Anti-corrosion coatings

Abstract

A coating comprising silicon-doped graphene layers wherein the graphene is in the form of horizontally-aligned graphene nanosheets.

Claims

1. A coating comprising silicon-doped graphene layers wherein the graphene is in the form of horizontally-aligned graphene nanosheets, and wherein the silicon content of the silicon-doped graphene is in the range 2 to 60 at %.

2. The coating as claimed in claim 1 wherein the graphene nanosheets have planar morphology.

3. The coating as claimed in claim 1, wherein the coating comprises or consists of a single monoatomic layer of silicon-doped graphene nanosheets.

4. The coating as claimed in claim 1 having a single-layer thickness in the range of 0.3 to 0.9 nm.

5. The coating as claimed in claim 1 having a total thickness in the range of 0.3 to 1.9 nm.

6. The coating as claimed in claim 1, wherein the silicon and/or second heteroatom are incorporated into the crystal lattice of the graphene.

7. The coating as claimed in claim 1, wherein the coating is metal-free.

8. The coating as claimed in claim 1, wherein said coating covers at least 50% of the surface area of the substrate.

9. The coating as claimed in claim 1, wherein said coating has a charge transfer resistance greater than 20 cm.sup.2.

10. The coating as claimed in claim 1, wherein said coating covers at least 80% of the surface area of the substrate.

11. The coating as claimed in claim 1, wherein said coating covers at least 95% of the surface area of the substrate.

12. The coating as claimed in claim 1, wherein said coating covers at least 99% of the surface area of the substrate.

13. The coating as claimed in claim 1, wherein the coating comprises or consists of multiple layers of silicon-doped graphene nanosheets.

14. The coating as claimed in claim 13, wherein the number of layers is in the range 1 to 10.

15. The coating as claimed in claim 13, wherein the number of layers is in the range 1 to 5.

16. The coating as claimed in claim 1, further comprising a second, non-metallic heteroatom dopant, which is not oxygen.

17. The coating as claimed in claim 16 wherein the second heteroatom is nitrogen.

18. A coated metal wherein at least a portion of the metal surface comprises a silicon-doped graphene coating as defined in claim 1.

19. The coated metal as claimed in claim 18, wherein at least 50% the metal is coated.

20. The coated metal as claimed in claim 18, wherein at least 80% of the surface area of the metal is coated.

21. The coated metal as claimed in claim 18, wherein at least 95% of the surface area of the metal is coated.

22. The coated metal as claimed in claim 18, wherein at least 99% of the surface area of the metal is coated.

Description

BRIEF DESCRIPTION OF FIGURES

(1) FIG. 1 shows an SEM image of vertical graphene nanoflakes showing a network morphology

(2) FIGS. 2A-2C show multilayer graphene nanoflakes (MGNFs) and carbon nanoflakes (CNFs). FIGS. 2(a) and 2(b) are different magnification TEM images of MGNFs. FIG. 2(c) is a high resolution TEM image of CNFs, showing the nanoflake has a knife-edge or conical tip structure with open graphitic planes. The inset of FIG. 2(c) is an EDS (energy dispersive x-ray spectroscopy) spectrum showing the chemical composition of MGNF films.

(3) FIG. 3. shows polarization curves after 1 h immersion for clean and annealed copper

(4) FIG. 4 shows polarization curves after 1 h immersion for annealed copper, graphene (G) and silicon incorporated graphene (G:Si 3 sccm TMS) coated copper foils.

(5) FIG. 5 shows optical photos of (a) bare copper, (b) graphene with silicon incorporated and c) graphene coatings of copper foils after annealing at 200 C. for 1 hour.

(6) FIGS. 6A-6C show SEM images of (a) bare Cu foil after annealing at 200 C. for 1 hour (b) Silicon doped graphene on copper foil before annealing and (c) Silicon doped graphene on cupper foil after annealing at 200 C. for 1 hour

(7) FIG. 7. shows raman spectra of (a) graphene and (b) Silicon incorporated graphene coated on Cupper before and after annealing in air at 200 C. for 1 hour

(8) FIGS. 8A-8C show impedance (8a), Nyquist (8b) and Bode (8c) plots of graphene and Silicon incorporated graphene coated on Copper foil

EXAMPLES

(9) In this work we have examined the behaviour of bare copper, annealed copper, graphene coated copper and Si incorporated copper in saline solution as well as after exposure to air at elevated temperature of 200 C. for 1 hour.

