.

[0058] All corrosion potential (E.sub.corr) measurements were recorded against the SCE reference electrode.

[0059] Potentiodynamic polarisation scans permit considerable amounts of information on electrode processes to be determined. Through this technique, information on corrosion rate, pitting susceptibility, passivity and anode/cathode behaviour of an electrochemical system may be obtained. During such scans, the driving force of the anodic/cathodic reactions (potential) is varied and the net change in reaction rate (the current) is measured. Tafel plots are usually displayed with the applied potential on the y axis and the logarithm of the measured current on the x axis, where the top half above the corrosion potential represents the anodic portion of the plot and the bottom half below the corrosion potential represents the cathodic portion of the plot. The Tafel region or active region is usually a straight line and represents electron transfer i.e. the metal oxidation reaction for the anode and the oxygen reduction process in the case of the cathode. The intersection point of back extrapolation of the anodic and cathodic Tafel slopes represents the corrosion current, from which a corrosion rate may be determined. The gradient of the Tafel slopes themselves is equivalent to the Anodic/Cathodic Tafel constants, measured in volts/decade, and these values are a measure of the degree of increase in the overpotential required to increase the reaction rate (the current) by a factor ten.

[0060] Beyond the Tafel regions, when an extended potential range is applied, additional useful features may be observed in the polarisation data. In the case of the anode, one such feature is known as the passivation potential. As the applied potential increases above this value, a decrease in the measured current density is observed until a low, passive current density is achieved; the point at which the current density undergoes no change with an increase in applied potential (passive region). Beyond this point, if the applied potential permits and is sufficiently positive, the current rapidly increases: the breakaway potential. For aluminium alloys, this breakaway potential may be due to a localised breakdown in passivity (pitting).

Example Data

[0061] The data in Table 3 demonstrates the electrochemical values obtained for samples which have scribe damage, and intact coatings. It shows the corrosion potential (E.sub.corr) The anodic and cathodic currents, and corrosion rate in m per year and mils per year. This data is used to construct Tafel plots which in themselves demonstrate whether the corrosion mechanism is by barrier, or passivation.

[0062] The Tafel plot showing passivation occurring with Formulation 2 when scribed is shown in Table 4

[0063] The near flat gradient of the upper curve in Table 4 is consistent with passivation occurring at the substrate in this case an aluminium alloy. When no scribe is present, the coating itself acts as a barrier, and no passivation occurs as water and oxygen are not present at the substrate. The Tafel plot showing passivation occurring with Formulation 2 when unscribed is shown in Table 5

[0064] In contrast an indication of barrier performance can be seen from Formulation 5. The Tafel plot occurring with Formulation 5 when scribed and unscribed are shown in Tables 6 and 7 respectively. There is little difference in the anodic and cathodic currents shown which is an indication that Graphene Grade 2 performs as a physical barrier, rather than controlling corrosion by passivation.

[0065] Barrier performance of the Graphene Grade 2 is also demonstrated with water vapour transmission testing. With five Formulations and a Control C as per Table 8 The epoxy was cured with a polyamide blend (epoxy: polyamide 5.36:1), and the panels were allowed to cure for a period of at least 7 days at a consistent ambient temperature.

[0066] Testing for the transmission of water through the film showed the results in Table 9. As may be seen, the data in Table 9 demonstrates a significant decrease in the transmission of water through the film as the loading of the Graphene Grade 2 increases.