Graphene anti-corrosion coating and method of application thereof

Abstract

A graphene composite coating on a metal surface with excellent corrosion resistance by electrophoretic or electrolytic deposition has been obtained. The composite coating was shown to significantly increase the resistance of the metal surface to electrochemical degradation. The graphene coating significantly reduces cathodic current, which is an indicator of the rate of corrosion at the interface between the cathodic material and the anodic material.

Claims

1. A method of coating a single layer of graphene flakes comprising the steps of: 1) loading objects to be coated into a rotating carousel; 2) immersing the rotating carousel into an electrolytic suspension of graphene flakes; 3) locating a graphite rod outside the rotating carousel and within the electrolytic suspension; 4) applying a dc potential bias between the rotating carousel and the graphite rod; and 5) rotating the carousel; thereby uniformly coating the objects on all surfaces with the graphene flakes.

2. The method of claim 1 further including the step of creating the electrolytic suspension of graphene flakes by exfoliating graphene flakes from the graphite rod.

3. The method of claim 1, further including the step of sonicating the coated object.

4. An electrophoretic deposition cell comprising: a power source, a tank, a rotating carousel within the tank including a porous basket for containing an object to be created, a graphite anode, an electrolytic suspension of graphene flakes in the tank and, the rotating carousel is a cathode, the graphite anode is outside the carousel, and a dc potential bias exists between the rotating carousel and the graphite electrode.

Description

CONCISE DESCRIPTION OF THE DRAWINGS

(1) Further features and advantages of the subject invention will become clear with the aid of the following description with reference to the accompanying drawings, in which

(2) FIG. 1 shows a schematic of a representation apparatus for deposition of a graphene coating according to the subject invention.

(3) FIG. 2 is a Tafel plot of coated and uncoated Ti rods.

(4) FIG. 3 is a graph showing the change in the corrosion potential of a coated Ti rod.

(5) FIG. 4 is a graph showing the change in the corrosion current of a coated Ti rod.

(6) FIG. 5 is a graph showing the change in the corrosion potential of a coated BeCu rod.

(7) FIG. 6 is a graph showing the changes in the corrosion current of a coated BeCu rod.

DETAILED DESCRIPTION OF THE INVENTION

(8) In this electroplating process the metal is the cathode and a graphite rod is the anode. Voltages tested so far were 5V and 10V. Electroplating times tested were 30 s and 60 s. Some coating procedures involved sonication, but the ones without sonication were still stirred at 400 rpm. For stainless steel, a hydrochloric acid bath was used to remove the anodic surface layer.

(9) A number of electrolytes were used, with voltage between the cathode and anode tested at values from 5 V to 30 V, and with coating times between 5 s and 60 s. Some samples were also coated using a complete electrophoretic deposition (EPD) method, where both electrodes were Ti and the electrolyte contained a suspension of submicron graphene flakes. Standard half-cell potentials were used to estimate the voltage that would be needed for ELD. Using, for example, Ti in K.sub.2SO.sub.4 solution, it was determined that the minimum ionization level followed the equation
Ti(s)+1.63 V.fwdarw.Ti.sup.2+,(0-1)
while the equivalent graphene reaction was 1.04 V. Thus, the minimum Ti reaction would require 2.67 V, plus whatever is needed to move the graphene flakes and overcome entropy. Thus 5 V was selected as the minimum value for coating.

(10) As stated above, the subject invention is a coating that significantly reduces cathodic current, which is an indicator of the rate of corrosion at the interface between the cathodic material (Ti, Cu, Fe) and an anodic material (usually Al). The coating system uses purely electrolytic deposition (ELD), in which a graphite anode provided the graphene flakes that were electrochemically bonded to the cathode (the materials used for the cathode were Ti 6/4, 316L CRES, and BeCu).

(11) Each sample coated was a rod, 150 mm long3 mm in diameter (7 mm.sup.2 face area). One face of each rod was polished and that end of the rod was coated. The entire coated length of the rod, plus about 10 mm, was then covered with a piece of Teflon heat-shrink tubing and cured at 350 C. for up to 30 minutes, after which the excess of the tubing was cut off, almost exposing the coated face but leaving the uncoated end of the rod to attach to electricity. Rods of the coated metals were cut to 10 cm and then coated with graphene.

(12) Tafel plots are slow scan linear voltammograms where the current is displayed as the logarithm of the current. If the graphene layer is effective in decreasing corrosion, then the corrosion potential, which is determined by the location of the dip in the Tafel plot, should move positive with the coating, which means that it is more difficult to start corrosion. The corrosion current, which is determined by the intersection of the linear fit of the cathodic and anodic currents, should decrease, meaning that corrosion is proceeding more slowly.

