Graphene: Potential corrosion inhibitor

Dr. Neil Canter, Contributing Editor | TLT Tech Beat July 2012

A new study shows this unique material acts as a corrosion inhibitor on copper and nickel.

KEY CONCEPTS
• Graphene exhibits corrosion-inhibiting characteristics on the surface of copper and nickel through a passivating mechanism.
• Graphene coatings were applied to copper metal through the use of chemical vapor deposition and transferred to nickel through the use of polymethyl methacrylate.
• Data generated through Cyclic voltammetry, Tafel analysis and Electrochemical Impedance Spectroscopy shows that multiple layers of graphene provide good corrosion protection.

ONE OF THE MOST INTRIGUING MATERIALS ACTIVELY BEING EXAMINED IS GRAPHENE, which is a single, two-dimensional layer of carbon atoms arranged in hexagonal structures. Each carbon atom is connected to three other atoms with the bond length between carbon atoms being approximately 0.142 nanometers.

In a previous TLT article, research was discussed demonstrating that graphene is the strongest material ever examined (1). This work was accomplished by applying a diamond tip connected to the cantilever of an atomic force microscope (AFM) onto a graphene layer until a break point was achieved. Graphene exhibited a break strength of 130 gigapascals, which is 200 times the strength of steel.

Corrosion continues to be a major cost problem, resulting in U.S. industries losing an estimated $200 billion annually. Kirill Bolotin, assistant professor in the physics and astronomy department at Vanderbilt University in Nashville, Tenn., says, “In working with graphene, we feel that it has the potential to act as a corrosion inhibitor because it exhibits a unique combination of properties.”

Graphene is very stable chemically and can be heated up to a temperature of 400 C without any difficulty. This material is also inert, very biocompatible, exceptionally transparent and conductive. A final important property is that graphene can be transferred onto a surface and will conform to the topography and features of that surface.

If graphene can be found to exhibit corrosion inhibiting characteristics, then this would just add to the versatility of this unique material. A new study has now been done that shows graphene can provide effective corrosion inhibition.

MULTILAYER GRAPHENE APPLIED BY CVD
Bolotin, in collaboration with his fellow researchers, conducted a series of studies to evaluate the ability of graphene to act as a corrosion inhibitor on copper and nickel. He says, “We chose to study copper and nickel for several reasons. Graphene can be directly grown on copper and then mechanically transferred to nickel. We found that both metals are relevant in today’s technology and can readily corrode in aqueous environments.”

The preferred approach for preparing graphene coatings was to use chemical vapor deposition (CVD). A copper foil is heated to 1,000 C under a flow of hydrogen in a fused silica tube that is heated in a hot wall furnace. The purpose of this step is to remove contaminants from the copper surface. Methane is then introduced with hydrogen gas to produce a graphene coating that can be varied from one layer to eight layers.

Mechanical transfer of graphene to nickel was achieved using polymethyl methylacrylate (PMMA). Bolotin says, “We deposited a layer of PMMA onto a single layer of graphene grown on a copper foil. After the material is placed in an etching solution to split out the copper, the PMMA/graphene stack is placed in a distilled water bath and then transferred onto a nickel metal surface. Acetone is added to the material to dissolve the PMMA, leaving a layer of graphene deposited on the nickel surface.” Bolotin indicates that the process can be repeated to prepare thicker graphene coatings.

Cyclic voltammetry (CV), Tafel analysis and Electrochemical Impedance Spectroscopy (EIS) were used to evaluate the corrosion-inhibiting characteristics of the graphene coatings. Bolotin says, “The CV test data showed that copper coated with graphene displayed minimal oxygen reduction compared to bare copper. This suggests that graphene forms a passivating barrier that protects the metal.”

Tafel analysis was conducted to quantitatively determine the rate of corrosion. Bolotin says, “We found that graphene grown by CVD significantly slowed down corrosion on both copper and nickel. Graphene-coated copper reduced corrosion by seven times as compared to the bare metal. For nickel, the rate was approximately 20 times better than the bare metal.”

The researchers also noted that the rate of corrosion reduction is equal to an organic coating that is five times thicker. Figure 2 shows the effect of two copper foils after being heated on a hot plate at 300 C. The sample on the right is bare copper that clearly oxidizes and changes color. Graphene-coated copper on the left was unaffected.


Figure 2. A graphene-coated copper foil (shown on the left) withstood heating up to 300 C without changing color. In contrast, an unprotected copper foil (shown on the right) changed color under the same conditions. (Courtesy of Vanderbilt University)

EIS was used to determine the mechanism for how graphene provides corrosion protection. Bolotin says, “We believe graphene acts as a noble metal that does not corrode. This material acts through a passivating mechanism. The only evidence for corrosion is seen when cracks are present in the graphene.”

This conclusion suggests that multiple layers of graphene will exhibit better corrosion protection because there is a greater chance that the entire metal surface will be covered with this material. Bolotin indicates that future work will focus on making the graphene coating more uniform. He adds, “We would like to scale up the coating process to minimize cracks and make the coating as pristine as possible.”

Additional information can be obtained from a recent article (2) or by contacting Bolotin at kirill.bolotin@vanderbilt.edu. Similar work evaluating the corrosion resistance of graphene-coated copper and copper/nickel alloys was also conducted by Dr. Rodney Ruoff and his research group at the University of Texas (3).

REFERENCES
1. Canter, N. (2009), “Graphene: The Strongest Material Ever Examined,” TLT, 65 (2), pp. 28-29.
2. Prasai, D., Tuberquia, J., Hart, R., Jennings, G. and Bolotin, K. (2012), “Graphene: Corrosion-Inhibiting Coating,” ACS Nano, 6 (2), pp. 1102-1108.
3. Chen, S., Brown, L., Levendorf, M., Cai, W., Ju, S., Edgeworth, J., Li, X., Magnuson, C., Velamakanni, A., Piner, R., Kang, J., Park, J. and Ruoff, R. (2011), “Oxidation Resistance of Graphene-Coated Cu and Cu/Ni Alloy,” ACS Nano, 5 (2), pp. 1321-1327.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.