Long-lasting corrosion protection is essential for materials used in vehicles and airplanes to ensure structural integrity under extreme operating conditions. Two chemical pretreatment processes are widely used in industrial settings to prepare coating adhesion and protect aluminum alloy surfaces against corrosion. Although highly regulated, both processes use large amounts of hazardous compounds with known environmental and health risks.
A multidisciplinary team of scientists from the Department of Energy’s Oak Ridge National Laboratory applied a laser interference patterning technique, or LIS, that is making significant progress toward eliminating the need for these dangerous chemicals. The new application of the LIS method responds to a call from the US Department of Defense for research projects that explore non-chemical alternatives for corrosion protection in military vehicles and aircraft systems.
The chromate conversion coating, or CCC, uses hexavalent chromium, a known carcinogen, to inhibit corrosion. Sulfuric Acid Anodizing, SAA, uses sulfuric acid, which can severely irritate the skin and eyes and, when inhaled, can cause permanent lung damage. Millions of gallons of used chemical solutions are disposed of as hazardous waste every year.
The military operates more than 12,000 aircraft, 10,000 tanks, hundreds of ships, and a host of other vehicles and weapon systems. The DoD owns and operates hundreds of industrial facilities that manufacture and repair these vehicles and equipment, spending more than $ 20 billion on corrosion protection annually. The agency’s Strategic Environmental Research and Development Program, or SERDP, planned and executed with the Ministry of Energy and the Environmental Protection Agency, is “focused on the development of alternative technologies to eliminate materials and processes that are of concern to the environment, ”said Robin Nissan, master program of SERDP and its sister program, the Environmental Safety Technology Certification Program.
“Our defense systems need repair and refurbishment,” he said. “Our programs invest in the development of alternative processes that can ensure robust performance, sustainable practices and eliminate environmental risks. “
In three successive publications, ORNL materials scientist Adrian Sabau and a team of chemists and manufacturing scientists described, demonstrated and analyzed an LIS technique and compared its performance to traditional solvent-intensive methods. Research co-authors included ORNL’s Jiheon Jun, Mike Stephens, Dana McClurg, Harry Meyer III, Donovan Leonard, and Jian Chen.
Sabau, who specializes in processing materials such as metal casting and solidification, and his team recently completed a project using LIS for bonding in automotive applications. When reading DoD’s call for research into solvent-free surface preparation, Sabau recognized that a similar technique could also be effective for adhesion of coatings.
In their experiments, they processed aluminum alloy sheets by dividing the primary beam of a pulsed nanosecond laser into two beams and focusing them on the same point on the sample surface. This process roughened the surface with periodic structures, altered the surface chemistry and the microstructure of the subsurface.
“In laser processing, you impact a lot of energy on the top surface, and we have to understand what is happening to the substrate. Is it damaged? Does it crack? Are there any microstructure effects that are not beneficial for corrosion protection? Sabau said.
Meyer, physico-chemist, and Leonard, microscopist, contributed to the characterization work described in Optics and laser technology. Meyer performed a chemical surface analysis using X-ray photoelectron spectroscopy, or XPS.
“XPS is a materials characterization technique that can determine what elements are on the surface – the first 5 to 8 nanometers – of solid materials,” Meyer said. “Prior to laser processing, XPS was used to determine the chemical composition of aluminum alloy sheets as received, which had high amounts of carbon. XPS was used again to determine if the laser treatment cleaned the surface. The results showed a significant reduction in carbon and was one of our main findings. XPS, along with the results of electron microscopy, have helped us understand how native oxide has been altered by laser treatment.
Sabau added: “In looking at the characterization of the subsoil, we found a beneficial aspect that we stumbled upon by accident. In the upper layer, we have seen the dissolution of precipitates rich in copper, where corrosion can start.
After cleaning an aluminum alloy sheet, surface energy often prevents the coating from sticking properly, a known problem in industrial surface coatings. The team’s next post, for the International Journal of Adhesion and Adhesives, examined the adhesion of the coating and found that the LIS method provided adhesion as well as industry standard and solvent intensive CCC and SAA techniques. A patent for the adhesion of coatings was issued in 2021 based on this LIS technique.
For the adhesion study, McClurg performed a material profilometry, a technique that maps surface contours and provides roughness measurements.
The third article, published in Corrosion: the journal of science and engineering, described the final tests the Sabau team conducted with an epoxy primer used by the U.S. military for aircraft wings and bodies.
Technician Mike Stephens performed the delicate and urgent task of applying spray coatings of primers and topcoats to exacting DoD specifications on alloy sheets that had been prepared with various treatments. He then exposed the samples to 2,000 hours of salt spray to examine corrosion resistance over several periods. Jun led the corrosion testing, studying how surfaces prepared by LIS compare to conventionally prepared alloy substrates, with and without a primer and topcoat.
“The substrate treated with laser interference exhibited higher corrosion resistance,” said Jun, who attributed the result to the dissolution of copper-rich precipitates. However, on samples coated with a primer or primer and topcoat, LIS did not perform as well as chemical solvent techniques, with some samples showing blisters within 96 hours after exposure to salt spray. However, these blisters were small and remained stable for hundreds of hours of exposure.
The team tested a second set of samples that were simply wiped off with acetone before the primer was applied, resulting in very little corrosion and blistering was delayed by hundreds of years. time.
Jun said that further investigation to optimize the SIL would be worth it.
“Our research approach, combining electrochemical measurements at the laboratory scale and ASTM adopted by industry [American Society for Testing Materials] the salt spray tests have been very successful and have helped to deeply understand the effects of laser interference treatment, ”he said.
“For a process that was carried out at room temperature without solvents, most of the samples performed extremely well,” Sabau said. “This technique is a big step in the right direction towards intensive non-chemical surface preparation for coatings. “
ORNL is managed by UT-Battelle for the Office of Science of the Department of Energy, the largest support for basic research in the physical sciences in the United States. The DOE Science Office strives to address some of the most pressing challenges of our time. For more information, please visit energy.gov/science.
The title of the article
Corrosion behavior of AA2024 structured by laser interference coated with epoxy primer containing chromate
Publication date of the article