(13) A first set of Tafel plots were taken of the graphene coated rods, then the rods were polished with 0.25 micron grit and then with 0.05 micron grit. They were then washed with deionized water, isopropanol, deionized water, and finally sonicated in millipore water for 10 min before a second set of Tafel plots for uncoated metal were taken.

(14) Tafel analysis records current and voltage, compared to a standard Ag/AgCl electrode. This takes advantage of the empirical equation

(15) $\begin{array}{cc}I=I_{\mathrm{ext}}+I\left(V=0\right)\left\{\exp \left[-\frac{nFV}{\mathrm{RT}}\right]-\exp \left[-\frac{\left(1-\right)nFV}{\mathrm{RT}}\right]\right\},& \left(0\text{-}2\right)\end{array}$
where V is the voltage at minimum current, is a transfer coefficient, F=9.648710.sup.4 C/mol is the Faraday constant, R=8.314 J/K mol is the molar constant, T is the absolute temperature, and n is the number of moles. I.sub.ext is the current due to external reactions between the Ag/AgCl electrode and other chemicals in the electrolyte; the voltage related to this contributes to the voltage at which V=0.

(16) The rods were tested in a solution of 3.5% NaCl in water. The water was not treated in any way to affect the oxygen content, so it is unlikely the water was oxygen-saturated, but it was definitely not deoxygenated. Direct measurement of corrosion current was also made.

(17) Raman spectroscopy was performed with a Fisher Scientific Optical/Raman microscope in order to prove whether the rods were in fact coated with graphene. Coated rods were cut to 2 cm and held in place in a PVC block with a set screw so the smooth surface could be analyzed. Microphotographs were taken to select locations for the various Raman scans.

(18) The spectra were taken with a 532 nm laser, 40 magnification, 5 mW power, and were corrected for fluorescence with a 6.sup.th order polynomial. 48 scans were taken at 10 s per scan, with another 48 scans for background.

(19) In the Raman scans, peaks near 1350 cm.sup.1, 1550 cm.sup.1, and possibly 2700 cm.sup.1 were observed in the scans of the coated rods but not in the scans of the uncoated rods.

(20) Raman point spectra of the coated Ti rods showed peaks at 1350 cm.sup.1 and 1560 cm.sup.1 which were not present in the spectrum of uncoated Ti, and correspond to graphene..sup.14 This indicates that the Ti was definitely coated with graphene successfully.

(21) Tafel plots of Ti coated with graphene oxide at 10 V, for 60 s, while sonicating, showed a reduction of corrosion current and increase of corrosion voltage that indicated they were essentially impervious to corrosion by rusting. As described above, the corrosion potential of the system is shown in a Tafel plot as the potential (horizontal coordinate) where the graph dips precipitously. The apparent corrosion potential for the coated rods is 0.2V vs Ag/AgCl which is the same as the O.sub.2/OH.sup. redox couple, indicating that the O.sub.2/OH.sup. redox couple is the only one present and there is no actual corrosion of the metal or the graphene coating. Previous to treatment, the corrosion potential of the metal is 0.15V vs Ag/AgCl. An example of this analysis can be found in FIG. 2.

(22) In FIG. 2 linear fits are included to show how corrosion currents are determined. With coating, the corrosion potential moves positive and the corrosion current decreases, thus showing that coating Ti with this method improves corrosion resistance, coating with sonication is best for Ti coating. This figure also shows the apparent corrosion current measurement by showing the linear fit of the cathodic and anodic currents in the Tafel plots of coated and uncoated Ti. The corrosion current is the current coordinate (vertical axis) where the lines meet on a Tafel plot and is the measure of the rate at which an electrochemical reaction such as rusting takes place. Here the apparent corrosion current from the Tafel plot of the coated Ti actually refers to both the rate at which the O.sub.2/OH.sup. reaction occurs and the rate of oxidation of this, not quite ideally coated sample.

(23) Several coating methods were attempted and compared for the Ti samples. Comparing FIGS. 3 and 4, and it is seen that the method with the greatest voltage, time, and with sonication resulted in the greatest shift in corrosion potential which likely indicates that it is the most effective barrier.

(24) FIGS. 5 and 6 are comparisons of different methods for coating BeCu. FIG. 5 shows graphene 5 V, 30 s, with no sonication, to be the most effective, but also the least reproducible.