(10) Sample Preparation

(11) A SEKI Technotron ECR-MPCVD system (2.45 GHz, 1.5 KW), was used to deposit Si incorporated graphene films. Briefly the system was pumped down to base pressure of 2.8 10-5 Torr and was operated in ECR mode. Cu foils 25 m thick (99.999%, Alfa Aesar No. 10950) were initially etched for 3-4 minutes at room temperature in an Ar plasma using microwave power of 100 W. Then the temperature was raised to approximately 900 C. and the samples are heated in a background pressure of 40 Torr of N.sub.2.

(12) Graphene and silicon incorporated graphene were synthesised in a gas mixture of methane and argon and TMS (tetramethylsilane-Si(CH.sub.3).sub.4 with gas flow rates of 6, 3 and 0 to 10 sccm (standard cubic centimeters per minute) respectively. Deposition was carried out on the pre-heated Cu foils at a gas composition pressure of 7.5 10.sup.4-17 10.sup.4 Torr for a duration of 90 sec at a microwave power of 100 W.

(13) Corrosion resistance is profoundly influenced by the microstructure of the metal as well the presence of native oxide. The native oxide of copper was mechanically removed with SiC paper and cleaned in ultrasonic bath of acetone and ethanol (termed cleaned). Samples termed annealed were subjected in all the steps required for graphene deposition, without been coated.

(14) Electrochemical Tests

(15) Electrochemical measurements were carried out by using a self-made 10 mL cylinder cell with three electrodes (specimens with an exposed area of 0.196 cm.sup.2 acted as the working electrode, platinum wire as counter electrode and a Ag/AgCl electrode as the reference electrode). Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed in 0.1 M sodium chloride (NaCl) solution using an Autolab potentiostat.

(16) All the experiments were performed at room temperature. Open circuit potential was monitored for 1 h to confirm its stability with time. Any fluctuation of the open circuit potential less than 10 mV for a period of 1000 s was considered as a stable potential before carrying out the corrosion tests. Potentiodynamic polarization tests were carried out at a scan rate of 0.5 mV/s. The impedance tests were carried out by applying a sinusoidal potential wave at Open Circuit Potential OPC with an amplitude of 10 mV Impedance response was measured over frequencies between 1 MHz and 10-2 Hz, at open circuit potential.

(17) We employ optical micrographs as one diagnostic for degree of oxidation; oxide films of increasing thickness lead to optical interference effects and pronounced color changes.

(18) Results and Discussion

(19) Polarization curve (FIG. 3) shows more positive Ecorr, corrosion potential value and lower current densities in both anodic and cathodic regions for the annealed copper, which implies a higher corrosion resistance. The role of graphene and silicon incorporating graphene coating in suppressing Cu corrosion is shown in FIG. 4. It is clear that silicon incorporation (3 sccm TMS during deposition) causes a positive shift on the Ecorr and a marked decrease in the anodic current densities, compared to pure graphene, which is further enhanced at higher TMS flow rates.

(20) FIG. 5 shows photograph images of Cu foil surface, graphene coating, and silicon incorporated graphene coatings after annealing in air at 200 C. for 1 hour. Before annealing, the surface colours of bare copper, graphene and silicon incorporated graphene coated Cu foils are almost similar as the graphene film is highly transparent. After annealing in atmospheric air, the uncoated Cu surface changes significantly to red due to significant oxidation of copper. The change of surface colour of Silicon incorporated graphene coated Cu foil is negligible whereas that of graphene rather more noticeable, which indicates that silicon incorporation creates a greater diffusion barrier to prevent oxidation. In addition no significant change was observed in the SEM images (FIG. 6) as well as Raman spectra of the graphene and silicon incorporated graphene before and after annealing in air as revealed in FIG. 7.

(21) FIG. 8 shows Nyquist plot (Zreal vs. Zimg) and Bode magnitude plot (|Z| vs. log of freuencency) and phase plots for a graphene and silicon incorporated graphene coated cupper. The figures demonstrate that Silicon incorporation increases the impendence of graphene. Therefore the improved corrosion resistance of Silicon incorporated graphene coating can be attributed to its enhanced charged transfer resistance.