Nanotechnology Helps Army Usher In Coatings Evolution
By Matthew V. Veazey
 |
 |
These images show a fuel tank at Ft. Bragg before (left) and after being applied with an epoxy and polyurethane coating formulated with carbon nanotubes. The coating is commercially available. Photo courtesy of Army Corps of Engineers ERDC CERL. |
The Department of Defense uses zinc-rich primers to protect steel structures such as bridges, metal buildings, utilities, and numerous other assets from corrosion. A key attribute of these coating system components is that zinc dust particles present in the primer "sacrifice" themselves, undergoing corrosion rather than the underlying steel surface that is being protected. Although exhibiting this process of "cathodic protection" (CP) is a desirable feature for keeping structures and equipment in working order, zinc-rich coatings are not without their disadvantages.
"Protective coatings are the first line of defense for the prevention and control of corrosion on steel," said Susan Drozdz a chemist with the U.S. Army Engineer Research and Development Center Construction Engineering Research Laboratory (ERDC CERL) in Champaign, Illinois. "Coatings provide a barrier between the substrate and the surrounding environment. However, virtually all coatings are susceptible to defects, and when the barrier is breached corrosion is likely to occur."
Reducing Zinc Dust
Drozdz and her fellow researchers at ERDC CERL's Paint Technology Center are testing a coating system that is designed to have improved physical and mechanical properties—and thus a longer service life—while maintaining the corrosion protection offered by a traditional zinc-rich primer. "To achieve electrical conductivity, zinc-rich coatings rely on tangential contact between zinc dust particles," Drozdz said. "The high pigment volume impacts physical and mechanical properties. Judged against other pigmented coating films, zinc-rich primers have reduced durability, adhesion, flexibility, and impact and abrasion resistance."
Drozdz also pointed out that the manner in which zinc-rich coatings protect the substrate actually limits how well they can protect the steel against corrosion. "As the zinc dust protects the substrate in the zinc-rich system, the zinc becomes oxidized," she explained. "The zinc dust in closest proximity to the substrate sacrifices itself first, thus oxidizing first and therefore creating a non-conductive barrier between themselves and other tangential zinc particles in the coating system." In effect, the CP stops when the zinc dust adjacent to a coating becomes oxidized.
Carbon Nanotube Coating
The epoxy and polyurethane technology that ERDC CERL is testing, which is commercially available, forms a strong protective barrier against scratches and other mechanisms that can exacerbate corrosion by exposing the steel substrate to the environment. It achieves the latter capability by integrating carbon nanotubes to reinforce and stiffen the coating, which provides excellent tensile strength and improved flexibility.
 |
 |
The epoxy and polyurethane coating is used on the supply line running to the Ft. Bragg fuel tank (left). Pictured (at right) is the piping exerior surrounding the tank. Photo courtesy of Army Corps of Engineers ERDC CERL. |
Carbon nanotubes are novel, manipulated atomic- or molecular-scale carbon structures that exhibit extraordinary strength and unique electrical properties; they are also efficient conductors of heat. So strong are tiny carbon nanotubes—each is a mere 1 to 100 nanometers in length (1 nanometer equals one billionth of a meter)—that their bonding structure is stronger than the bonds found in diamonds. "Nanotubes naturally align themselves into 'ropes' held together by Van der Waals forces," Drozdz explained, referring to the attractions among atoms, molecules, and surfaces that result from the fluctuating polarizations of nearby particles.
"Our research and development goal is to develop organic coatings that are easily applied, producing the toughest, most resilient barrier properties," Drozdz continued. Should a break in this barrier occur, the coatings also must inhibit the corrosion of the exposed steel substrate.
"In order to accomplish this endeavor a basic problem associated with organic coating systems needs to be overcome: they are non-conductive," she said. "In order to achieve long-term corrosion protection, a conductive electron path must be established through a non-conductive binder. Carbon nanotubes and inherently conductive polymers provide this function."
Drozdz also pointed out that the lower zinc loading levels in the epoxy and polyurethane coating yield a strong, stable coating film that is well optimized under the resin's critical pigment volume concentration. "The strong and conductive network of carbon nanotube ropes strengthens and stiffens the film while building an electron path through the binder system," she explained. "Essentially, less zinc provides more availability for CP to damaged coating areas via the carbon nanotube ropes and inherently conductive polymers."
Helping the Environment
Aside from exhibiting improved physical and mechanical properties as well as superior CP, the coating formulated with carbon nanotubes offers important environmental benefits. "Longer service life equates to waste reduction," Drozdz said. She explained that the high per capita rates in which the world's industrialized countries consume coatings annually translate into large amounts of waste produced with high demands on raw materials and energy. "Extending service life has a positive environmental and economic impact on waste reduction, energy, and raw material consumption.”
Also benefiting the environment are the coating's lower zinc levels, which Drozdz said yield correspondingly lower levels of toxic-heavy metals such as cadmium and lead that are always present with zinc. "Total elimination of all heavy metals is possible utilizing an aluminum system variant" of the coating, she added.
'Evolutionary Benefits' Ahead
The Army’s Engineer Research and Development Center has tested several coatings formulated with carbon nanotubes or inherently conductive polymers. Drozdz said the results to date have been encouraging. Researchers apply various primer formulations and an epoxy polyamide topcoat—three coats in all—to a series of steel test panels, subsequently scribing the panels to expose the steel substrate and immersing them in both fresh water and salt water in accordance with ASTM D870. In addition, investigators are conducting outdoor weathering and exposure testing in accordance with ASTM D1014 in Champaign, Illinois; Fort Bragg, North Carolina; Fort Lewis, Washington; and the Army Pier at Duck, North Carolina. In addition, researchers have completed demonstration projects of the three-coat system at Fort Bragg and Fort Lewis.
Drozdz notes that researchers periodically evaluate the corrosion resistance and weathering of the scribed panels in accordance with ASTM D1654. After more than 4.5 years and 40,000 hours of testing, the scientists grade the panels "10" on a scale of 1 to 10. Control panels consisting of a zinc-rich system (conforming to the military specification MIL-DTL-24441) that is commonly employed by the Army Corps of Engineers serve as a benchmark. "Carbon nanotube and conductive polymer formulations utilizing zinc or aluminum as the sacrificial pigment are performing better than or equal to the control panels," Drozdz reported.
The carbon nanotube coatings, which are offered by Tesla NanoCoatings Limited, are available in several variants under development, which are derived from the initial product platform. The three-coat systems tested and used at Fort Bragg and Fort Lewis are available for commercial and government uses, and a two-coat system will be available later this year.
Although Drozdz is focused on military applications of the carbon nanotube technology, she expects it to lend itself to myriad commercial and industrial applications such as:
- Marine coatings
- Bridges
- Lightweight performance coatings
- Plating alternatives
- Steel hardware and structures
- Petrochemicals
- Offshore rigs
- Oil tankers
- Pipelines and transmission lines
- Drilling, refinery, and plant maintenance coatings
- Locks and dams
"We anticipate that the revolutionary properties of carbon nanotechnology will provide evolutionary benefits to the protective coatings market," Drozdz concluded, adding that nanotechnology benefits have already resulted in coatings with improved adhesion, barrier, and corrosion-resistant properties. "In the very near future, these coatings will become the standard for corrosion protection of metal substrates."
|
